Advances in Agronomy, Volume 40

ADVANCES IN

AGRONOMY

VOLUME 40

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ADVANCES IN

AGRONOMY Prepared in Cooperation with the AMERICAN SOCIETY OF AGRONOMY

VOLUME 40 Edited by N. C . BRADY Science and Technology Agencyfor International Development Department of State Washington,D.C.

ADVISORY BOARD H. J. GORZ,CHAIRMAN M. A. TABATABAIT. M. STARLING

E. J. KAMPRATH R. J. KOHEL G. E. HAM G. H. HEICHEL E. L. KLEPPER R. A. BRICCS,Ex OFFICIO ASA Headquarters

1986

ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers

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CONTENTS PREFACE............................................

vii

AGRONOMY OF WHITE CLOVER

J . Frame and P. Newbould I. Introduction ..........................................

I1. Morphology and Function...............................

Environment ......................................... Culture .............................................. Nitrogen Fixation., .................................... Quality .............................................. Grass/White Clover Dynamics ........................... Production and Management ............................ Potential Production ................................... X . Management Guidelines ................................ XI . Conclusions .......................................... References............................................

111. IV. V. VI. VII. VIII . IX.

1 4 10 20 38 41 45 49 67 69 70 75

AGRONOMIC VALUE OF UNACIDUIATED AND PARTIALLY ACIDULATED PHOSPHATE ROCKS INDIGENOUSTO THE TROPICS

L. L. Hammond. S. H . Chien. and A . U . Mokwunye I . Introduction .......................................... I1. Indigenous Phosphate Deposits in the Tropics............... I11. Agronomic Potential of Phosphate Rock for Direct Application .................................. IV . Physical Factors Influencing Expression of Agronomic Potential ................................. V. Soil Factors Influencing Expression of Agronomic Potential . . . VI . Climatic Factors Influencing Expression of Agronomic Potential ................................. VII. Partial Acidulation of Phosphate Rock ..................... VIII. Regional Findings on Direct Application of PR and PAPR .... IX . Summary and Conclusions .............................. References............................................ V

89 90 93 101

104 110 112 116 134 137

vi

CONTENTS CROP SIMULATION MODELS IN AGRONOMIC SYSTEMS

F. D. Whisler, B. Acock, D. N. Baker, R.E. Fye, H. F. Hodges, J. R. Lambert, H. E, Lemmon, J. M. McKinion, and V. R. Reddy I. Need for Crop Simulation Models and Types of Models. ......

11. Model Building. .......................................

111. Model Testing ........................................ IV. Model Applications .................................... V. Summary.. .......................................... References. ...........................................

141 146 175 180 204 204

UREA TRANSFORMATIONS AND FERTILIZER EFFICIENCY IN SOIL

W. D. Gould, C. Hagedorn, and R. G. L. McCready I. 11. 111. IV. V.

Introduction. ......................................... Problems Associated with Urea Fertilizers. ................. Urea Transformations .................................. Methods to Alter the Efficiency of Urea. ................... Summary ............................................ References. ...........................................

209 21 1 216 220 23 1 232

LUPIN CROP AS AN ALTERNATIVE SOURCE OF PROTEIN

L. Lopez-Bellido and M.Fuentes I. Introduction.. ........................................ 11. Botany and Ecology of Lupin ............................ 111. Plant Breeding ........................................ IV. Plant Production ...................................... V. Chemical Composition and Use of Lupin Seed .............. VI. Perspectives .......................................... References............................................

INDEX.....................................................

239 242 260 268 284 288 290

297

PREFACE The contributions to this volume 0fAdvance.sin Agronomy cover a range of topics of current interest to crop and soil scientists.Two are crop specificand deal with important legumes. The first focuses on white clover (Frame and Newbould: Agronomy of White Clover), the important pasture legume in temperate regions. What is known about this crop, as well as its limitations, is given coverage. Another chapter is a review of the current and potential role of lupins as sources of plant protein for human and animal consumption (Lopez-Bellido and Fuentes: Lupin Crop as an Alternative Source of Protein). Two other contributions address issues involving soils. Hammond et al. (Agronomic Value of Unacidulated and Partially Acidulated Phosphate Rocks Indigenousto the Tropics) focus on the role of phosphorus in tropical agriculture and on practical means of using indigenous phosphate deposits to increase agriculturalproduction. Gould et al. (Urea Transformationsand Fertilizer Efficiency in Soil) review the transformations of urea and their implications for nitrogen supply and utilization. The worldwide significance of these transformations is shown by the serious losses of nitrogen resulting from the transformation of urea nitrogen to other forms. The article by Whisler et al. (Crop Simulation Models in Agronomic Systems) deals with the building and testing of crop simulation models for cotton, soybeans, and wheat. This pioneering work may well represent the beginning of a new area of computerized models to govern on-farm management decisions in not only the industrialized nations but also the Third World. I thank the contributorswho prepared the five chaptersthat constitute this volume. Researchers worldwide will appreciate their efforts. N. C. BRADY

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ADVANCES IN AGRONOMY. VOL. 40

AGRONOMY OF WHITE CLOVER J. Frame' and P. Newbould2 'The West of Scotland Agricultural College, Auchincruive, Ayr KA6 5HW, Scotland *The Hill Farming Research Organisation, Bush Estate, Penicuik, Midlothian Eli26 OPY, Scotland

I. INTRODUCTION White clover (Triflium repens L.) is the most important pasture legume in temperate zones of the world. It is of value because of its wide climatic range, the high nutritional quality and digestibility of its herbage, and the significant contribution it makes to the economy of grass/white clover pastures by fixation of atmospheric nitrogen, especially in the absence of fertilizer nitrogen. While white clover is grown across a broad spectrum of climates, it is most frequently used in temperate mild to temperate cold climates; lacking a high density of roots (Evans, 1978) it is not productive during hot summer months (Russell and Webb, 1976), nor can it withstand extreme cold (Burdon, 1983). White clover is normally described as a creeping, much-branched perennial (Turkington and Burdon, 1983), but Hollowell ( 1966) has postulated that in some environmental and management situations it behaves like an annual, or even as a mixture of plants of both types. In studies of the whole Trifolium genus, both annual and perennial species are found with either simple tap-rooted stoloniferous or rhizomatous root habits which may be related to chromosome number (Taylor et al., 1979). Thus, the variation in longevity noted by Hollowell may have genetic as well as environmental connotations. The literature on all aspectsof the growth and performance ofwhite clover is enormous and the rate of publication continues to increase annually. This review is based largely on work in northwestern Europe and New Zealand, but literature on work from other countries is also cited where relevant. For example, the role of all clovers in the United States, but especially white clover, was reviewed recently by Carlson et a/.(1985) and by Gibson and Cope ( 1985). Thus, on the use of white clover in the United States and other recently reviewed subjects such as nitrogen fixation (La Rue and Patterson, 198 1) and competition in grass/legume pastures (Haynes, 1980), only brief mention is made in the present article for the sake of coherence. The aim is to describe the origins and morphology of the plant and its nutrient and enviI Copyri&t 8 1986 by Academic Resq Inc.

AU rights of reproduction in any form TcscTvcd.

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J. FRAME AND P. NEWBOULD

ronmental requirements, but major emphasis is given to the agronomic and management aspects of the plant and of swards containing it. A. ORIGINS

The TrijXum tribe of the Papilionoideae subfamily of the Leguminosae or Fabaceae consistsof about 250 speciesdistributed throughout the temperate and subtropical regions of the north and south hemispheres,particularly in Europe, northwest and central Asia, northeast Africa, parts of tropical Africa and South Africa, and western North and South America (Polhill et al., 1981; Heyn, 1981). About 25 species are of significance as food for grazing animals, of which 10 are agriculturally important (Evans, 1976). Wild white clover is the most important pasture legume in many parts of the temperate zones (Turkington and Burdon, 1983) and is also found in many regions outside the temperate zone where the climatic conditions are mild without extremes in temperature and precipitation (Ostrowski, 1972). There is controversy over the precise center of origin of white clover; Evans (1976) and Turkington and Burdon (1983) favor Eurasia, Zohary ( 1972) favored North America, and Polhil et al. (198 1) favored both Eurasia and North America. Taylor et al. (1979), expanding the centers of diversity, and therefore possibly the origins too, included Africa. Such confusion arises because wild clovers are distributed in patterns determined primarily by climatic and natural biotic pressures, while cultivated forms have been selected by man and his grazing animals. The dates of onset ofdeliberatepasture management and the introduction of sown legumes into European agricultureare unclear. Evans (1976) stated that red clover (Trifoliumprutense L.) was cultivated in southern Europe in the third and fourth centuries AD and was introduced into Spain in the sixteenth century, and through Holland and Germany to England in 1650. It is probable that regular use of white clover began in England at about the same time, since it was introduced or recognized in pastures in the early sixteenth century (Fussell, 1964). The settlement by Europeans of North America, Australia, and New Zealand resulted in a great increase in the distribution of forage legumes, and white clover in particular. However, until the stimulus provided recently, mainly from Australia, exploitation of the natural legumes in Mediterranean zones and in tropical areas of the American continent has commenced only in recent times (Tothill, 1978). B. PRESENT USE IN WORLD AGRICULTURE White clover grows on roadside verges and in natural pastures throughout the world where climatic and soil conditions are suitable (see Section II1,A

AGRONOMY OF WHITE CLOVER

3

and B), but it is seeded deliberately in pastures mainly in North America, New Zealand, and northern Europe. White clover is used principally as a component of mixed grass/clover swards, which are usually grazed in situ. It is not grown in monoculture because of the difficulties of keeping such swards weed free, low annual herbage production, its short growing season, and concern about bloat and possible reproductive problems in grazing livestock. It was rarely used as a conservation crop (hay or silage)because of the brittleness of its leaves when dried as hay, lack of bulk production, and problems with making well-fermented silage from it. The difficulties of making silage from clover alone or grass/clover mixtures have now been overcome by wilting, chopping, and the use of acid additives (Frame, 1976a; Castle ef al., 1983). It is estimated that half of 45 Mha of humid or irrigated pastureland in the United States contains white clover but there is uncertainty concerning the amount of seed used each year. Van Keuren and Hoveland ( 1985)noted that its area of adaptation includes most of the United States, although its companion grass vaned with differing soil and climatic conditions. Most of the home-grown Ladino clover in the United States is produced in California; annual production in recent years has been 1360- 1590 metric tons* (Rincker and Rampton, 1985). New Zealand provides the bulk of the United Kingdom [and European Economic Community (EEC)] white clover seed supply and that country exported about 4000 tons of the variety Grasslands Huia in 1980 (Lancashire, 1985). In the period 1975- 1984, between 50 and 60% of certified permanent pasture white clover produced was also exported. Since the variety GrasslandsPitau was released in 1978 about one-third of its total seed production has been exported (305tons). Figures for actual seed sales are hard to obtain but it appears that New Zealand exports about 4400 tons of white clover seed per year. Differences between production and exports suggest that the New Zealand farmers sow 1000- 1300 tons of white clover seed annually. In the same article, Lancashire (1985)indicated that if all the potential areas in New Zealand for sowing with Grasslands Pitau and the recently developed Grasslands Tahora white clovers were seeded, about 13,500 tons of seed would be required. In the EEC countries, about 3000 tons of white clover seed is used each year, of which the United Kingdom sows about one-third (Fitsen, 1985). The sources of the white clover seed supply in the United Kingdom, shown in Table I, help to illustrate the main seed-producing(and presumably seedusing) areas of the world (Anonymous, 1983a).White clover is also used as an amenity legume on roadside verges and other predominantly marginal land, and for permanent pastures which are grazed in sifu.The importance All tons mentioned in text are metric tons.

4

J. FRAME AND P. NEWBOULD Table I Sources of White Clover Seed Supply in 1980 in the United Kingdoma

Country New Zealand Denmark United Kingdom United States Australia Netherlands Other countries (Belgium, Italy, West Germany)

Metric tons 709 378 61 47 20 15 4

96 57.5 30.6 4.9 3.8 I .6 I .2 0.3

From Anonymous (1983a).

ofall legumes to man was well described recently by Iseley (1982)but, just as with the grain legumes, the full potential of forage legumes remains to be realized (Gladstones, 1975; Rogers, 1976). Although use of white clover is substantial in some parts of the world, this review discusses the many soil, botanical, and cultural problems which hinder its wider exploitation.

II. MORPHOLOGY AND FUNCTION A. ROOTS

White clover plants develop an extensivelybranched tap root system from the primary seedling. Thereafter, adventitious roots with numerous lateral branches arise from the nodes of the stolons which develop from the mother plant (Erith, 1924; Hector, 1936). Early in the root’s life, rhizobial infection takes place and large numbers of nodules develop, mainly on the finer branches of the root system. These nodules may be club shaped, ellipsoidal, or sometimes palmate. The clover tap root system is short lived (Stuckey, 1962; Gibson and Trautner, 1965;Carlson et al., 1985),so the plant is eventually dependent on the roots which develop from stolon nodes (Hollowell, 1966). Tall-growing clover types such as Ladino or Grasslands Pitau with large leaves develop a well-defined tap root system compared with prostrate, small-leaved varieties such as Kent Wild White (Caradus, 1977). This may render the small-leaved types less vulnerable to loss of their tap root system; since they are also highly

AGRONOMY OF WHITE CLOVER

5

stoloniferous, their persistence will be enhanced relative to the larger-leaved types. Work by Haycock (1982) has indicated that both types of root systems have similar potentials for supporting leaf growth. Estimates of tap root longevity vary from less than 1 year to 2 years (Westbrooksand Tesar, 1955; Spedding and Diekmahns, 1972). Thus, Hollowell (1966) has described the white clover plant in terms of an annual which may behave as a perennial through asexual propagation. White clover explores the upper layers of soil but has the disadvantage compared with grasses of shorter root hairs and a smaller “root hair cylinder” (Evans, 1977). Evans (1978) found that white clover roots reached the same depth as those of perennial ryegrass and cocksfoot, with most roots of all species occurringin the upper (0-200 mm) soil layers. However, white clover had much less root mass in these upper layers, from which the bulk of soil nutrients and moisture is extracted (Bland, 1968; Evans, 1978). In New Zealand, Caradus and Evans (1977) found new root production peaked in autumn and fell gradually throughout winter to a minimum in spring and summer. Possibly because of lower winter temperatures in England, the production of new roots was highest in spring, but there was also a smaller peak in autumn (Garwood, 1968). Severe shading can lead to a marked loss of both roots and nodules, with little subsequent new growth (Butler et al., 1959;Chu and Robertson, 1974). With recurring defoliation, a balance between death and regeneration of roots occurs, leading to a rapid turnover of root and nodule tissue (Evans, 1973); defoliating white clover every second day drastically reduced root elongation.

B. LEAVES The first photosynthetic organs are formed after germination from the two cotyledons in the seed. The first true foliage leaf is unifoliate and round, but later emerging leaves consist of three leaflets, ovate to circular in shape, and with the edges either entire or serrate. There is a pair of leaflike stipules at the base of each petiole (Hector, 1936). In most genotypes there are inherited characteristic marks, whitish or red, on the upper surfaces of the laminae. The leaves are arranged alternately on the main stem, and then arise from stolons, one per node. Successive petioles are progressively longer until they reach normal length (Brougham, 1958a,b; Wilman and Asiegbu, 1982b). Leaflet size and shape are functions of the clover variety, and the position of the leaflet on the stolon. Leaflet size is currently used for classifying varietal type (Anonymous, 1985b). Further detailed accounts of the morphology of leaves (and other parts) may be found in Erith ( 1924), Hector ( 1936),Gibson and Hollowell ( 1966),Spedding and Diekmahns ( 1972), Burdon ( 1983),and Carlson et al. (1985).

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J. FRAME AND P. NEWBOULD

The process of leaf initiation, development, growth, and senescence is affected by the prevailing temperatures and light. The processes which influence numbers and development of the leaves and stolons are markedly temperature dependent, whereas the processes governing differentiation and size are mainly light dependent (Brougham, 1962). The longevity of clover leaves, which determine their photosynthetic usefulness to the plant, was reported by Brougham (1958a,b) to be between 28 to 38 days from time of emergence. In late autumn and winter the leaves in the older parts of the stolons generally die away, leaving mainly undeveloped leaves near the stolon apices (Davies and Evans, 1982), but there can be a steady but slow production and loss of leaf throughout winter. This is followed by a net increase in dry weight when spring temperatures and radiation improve. One of the factors which contributes to clover’s reputation for unpredictability is its greater variation than grass in number of leaves per unit area of ground, from site to site, from year to year, and from one time of year to another (Hollington and Wilman, 1985). The ability of white clover to achieve a high level of leaf expansion at low temperatures and light intensities is an important breeding objective (Cooper, 1969; Breese, 1979; Rhodes, 1984, 1985). This ability is particularly desirable in colder environments such as the uplands of the United Kingdom (Munro and Hughes, 1966). Genotypes adapted to low temperatures have been identified (Eagles and Othman, 1981;Ollerenshaw and Baker, 1981; Ollerenshaw, 1983)and considerable differences in the response of different genotypes to changes in temperature have been observed (Beinhart, 1964; Williams and Hoglund, 1978). White clover leaves growing in a grass/clover sward are not significantly shaded, since successivepetioles extend longer and keep pace with increasing height of sward canopy (Mitchell, 1956b;Dennis and Woledge, 1982, 1983). The photosynthetic capacity of the clover laminae is apparently not reduced even in the mixed swards of high leaf area commonly used in the United Kingdom. Dennis et al. (1984) concluded that clover production was more affected by the number of growing points at the beginning of a growth period than by any differences in sward structure or light environment during regrowth. The removal of leaves and/or petioles and stolons in cutting or grazing of white clover plants occurs during utilization. Knowledge of the physiological responses of the plant to defoliation is needed to understand agronomic responses (see Section VII1,D and E). The phenotypic plasticity of the plant is well known (Hill, 1977); King ( 1963)showed that heavy grazing of indigenous white clover populations resulted in small-leaved prostate forms of white clover. In subsequent work King et al. (1978) found that acropetal movement of assimilates occurs in white clover such that partial defoliation

AGRONOMY OF WHITE CLOVER

7

has only a small effect on the growth of terminal leaves, which are the main source of clover production (Beinhart, 1963). This suggests that growth and spread of white clover plants will be more rapid if defoliation of the central leaves can be avoided. More importantly, the rate offormation, number, and size of internodes were not significantly depressed by partial defoliation. Recently, Boatman and Haggar (1984) concluded that “white clover seedlings have a considerable ability to recover from leaf removal, especially if only laminae of old leaves are removed, but the growth reduction following removal of petioles as well as laminae appears to be especially severe.” Further detailed work on this subject and on the light responses of white clover leaves formed at different positions in the canopy of a mixed grass/ white clover sward is required, but the complexity of the interacting factors involved is clearly evident. Studies are especially needed of the developmental morphology of modern white clover varieties (Arnott and Ryle, 1982). C. STOLONS The seedling develops a short primary stem with a number of nodes. Stolons emerge from the axils of the leaves and if growth is unhindered, the clover plant forms a rosette with a network of freely branched stolons. Barley (1953) showed photographically that stolons are present on and just below the soil surface. As the center of the plant dies, individual stolons take on an independent existence (Erith, 1924;Spedding and Diekmahns, 1972;Chapman, 1983). Growth and survival of white clover are therefore strongly dependent on stolon development and replacement (Knight, 1953a,b; Beinhart, 1963;Schillingerand Leffel, 1964;Gibson and Hollowell, 1966; Hollowell, 1966). The buds which develop in the axils of the stolon-borne leaves bear secondary stolons or inflorescences, depending on the season of the year. Stolon production is influenced by temperature, light, and flower initiation (Brougham, 1962, 1965; Beinhart, 1963). Since each inflorescence produced eliminates the potential of a stolon bud to proliferate (Knight, 1953b), profuse flowering can be detrimental to plant persistence; conversely, if seed is set and shed, the longevity of the clover stand will be assisted. Severe grazing by sheep of stolons and therefore removal of terminal apices gives stolons less chance to assimilate material and exploration of temtory may be curtailed (Clark et af.,1984a). Wild white clover types, which are strongly stoloniferous, will have better potential persistence than less stoloniferous large-leaved types (Davies, 1970; Munro el a!., 1975;Baines et af.,1983). This gives the wild white types an advantage under severe grazing, exemplified by continuous sheep stocking (Evans and Williams, 1984) and a better ability to colonize bare spaces

8

J. FRAME AND P. NEWBOULD

(Burdon, 1983; Turkington and Burdon, 1983). It has been suggested that a grass/clover mixture provides a more stable association than a grass monoculture due to the ability of clover to proliferate by stolons and colonize bare ground (Hams and Thomas, 1973). Increased length, diameter, and weight of stolons were factors contributing to the positive effect on white clover performance of increased interval between defoliations ( Wilman and Asiegbu, 1982a,b;Frame, 1985~).Fertilizer N application adversely affected stolon weight (Boyd and Frame, 1983; Frame, 198%). The length and diameter of stolons were reduced less in large-leaved than small-leaved varieties (Wilman and Asiegbu, I982b). Vigor of clover growth in spring was related to the amount of stolon which overwintered (Hams et al., 1983). Two genotypic extremes with differing stolon-branchingability were referred to by Beinhart et al. (1963) and Gibson et al. ( 1 963). The stolons of “non-viney” genotypes were freely branching and spread outward radially while “viney” genotypes were less stoloniferous and sparsely branched. In Switzerland, Boller and Nosberger (1 983) noted that an alpine clover ecotype allocated a higher proportion of dry matter to the stolonsthan did a valley ecotype. In perceiving a model ofwhite clover for New Zealand hill country, dense stolon branching, small leaves, and a prostrate habit were identified as desirable characteristics (Caradus and Williams, I98 1). Smith (1 949) found a stronger association of winter injury with the large Ladino clover stolons compared with the smaller stolons of common white clover types. Recent studies in New Zealand have increased awareness of the distribution pattern of stolons in grazed pastures. Hay (1983) and Hay et al. (1 983) reported the effect of season or time of year on three “vertical classes” of stolon: aerial, surface,and buried. Buried stolon formed a large percentage of stolon total weight, and increased from a minimum in autumn to a maximum in spring. Earthworm casting rather than stock trampling was the more important factor involved. Aerial and surface stolon both peaked in summer and were least in early spring. Environmental conditions such as drought reduced the amount of stolon and modified its distribution in favor of buried stolon (Hay and Chapman, 1984). Total stolon weight and weight per unit length were greater under cattle grazing than sheepgrazing (Hay et al., 1983; Hay and Chapman, 1984). Further longer term work (Hay, 1985) confirmed that white clover underwent an annual cycle of burial of stolons in winter, reemergence of growing points in spring and surface stolon development over summer (Fig. 1). Hay (1983) identified three ways buried stolon assisted white clover persistence: initiation of subsurfacebranch stolon capable ofgrowing vertically to the soil surface and redirection of stolon apices to the surface; initiation of branch stolons where a vertical branch reached the soil surface; production of budlike structures, with several growing points, from vertically grown stolons.

AGRONOMY OF WHITE CLOVER

9

100 1

1981

1982

1983

FIG.1. Mean percentageby weight of stolon in aerial (U),surface(e),and buried (0)classes at each sampling, autumn 1981 (March)to summer 1983 (January) in New Zealand. (From Hay, 1985.)

Clearly, detailed investigations into stolon distribution strata are warranted in grazing studies on grass/white clover swards in future. In the past, clover regeneration following declines in contribution, and sometimes apparent disappearance, has been largely attributed to germination of buried seeds or from plants which had become dwarfed for self-preservation.

D. INFLORESCENCES Globular inflorescencesare borne on long peduncles arising from the axils of the leaves, and are produced from the nodes of the stolons, although not at the basal nodes (Erith, 1924;Turkington and Burdon, 1983).The peduncles are usually about twice as long as the petioles of the leaves in whose axils they develop. The flowers, 20-40 per flowerhead, are white or pink and are typically arranged in a raceme. The flowering phase and inflorescencesare less important as determinants of herbage production in clover than in grasses, but persistence is adversely affected by profuse flowering (Knight, 1953b; Gibson, 1957). Flowering, which extends over several weeks, is influenced by many factors including genotype, temperature, photoperiod, and management. It is enhanced by long days (14- 16 hr) and - 20°C optimum temperatures following short days and low temperatures (Norris, 1984a;Carlson et al., 1985).However, there is considerablevariation among varieties in their temperature requirements for floral development (Norris, 1984b, 1985). Flowering can be delayed by cutting or grazing (Zaleski, 1961). In the United Kingdom, inflorescences are formed from May to July

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J. FRAME AND P. NEWBOULD

(Speddingand Diekmahns, 1972). Cross-fertilizationby insect pollination is enforced by self-incompatibility. The seed ripens 3 -4 weeks after pollination. The seeds are heart shaped and mainly yellow to brown in color. A proportion of the seed has impermeable coats, referred to as “hard” seed, and these can remain viable in the soil for a number of years. Burdon ( 1983) gave the seed size of indigenous clover in the United States as 1.O X 1.1 X 0.6 mm with a mean 1000-seed weight of 0.5 -0.8 g. Duke (198 1) cited 1,764,000 seeds/kg for white clover, while Wheeler (1950) had cited 1,543,000. Our own investigations led to values of 1,374,000- 1,642,000 seeds/kg for a range of varieties of differing leaf size types. However, data on seed size and many other morphological characteristics of the host of white clover varieties used in agricultural practice are scarce. This is not surprising since, for example, 43 varieties are listed in the EEC Common Catalogue (Anonymous, 1985~).

Ill. ENVIRONMENT As indicated in Section I,B, white clover is adapted to widely vaned environmental conditions. Its responses to these and to cultural influences are largely determined by physiological characteristics and processes, a theme recently reviewed for clovers, including white, by Kendall and Stringer (1985). The objective of this section is to describe and discuss briefly the relationships between selected environmental factors and the agronomic responses of white clover.

A. CLIMATE

In company with most plants, the growth and flowering ofwhite clover are affected by radiation, day length, temperature, soil water, and, in exposed places, wind. For growth within Britain, Burdon ( 1983)concluded that white clover was unrestricted by general climatic trends although areas subject to severe frosts or to prolonged droughts were not conducive to its growth and survival. Its distribution is widespread over all aspects and slopes though generally restricted to unshaded sites, but restricted at one extreme by intolerance of drought and at the other by the combined effects of low temperature on growth, N fixation, and the availability of plant nutrients. The broad requirements for white clover to grow well, as far as they are known, are described below under separate headings. While little can be done to alter most of the climatic factors, i.e., they are permanent limitations, understanding ofthe effects is necessary for plant breeders and agronomists so that the plant’s potential can be fully exploited.

AGRONOMY OF WHITE CLOVER

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1. Light

The early work of Blackman ( 1938),Black ( 1957),and Brougham ( 1958a) indicated that white clover was essentially a sun-loving plant to which shade was inimical. Most of the evidence suggests that the quantity of light energy, rather than its quality or intensity, is the most important factor (Black, 1957).Once the leaf area index (LAI) of white clover has reached 3 - 4 and all the light is intercepted, the amount of light energy may be a limitation to production. There is some evidence that legumes, and white clover in particular, require more light at the red end of the spectrum than, say, grasses (Wamngton and Mitchell, 1976). It is certainly our observation that white clover does not grow well in a growth chamber with warm white fluorescent lamps alone; a small number of tungsten filament bulbs must also be in position. The photosynthetic efficiency of white clover leaves is affected by the light environment in which they are formed, just as in grass (Woledge, 1978). However, if shaded, white clover has the ability to extend its petioles and thus place the developing leaves in a high light intensity (Dennis and Woledge, 1983). It has also been suggested that white clover leaves at the base of a mixed sward have the ability to find sun flecks, so improving its chances of survival in the presence of taller growing companion grasses (Boller and Nosberger, 1985). Both radiation and temperature influence the growth rate of white clover in spring (Davies and Evans, 1982); this is shown by the rapid growth in February and mid-March in Wales of the many new, small, branch stolons formed over winter. The critical day length for white clover is 13.5 hr (Spedding and Diekmahns, 1972). On the basis of flower initiation, Thomas ( 1961) described white clover as a short-long-day plant. Flowering, a prerequisite for seed production, is influenced by both temperature and day length to contrasting degrees for different varieties. With a 16-hr day length at 10°Conly 10%of a group of varieties flowered, whereas at 18 and 26°C most plants flowered (Norris, 1985). Thomas ( 198la) has shown in the field and with six varieties ofwhite clover, that defoliation has a stimulating effect on inflorescence initiation in plants that have stopped initiation in long days.

2. Temperature Temperature has effects on the availability of soil nutrients, germination, the growth of white clover roots and shoots, winter survival, photosynthesis and respiration, and nitrogen fixation. It is difficult to identify which process is most affected at any one time, but it is commonly believed that the minimum temperature for shoot growth ofwhite clover is 5.8"C,as for most grasses, but that active N fixation requires a temperature of 9°C (Martin,

12

J. FRAME AND P. NEWBOULD

1960). Thus, Munro (1 970) was able to conclude that the temperature requirement for white clover to grow and fix N ranged from 9 to 27' C with an optimum of 25 "C. In temperate zones, white clover is distributed widely on an altitudinal basis despite considerable variations in temperature with altitude. For example, in the United Kingdom it appears from sea level to over 900 m (Snaydon, 1962). In Norway and Switzerland it persists at 1200 and 1800 m, respectively (Erith, 1924); in Colorado, it grows at 3350 m (HultCn, 1970), and at 6000 m in the Himalayas (Turkington and Burdon, 1983). Cold-hardiness during establishment can be critical (Spedding and Diekmahns, 1972), since survival at - 10°C is minimal (Rachie and Schmid, 1953). It also appears that temperature may affect sward photosynthesis in the field since photosynthetic rates at 15"C were about twice those at 5 "C ( Woledge and Dennis, 1982). Respiration, i.e., senescence losses, are also sensitive to temperature but both white clover and perennial ryegrass, a common companion grass, appear to respond in the same way. The climate of the United Kingdom results in a short growing season since air temperatures do not reach the minimum level until late May and they decline again in late September (Williams, 1970). Woledge and Suarez (1983) concluded that white clover could be at a disadvantage compared with grass in spring because of its production of a large root :shoot ratio and small leaf area at low temperatures. The shoots of white clover grow best at about 24°C (Mitchell, 1956a). At this favorable growth temperature, 14C-labeled assimilates move at about 30 cm per hour, but at 10°Cthey move at about 20 cm per hour. When plants grown at 20°C were transferred to a lower temperature the rate oftranslocation was reduced and almost ceased at 5 "C; however, plants grown at 5 "C were adapted to lower temperatures in terms of translocation (Hoshino et al., 1966). There are, unfortunately, only limited studies on the response of white clover to temperature. The results demonstrated good growth in the range 18-30°C with an optimum of24"C (Brougham et al., 1978).The temperature range encountered during the British growing season, both lowland and upland, is therefore suboptimal for the most part, but particularly in early and late season. Generally, white clover is observed to start growth 2-3 weeks later and to cease earlier than many ryegrasses used in the United Kingdom (Williams, 1970). Kleter ( 1968) concluded that the percentage of white clover in permanent grassland in the Netherlands was strongly related to spring temperatures, particularly in April. There is general agreement that day temperatures influence growth more strongly than night temperatures and that growth is best when night temperatures are equal to or only slightly lower than day temperatures; also, the optimal temperature for growth falls as light intensity falls (Spedding and

AGRONOMY OF WHITE CLOVER

13

Diekmahns, 1972). Davies and Evans (1982) investigated earlinessofgrowth in two extreme types of clover and found differences due to lamina size and petiole length rather than differing ability to grow at low temperatures. Laissus (1983), working in Normandy, France, noted that decreases in soil temperature at 100 mm were related to a decline in white clover production. Consequently, he concluded that compact or cold soils (with a high waterholding capacity) were not suited to white clover. There is little work on selecting white clover material with ability to make significant growth at low temperatures (Ollerenshawet al., 1980; Eagles and Othman, 198 I ;Haycock and Ollerenshaw, 1982). Obviously such a characteristic would be valuable for hill and upland areaswhere spring growth is at a premium. Fertilizer N use in these areas is low, and vigorous clover growth and rhizobial N fixation are vital to sward production, particularly in grass/ clover reseeds sown to supplement the frequently clover-deficient native swards. Alcock et al. (1974) noted that on sheltered hill plots, rapid increases in LA1 and dry matter of Aberystwyth S 1 84 white clover could be due to soil heating effects. Information on the winter hardiness of white clover varieties is scarce but there are indications that Grasslands Huia, the most widely sown variety in the United Kingdom, has limitations in this respect (Hunt el al., 1965). Vigor of early clover growth in spring was related to the amount of stolon which overwintered satisfactorily (Harris et al., 1983); the low stature and other morphological features of Aberystwyth S 184 enable it to enter spring with more stolon and leaf than other varieties. Norris (1981) stated that white clover leaves were produced from nodes and lateral buds at the expense of food reserves in the stolons. The hazard of winter damage in upland areas due to low temperatures can be a major factor influencing why grassland potential is lower there than in the lowlands (Munro and Davies, 1973). Davies and Young (1967) noted the relationship between morphological adaptation and winter hardiness. Cold-tolerant plant populations from northern Europe were always smaller leaved, more prostrate, and more stoloniferous than Mediterranean populations, which were often erect types with small numbers of large plant parts. The Ladino type from southern Europe is particularly frost sensitive (Garaud, 1984).

3. Drought Strictly, this is a soil physical factor, but there is an obvious relationshipto climate. Foulds (1978) noted few indigenous white clover plants in areas subject to severe drought. The marked response of white clover to irrigation (Low and Armitage, 1959; Low and Piper, 1960; Stiles, 1966; Cowling,

14

J. FRAME A N D P. NEWBOULD

1982) has been interpreted as clover sensitivity to drought and in mixtures more so than grass. However, Kleter (1968), who surveyed Dutch permanent pasture, discovered no relationship between percentage clover in grass/ clover swards and summer rainfall. No consistent relationship was found between several growing-season weather parameters and total herbage or clover production since production was affected by a complex of interactive factors (Frame and Boyd, 1984). Recently Thomas ( 1984) showed that droughting28 - 56 days after sowing reduced the shoot growth of grass and clover monocultures equally but affected clover more than ryegrass in mixtures, possibly because the clover has less root mass in the upper soil layers (Evans, 1978). Thomas ( 1984) concluded that during drought, the clover content of a mixed sward could be maintained by frequent defoliation, albeit Evans ( 1978) noted that defoliation of clover every 2 days reduced root elongation. In Australia, Hartridge (1979) stressed that the amount of rain and the rainfall pattern determined the white clover content in a pasture. He considered that soil moisture content was the most important environmental factor limiting the production of pasture plants while temperature had the greatest effect on their persistence.

4. Wind White clover may suffer wind stress in exposed situations. It is, however, likely to be protected, at least partially, by associated grasses as it is commonly grown in mixtures, and by its waxy leaf cuticle. Taylor (1976) contrasted lowland and upland climates in the United Kingdom, and described the upland climate in terms of severe wind exposure, low temperatures, excessive precipitation, continual ground wetness, humidity and cloud cover, persistent winter frost and snow cover, sunshine deficiency, and low evaporation. Limited work on the effect of altitude on swards, especiallythe white clover component, has been conducted. In wind tunnel studies, Morse and Evans (1962) showed that clover plant growth was retarded at wind speeds above 0.3 m/sec. Excessive wind speeds (and hail or heavy rain) can induce nastic closure of the leaflets. Wind effects on grass have been demonstrated in wind tunnel experiments by Grace and Pitcairn (198 1). Wind caused increased leaf transpiration and reduced leaf extension, relative growth rate, and leaf area ratio; soil water stress exacerbated the effect. Wind shake of the grass also reduced grass leaf extension. Such effects in the mixed sward conceivably could assist white clover growth due to reduced grass competition but, although no experimental work has been done, it is likely that clover plants within the association could also be adversely affected.

AGRONOMY OF WHITE CLOVER

15

Damage to swards by wind-borne salt spray, particularly desiccation of foliage, can occur in exposed coastal areas (Copeman, 1979),and if clover is a sward constituent then it is likely to be damaged. Exposure to prevailing winds at the site of origin appeared to be an important factor associated with the frost tolerance of natural white clover genotypes (Ollerenshawand Haycock, 1984). In wind tunnel experiments with Grasslands Huia white clover, Bircham (1978) showed that the relative growth rate during wind was higher at 5.0 m/sec than at 10.0 m/sec. He attributed this to water stress; although wind had no direct effect on rate of transpiration, it closed the stomata of the most prominent leaves.

B. SOIL Many soil factors (parent material, texture, bulk density, drainage class, pH, organic matter content, availableplant nutrients, and microbial constituents) affect the growth of white clover and, because these factors interact, it is difficult to determine the dominant influencesfrom published data. However, it is generally accepted that white clover has more specific soil physical and chemical requirements than the accompanying grass (Jackman and Mouat, 1972; Whitehead, 1982; Rangeley and Newbould, 1985; Blue and Carlisle, 1985). 1. Physical

White clover is found on soils ranging in texture from coarse sand to heavy clay with very variable amounts of organic matter in the profile, but rarely in unamended peat ( Burdon, 1983).It is absent from continuouslywaterlogged soils and from shallow drought-prone soils and from strongly saline soils. In the United Kingdom the plant tolerates a range of soil pH from 5 to 6, but rarely occurs if the soil pH is less than 4.5 (Snaydon, 1962). Recent grassland surveys in the United Kingdom indicated that the proportion of clover in swards decreased as soil drainage class went from imperfect to poor to bad (Forbes et al., 1980);age of sward, soil pH, and available soil nutrients had less influence on clover content. McAdam (1983a) found in grazed swards in Northern Ireland that the proportion of swards with a continuous network of stolonsdeclined from 74 to 30% as the drainage class went from good, imperfect, poor, to bad. Clover growth appears to be better in sandy than clay soilsprovided there is no restriction in water supply (F. X. de Montard, personal communication). In New Zealand, there were indica-

16

J. FRAME AND P. NEWBOULD

tions of white clover decline as soil bulk intensity increased with grazing and treading (Edmond, 1974). In a survey of surface seeded pastures in the crofting areas of northern Scotland, Younie and Black (1979) found that the age of swards up to 17 years had little effect on the proportion of white clover, but indigenous vegetation type and soil organic matter content had big influences. A grazed, low-fertility soil previously growing Molinia caerulea had 5.6% clover while a peaty podzol growing Calluna vulgaris had 18%; the higher the percentage soil organic matter the lower was the percentage of white clover. 2. Soil p H Few workers have attempted to define the optimum soil pH needed in soil solution to maintain vigorous growth of white clover, although it is considered to be more sensitive than grass to low pH (Caradus, 1980). Andrew ( 1976), working in sand culture, found that growth of white clover under symbiotic conditions was almost nil at pH 4.0 and increased linearly from pH 4.0 to 6.0; nodulation was reduced below pH 5 . The addition of N to the cultures increased growth in the pH range 4-6 by about 60% provided calcuim ions were present. Most of the studies on this topic have been carried out in water or sand culture (Blue and Carlisle, 1985) so that the influences of A13+toxicity and soil acidity on rhizobia and plant roots have not been assessed. Present thinkingis to add sufficient lime to displace and hydrolyze the toxic amounts of soluble aluminum (Coleman and Thomas, 1967; Logan et al., 1985). In areas where the quantity of soluble aluminum falls below the levels that are toxic, the soil pH varies with soil type but ranges from 5.2 to 5.5. Rorison ( 1958) states that aluminum toxicity to the plant is not a problem if soils are limed to above pH 5.5. Thus the suggestion of Chestnutt and Lowe (1970) promulgated by advisers in the United Kingdom (Anonymous, 1985a) that white clover requires a pH not lower than 5.5 coupled with adequate calcium still seems the best policy.

3. Available Plant Nutrients (P and K) Chestnutt and Lowe (1970) state that a soil P status not lower than 20 ppm available P and a soil available K status of 170 - 200 ppm were required to establish and maintain the growth of white clover. More recently, Goodman and Edwards (1983) reported that a P concentration of about 2.5 ppm was required by the varieties usually grown. These workers gave no figure for K but they emphasizedthat legumes “have a great demand for potassium” and the seedlings fail if K is not added to low-potassium-absorptivesoils.

17

AGRONOMY OF WHITE CLOVER

Even the recent review paper on clovers growing in soils in the United States (Blue and Carlisle, 1985) failed to give a target figure for available soil K. Most workers have tried to describe the quantities of the major nutrients needed to bring about responses in growth of white clover based on cutting experiments (Rangeley and Newbould, 1985) or on critical concentrations in the herbage (Whitehead, 1982). The requirements for satisfactory clover production and maintenance under grazing have received little attention. In mixed swards, clover is more sensitive than grasses to pH decrease or reduction in P (Caradus, 1980) and K supply (Floate et al., 1981). Thus, failure to maintain an adequate pH or supply of available P and K would express itself initially in clover decline. Care must also be taken to balance the input of the major nutrients since interactions between lime and P, lime and K, and K and P have been reported (Floate et al., 1981; Rangeley and Newbould, 1985). The former interaction was reviewed in detail by Haynes ( 1984).The response found for the interaction between P and K when white clover was grown in a peat soil in pots was considerable (Rangeley and Newbould, 1985);without added K there was no response to added P and the maximum production of shoots was found when the highest level of both P ( 1 50 kg/ha) and K (320 kg/ha) were used (Fig. 2). The vital importance of kg K/ha 1

41

1 h Y

L z

U

1

1

50

I

100

1 150

Phosphor u s (kg/ha

FIG.2. The response of white clover to potassium and phosphoruswhen grown for 9 weeks in pots of dry peat soil. (From Rangeley and Newbould, 1985.)

18

J. FRAME AND P. NEWBOULD

this interaction in the grazed situation, where nutrient cycling through the grazing animal can occur, was shown by Floate et al. (198 1). It is generally accepted that maintenance dressings of K are needed mainly when herbage is cut and removed from conservation. However, on a peat soil, despite the extra return of K (and N) in urine, there was little retention of the recycled K in the soil so that higher levels of K than usually prescribed were necessary on this soil. Thus, when P and K were applied together, large responses in herbage production, especially of white clover, were observed; but it should be noted that some of this extra production was due to the N recycled in the urine (Floate et al., 1981).

4. Nitrogen A small supply of soil mineral N is needed by clover until nodules are formed and N fixation commences; thus the use of “starter” fertilizer N is essential in soils of low fertility, such as hill soils (Haystead and Marriott, 1979). However, too much mineral N can depress nodule initiation and development (Sprent, 1979). Animal slurry N and urinary N apparently inhibit clover less than mineral N (Heniott and Wells, 1962; Drysdale, 1966). Further quantification of the breakdown and buildup of organic N in soil and of how these processes are influenced by soil conditions and farming practice is needed (Anonymous, 1983b). Knowledge of the interactions of soil N and N fixation by white clover is also important, and this subject (Section V,B) together with the effect of fertilizer N on the growth of grass/ white clover swards (Section VII1,C) are discussed later.

5. Trace Elements The trace element needs of white clover and/or Rhizobium, especially for the N fixation process, include Co, Cu, Mg, Mn, Mo, Ni, B, Zn, S, and possibly Se (Chestnutt and Lowe, 1970). Some soils in Scotland aredeficient in Cu, Co, or Mn but there is no evidence of Mo deficiency; by contrast some soils contain a small excess of Mo (Reith, 197 1). Experiments have shown that lack of Mo, Mg, and Mn under certain conditions of soil pH, calcium, and major nutrient availability may affect the growth and performance of white clover (Floate et al., 198 1). To ensure that no problems from trace element deficiencies interfered with tests of Rhizobium for inoculation on white clover seeds, Newbould et al. (1982) added the following mixture (in kilograms per hectare) to all their trial sites: 10 CuSO,, 2 CoSO,, 2 ZnS04, 0.2 Na2Mo04 2H20, 5 Na2B4O7,with the addition of 10 MnS04 for deep peat humist soils. The addition of lime to raise the pH to 5.5 and above can

-

AGRONOMY OF WHITE CLOVER

19

markedly affect the availability of some trace elements (Mengel and Kirkby, 1982). Mo is particularly affected by this soil amendment and while not interfering with the growth of white clover, enhanced uptake of this element together with S can induce copper deficiencyin livestockgrazing the herbage (Evans, 1984). A recent study with cores of acid soils from the northeastern United States showed that the use of lime-pelleted seed with a fertilizer combination of K, Mg, Mo, and P improved establishment and growth of white clover, and that application of B, Cu, and Zn depressed nodulation and production possibly because too much was added (Murphy et al., 1984). In Australia and New Zealand, deficiences of Co, Mo, S, and Zn are widespread, and the response of legumes to applications of one or all of these elements can be dramatic (Robson and Loneragan, 1978).

6. Microorganisms One ofthe unique featuresof legumes such as white clover is the formation of a symbiosis between the plant and soil bacteria of the genus Rhizobium which results in fixation of atmospheric nitrogen. Many factors influence infection of the root by the bacteria and the growth and functioning of a nodule but, in the absence ofrhizobia effective at forming nodules and fixing nitrogen, no symbiosis can take place. Rhizobia are only abundant in soil when associated with their host legume and they are strongly affected by adverse conditions such as heat, drought, and acidity. Thus, some soils, especially acid deep peats, contain no rhizobia and others, e.g., acid brown earths (ochrepts)may contain rhizobia, of which only a few are effective at forming functioning nodules (Holding and King, 1963). Methods to study the ecology of rhizobia in soils have developed considerably in recent years (Jones, 1983) but it is still relatively difficult and tedious to determine the number ofeffectiverhizobia in soilswhere it is proposed to sow white clover. Wood and Cooper (1985) demonstrated that some rhizobia are more adapted than others to tolerate acid conditions and in a recent survey of 192 sites in Northern Ireland, Wood et al. (1985) confirmed that clover rhizobia were generally absent from peat sites. However, the data from the same survey revealed large populations ( 106/gdry soil) of rhizobia in mineral soils but only 79% of the isolates were effective on the variety Grasslands Huia. These workers were able to correlate the number of clover rhizobia at each site with soil pH, exchangeable Ca, base saturation, and A1 saturation, but the effectiveness of the rhizobia was not correlated with any soil chemical property. The relevance of this information will become apparent in the section on inoculation (Section IV,C). Another microorganism which can infect the roots of white clover and

20

J. FRAME AND P. NEWBOULD

confer nutritional advantagesto the plant in return for photosynthatesis the vesicular arbuscular mycorrhizal (VAM)fungi (Hayman and Mosse, 1979). The significance of mycorrhizae to the nodulation of N-fixing plants was well described recently by Barea and Azcbn-Aguilar( 1983).This microorganism is not as easily studied as rhizobia since it cannot as yet be cultured in the absence of its host, nor is it easy to identify different strains. Small populations of VAM fungi capable of infecting white clover and scavenging soil P for the plants (Sanders and Tinker, 1971) are widely distributed in soils. Even the acid deep peat hill soils in the United Kingdom, shown not to contain rhizobia, contained a small population of mycorrhizal fungi (0.3 propagules/g soil). On a soil volume basis this population was 100times less than in a brown earth soil but the roots of white clover plants grown in it after the addition of lime had about 15%of their root length infected with mycorrhizal fungi (Rangeley et al., 1981). The ability of VAM fungi to transport soil P from soils where it is in short supply to the clover plant has important consequences. With a supply ofP, both nodules and plants can grow with the former being able to fix atmosphericN, so further helping plant growth. The stimulation of this double symbiosis by inoculation of the white clover seeds or seedlings with either or both rhizobia and VAM fungi is theoretically attractive but, as will be described later, has not yet become of general and predictable application.

IV. CULTURE A. VARIETIES

Van Bockstaele (1985) classified the white clover varieties in western Europe into (1) small-leaved wild white, which is prostate and has thin, much-branched stolons; (2) small-, medium-small-, and medium-largeleaved varieties from northern and western Europe, belonging mainly to the “hollandicum” group and with shorter, less branched stolons and larger petioles than the indigenous wild white type; and (3) large-leaved Ladino type with thick stolons and a robust root system, which is a short-day plant and more frost sensitive than the other types. Varieties are classified in the United Kingdom into four groups according to leaf size, as measured on spaced plants (Anonymous, 1985b):(1) small leaved; (2) medium leaved; (3) large leaved; (4) very large leaved. In the United States,a range of clover leaftypes, small to large, is available, including foreign varieties. Louisiana and Louisiana S- 1 are notable American varieties with intermediate leaf size. Ladino, a large-leaved type, is de-

AGRONOMY OF WHITE CLOVER

21

rived from an Italian ecotype introduced in the early 1900s. It is thus regarded as both a variety and type (Carlson et al., 1985; Gibson and Cope, 1985).Its use in the United States was reviewed by Ahlgren and Fuelleman ( 1950).The variety, Pilgrim, was developed from Ladino in the 1950s.Other varieties, developed from Ladino and new plant introductions, include Merit, Regal, and Tillman. In Europe, member countries of the European Economic Community are required by statute to publish National Lists (NL) of herbage varieties. Only varieties listed in a country can be traded commercially in that country, unless the varieties have subsequently been included in the EEC Common Catalogue,when they can be sold in any member country. To qualify for the Common Catalogue, a variety must be included on a National List for 2 years. Entry to a National List requires independent evaluation, which may vary slightly from one member nation to another. An outline of the testing procedures in the United Kingdom will serve as an example. The evaluation of white clover varieties released by plant breeders is an ongoing exercise. Botanical assessments are made at three centers throughout the United Kingdom to determine the distinctness, uniformity, and stability (DUS tests) of the new varieties. Simultaneously, the varieties undergo comprehensiveassessment of their value for cultivation and use (VCU trials)at 1 1 centers; one to six new varieties may be under test annually. Each variety is sown for 2 consecutive years, but production is measured for NL purposes in full harvest years 2 and 3, and not in the year followingestablishment. The varieties are sown with a perennial ryegrass companion (cv. Premo) and cut five to seven times per season at production levels of - 1.5 tons DM/ha per cut. Two fertilizer N regimes of 0 and 200 kg/ha per year have been used from the inception of the white clover NL trials in 1974, but from 1983 sowings only the high N regime is being applied. Further plots of the varieties also sown with Premo are cut over every 10- 14 days during the growing season to assess clover persistence under frequent defoliation. Varietal performance is compared with that of standard control varieties using the criteria of seasonal and annual DM production of total herbage and of clover persistence, winter hardiness, and resistance to pests and diseases. The principal purpose of NL testing is to report on each variety for NL acceptance, and resultsare published in due course in the Plant Varietiesand Seeds Gazette (Her Majesty’s Stationery Office, 1977 et seq.). However, advisory services also use the results to recommend varieties (Anonymous, 1985b,e)to grassland farmers for use in seed mixtures. These advisory publications are revised annually as information on new varieties, or the results of retests of older varieties become available. Ofthe 2 1 white clover varieties on the National List in September 1985 (Table 11) 12 are recommended in Scotland (Anonymous, 1985e)and 14 in England and Wales (Anonymous, 1985b).

22

J. FRAME AND P. NEWBOULD Table I1 White Clover Varieties on the United Kingdom National List, October 1985 ~

Small leaved

~

Medium leaved ~

A b e r y s w h S 1 84a.b Kent Wild WhiteRb Rivendel"

~bevstwyths 1 0 0 4 DonnaRb Grasslands HuiaQb Mennaanb Pajbjerg M i l k

Large leaved

Very large

~~

Aliceb Blancaa.b Grasslands Pitau Kersey Whiteb MilkanovaRb N e d Olwenb Retor" Ross Sabeda

Aran4,b

Sonja4sb Recommended in Scotland. Recommended in England and Wales.

A number of the varieties have been in commerce for many years. For example, Grasslands Huia was developed in New Zealand in the 1930s although improvements were incorporated up to 1960 (Barclay, 196 1). Prior to 1956 it was known as New Zealand white clover. The New Zealand variety Grasslands Pitau, released in 1975, was developed for better cool-season growth than Huia (Barclay, 1969); it also has better growth from midsummer to autumn (Brock, 197 1). Aberystwyth S 184 wild white clover was released in 1942. Some newly introduced varieties have shown marked production superiority to older varieties (Ingram and Aldrich, 1982; Davies, 1984; Frame, 1985a) but seed supplies have been scarce, particularly of the British and European releases. Of the 900- 1000 tons ofwhite clover seeds used annually in the United Kingdom only around 4% is home produced, whereasbetween 80 and 90% is imported from New Zealand (Davies, 1982, 1984). Some of the new releases were developed for lowland rotationally grazed swards receiving fertilizer N, but have also exhibited good productive ability without N (Davies, 1984; Rhodes, 1984; Connolly, 1985). However, following the energy crisis in the early 1970s and a heightened appreciation of the fossil fuel energy required in the manufacture of fertilizer nitrogen for modem high technology agriculture (Pimental et al., 1972), breeding has concentrated on white clover potential per se. Rhodes (1984, 1985) listed the current breeding objectives in the United Kingdom as (1) to breed varieties which are persistent and productive under continuous grazing and suitable

AGRONOMY OF WHITE CLOVER

23

for hill and upland conditions, (2) to improve predictability of yield in upland and lowland conditions through disease and pest resistance, grass/ clover compatibility, and better spring growth of white clover, (3) to ensure high seed yield potential, and (4) to improve Rhizobiumlclover symbiosis. Van Bockstaele (1985) and Gibson and Cope (1985) have listed the characters related to persistence and productivity which they considered of relevance and value in breeding programs in Europe and the United States, respectively. There is a dearth of information on clover varietal response to winter cold and frost stress (Davies, 1970). Hams et af.(1983) found varietal differences in which vigor of early clover growth in spring was related to the amount of stolon which overwintered satisfactorily.The low stature and “dwarf” morphological features of small-leaved varieties allow them to enter spring with more stolon and leaf than larger leaved varieties. The widely used variety Grassland Huia may sufferwinterkill or damage in severe winters albeit it has potential to recover; Hunt et af.(1 965) noted that the length and weight of stolon in Huia were very low in spring followingwinter frost damage. The Huia variety is not adapted to hill country and severe grazing in New Zealand (Lancashire, 1974; Suckling and Forde, 1978; Charlton, 1984), but the recently bred variety GrasslandsTahora (Williamset af.,1982; Charlton and Giddens, 1983) is likely to be better adapted to predominantly sheep-grazed hill country, especially under moist and wet conditions. Clover varietal winter hardiness needs to be examined further to identify the best varieties for upland or exposed areas and for colder latitudes generally (Copeman, 1979; McAdam, 1984). Because of the resources required for grazing work, clover varietiesare not routinely compared in terms of response to grazing, voluntary intake, or animal production. However, grazing experimentationis increasingly being conducted at research centers. Experiments involving cutting, rotational grazing with sheep, or continuous sheep stocking have revealed marked variety X defoliation system interactions (Evans, 1979; Evans and Williams, 1982, 1984; Connolly, 1985). The best varieties under cutting, usually large-leaved types, often performed less well or poorly under continuous stocking while conversely, small- or medium- leaved types were usually the highest producing under continuous stocking. The ranking order for varieties was similar under cutting and rotational grazing. Selective grazing of clover by sheep with consequent loss of stolon appeared to be a major factor contributingto a decreasein clover performance under continuous stocking (Evans and Williams, 1984). Since white clover is mainly a grazing species and emphasis in practice is on its use in the grazed sward, there is a need to expand varietal evaluation under appropriate grazing systems. However, the experimental techniques

24

J. FRAME AND P. NEWBOULD

used must be standardized so that results of different experiments can be adequately compared (Frame and Newbould, 1984). These authors also make a general observation that most agronomic and grazing work on the grass/white clover sward in the United Kingdom has been undertaken with clover varieties not alwaysreadily availablein practice; this implies a paucity of work with Grasslands Huia. B. LIMEA N D FERTILIZERS The quantity of lime and fertilizers needed to create the optimum soil conditions for the growth of white clover swards is normally based on soil analysis. However, the latter can be a poor guide, especially in acid, organic matter-rich hill soils in the United Kingdom (Pimplaskar et al., 1982). Sufficient lime should be used to reduce the quantity of soluble aluminum, where this is present, and to raise the soil pH to 5.5 but no higher than about 6.0. Account must be taken of cation exchange capacity and quantity of soil organic matter in determining the lime requirement (Logan et al., 1985).In Scotland, 5 - 7 tons of lime per hectare are usually required (Anonymous, 1983b)with the higher amount being applied to the deep peat (humist) soils in the wetter west. This amount is adequate to raise the pH of the top 5 -7.5 cm of soil so that white clover can establish successfully. Subsequently, maintenance amounts of lime, 2-3 tons/ha at 3- to 5-year intervals, is recommended to ensure that the conditions continue to favor growth of white clover. Present recommendations for the nutrients needed to establish grass/ white clover swards in Scotland (Anonymous, 1983b)are to use 50,40, or 25 kg P/ha for soils of low, moderate, and high status, respectively,and 100,75, and 50 kg K/ha for soils of similar relative status in available K. For very nutrient-deficient deep peat soils in Scotland, Floate et al. (1981) recommended 60 kg P and 80 kg K/ha to establish a perennial ryegrass/white clover sward. C. INOCULATION

The significance and application of rhizobial inoculation in agriculture have been reviewed recently by Peterson and Loynachan (1982), Newbould (1983),and Burton (1985).The latter authorgivesadetailed account ofother aspects of rhizobia relationships in addition to inoculation. Inoculation by transfer of soil in which legumes had been grown to fieldsabout to be used for legumes was practiced in 9000 BC. At the present time, clover seeds are

AGRONOMY OF WHITE CLOVER

25

treated with a layer of bacterial cells suspended in a mixture of finely ground peat and glue immediately prior to sowing. Widespread trials with this method for white clover to be grown in acid hill soils where no clover had been grown previously gave disappointing results (Newbould et al., 1982). Only in the deep peat and wet peaty podzol soils were there consistent and significantresponses to inoculation in shoot growth (Table 111). The practice of spraying solutions of rhizobia onto young white clover seedlings has been tested in Wales (Young and Mytton, 1983; Mytton and Hughes, 1984). These authors showed that the strain of rhizobia must be matched with the variety of white clover and that this method of inoculation was also unpredictable in its success. The causes for poor clover response to inoculation in many soils are not hard to find. The small numbers of relatively ineffective indigenous organisms are highly competitive to the introduced strains. The latter have usually been selected after screeningfor effectivenessin monoculture and on one variety of white clover. Recent work by Jebb (1984) has studied competition between indigenous strains of rhizobia and considerable differences were found. However, the highly competitive strains were often of only moderate effectiveness at fixing nitrogen. It is hoped that microbial genetics can transfer nitrogen fixing genes to these strains, to make elite (competitive/effective)strains for use in practical agronomy. Much work is needed by rhizobiologists and by plant breeders to achieve this goal but its realization could have a big impact on the culture of white clover in pastures. Early suggestions from Powell ( 1976, 1977, 1979), working in New Zealand, that the growth of white clover would be considerably stimulated by inoculation of white clover seeds or seedlingswith VAM fungi have not been fully realized. Rangeley et al. ( 1981) obtained large responses in white clover shoot growth to inoculation with VAM fungi under laboratory conditions Table 111 The Response in Herbage Production of White Clover (cv. Grasslands Huia) to Rhizobiurn Inoculation in Year of Sowing" Herbage production (kg DM/ha) ~~

Soil type

No Rhizobium

With Rhizobium

~

Brown earths (ochrepts) Dry peaty podzol (orthods and humods) Wet peaty podzol (aquepts and aquods) Deep peat (fibrists and hemists)

77 126 65 61

From Newbould ef al. (1982). Significantly different at the I % probability level.

SED ~~~

94 129

14

93

26 20

120

28b

26

J. FRAME AND P. NEWBOULD

but were unable to reproduce these responses in the field. Hayman and Mosse ( 1979),using preinoculated clover seedlings in the field in hill soils in Wales, were able to show a response but this method of inoculation is not a practical proposition. Tests of an inoculation pellet containing nutrients, rhizobia, VAM fungi, and white clover seeds are now in progress under field conditions at several sites in the United Kingdom (Hayman, 1984). The advantage of inoculation with both rhizobia and mycorrhizal fungi has been described (Newbould and Rangeley, 1984)for an acid peat soil in the laboratory. With this soil, shoot production, nodulation, and N fixation by white clover were increased by 160, 130, and 85%, respectively, followinginoculation with mycorrhizal fungi in the presence of effective rhizobia. Further study to devise practically applicable methods of inoculation with elite strains of both rhizobia and mycorrhizal fungi appearsjustified. At the present time in the United Kingdom, inoculation of clover seed or seedlings with rhizobia is recommended for use on all hill soils as an insurance,but it is essential on deep peat and wet peaty podzol soils.

D. ESTABLISHMENT There is a lack of suitable assessment techniques for judging success or failure in clover establishment and long-term survival (Haggar et al., 1985a); Haggar et al. suggested about 150plants/m23 months after sowing, translating to 30%ground cover 12 months later, as a suitable target.

1. Relative Clover:Grass Seed Rates A high clover or low grass seed rate can improve white clover plant establishment (Hemott and Wells, 1960;Cullen, 1964b;Young, 1984). However, due to the ability of clover to proliferate by stolons and possibly by late germination of hard seed (those in which the testa is intact), this initial effect ofclover :grass seed ratio is ovemdden in the first or second full harvest year. Chestnutt and Lowe (1970) concluded that a ratio of clover to grass seed of 3.5 kg/ha to 22 - 25 kg/ha was adequate for obtaining a good clover :grass balance in a sward. Laidlaw (1 978) confirmed the short-lived effect of clover :grass seed ratio when he examined combinations of clover seed rates ( 1 - 9 kg/ha) and grass seed rates (3- 14 kg/ha). A constant clover seed rate with varying grass seed rates also has no significant effect on white clover performance (Cullen, 1964b Frame, 1985a).It may be concluded that there is scope for considerable flexibility in the clover :grass seed rate ratio in seed mixtures, but nevertheless there has been a scarcity of work on the critical

AGRONOMY OF WHITE CLOVER

27

seed rate of white clover necessary to establish specific seedling populations. Meantime, it would seem prudent to sow 3-5 kg clover seed/ha in the mixture to ensure a satisfactory clover presence in the early life of a stand. If this presence is not achieved, considerable management skill and favorable conditions will be needed to encourage satisfactory clover spread.

2. Time of Sowing There is scant recent published information on this topic. The optimal time of sowing for white clover is primarily related to the avoidance of climatic conditions which militate against its establishment. Midsummer is often associated with soil moisture deficits. Late-autumn sowing leaves insufficient time for adequate plant development before the onset of low winter temperatures. Mortality of autumn-sown clover can be high due to attack by Pythium and other fungi which cause “damping off’ of seedlings. Spring is therefore the best time for sowing. Very early spring sowing could have low-temperature limitations, although white clover seeds germinate and seedlings emerge more rapidly than grass even under unfavorable soil conditions such as low soil temperatures (Stapledon and Wheeler, 1948). Winter cereal growing has expanded considerably in recent years in certain areas of the world. If harvested early there is opportunity for direct seeding of grass/clover mixtures, therefore avoiding the hazards from nurse crop competition with undersowing techniques. However, there is a danger in colder latitudes that the period after harvesting winter cereals is too short for satisfactory plant development before winter onset (Harris et al., 1983; Younie et al., 1984). Stress and damage to grassland plants in winter are caused by a number of interactive factors including low temperatures, fluctuating temperatures, snow cover, low light intensity, and wind (Copeman, 1979). In practice, several factors including the farming system and local weather patterns will influence the timing of sowing. In principle, direct seeding in spring offers the best chance of entering the first full harvest year with a balanced grass/clover sward since there is time for manipulation of grazing management, weed control, and other measures to encourage clover spread.

3. Sowing Depth Because of its small seed size and consequent limitations in food reserves, white clover seed should be sown no deeper than 13 mm (Cooper, 1977). Deeper sowing, while possibly ensuring a better moisture supply, delays

28

J. FRAME AND P. NEWBOULD

emergence, weakens seedlings, and reduces their competitiveability against companion grasses or unsown species. Companion grasses, depending on the species, can be smaller in seed size (e.g., timothy) or several times larger (e.g., tetraploid perennial ryegrass). Undoubtedly, sowing white clover too deeply is a major cause of poor establishment. Stapledon and Wheeler ( 1948)concluded that optimum establishment of herbage seeds required different fractions of a seed mixture to be sown at depths suited to the different seed sizes, a practice difficult to achieve with commercial sowing equipment. When white clover (and timothy) seed were broadcasted, and perennial ryegrass drilled, a 50% better establishment of white clover resulted than from either drilling or broadcasting the whole seed mixture (Hemott, 1970). The technique employed in Hemott’s work of using a seed box for clover seed mounted on the rear of a seed drill should be exploited; alternatively, Cooper ( 1977)suggested that a drill could be developed which provided simultaneous seeding of grasses and clovers in alternate rows at different depths. To obtain reasonably precise sowing depths, a fine, firm soil tilth with an even soil surface is required. Soil cover and seed/soil moisture contact are more readily obtained with a fine than a coarse soil tilth. The technique of smoothing the soil surface with a soil leveller is one which is often neglected in farming practice. Good consolidation, subsequent to seeding, but avoiding soil “capping,” will increase contact between soil and seed and, with autumn sowing, will reduce the chances of damage to young plants by frost heave (Anonymous, 1983b). 4. Cover Crop A direct seeded grass/white clover sward is not as productive in its sowing year as an established sward. Thus a spring cereal grain cover crop has been commonly used to augment productivity per unit area and to give an immediate cash return. However, because of competition from the cover crop, the chances of poor clover seedling establishment, or even of clover failure, are greater under a cover crop than with direct sowing (Cullen, 1964a;Spedding and Diekmahns, 1972; Younie et al., 1984). Winterkill of poorly developed clover plants is also likely. Poor clover establishment is likely to lead to low clover presence in the sward, at least in the early harvest years. It can thus be a contributory factor in the production variabilityassociated with grass/clover swards. An arable silage (cereal/forage pea) crop, with subsequent early removal, is an alternativeto a cereal grain crop (Whytock and Frame, 1985), and fundamentally, direct seeding is the best option for clover.

AGRONOMY OF WHITE CLOVER

29

E. COMPANION GRASS One of the major guidelines for the encouragement of a significant white clover proportion in grass/white clover swards is the choice of suitable companion grasses (Chestnutt and Lowe, 1979;Frame and Newbould, 1984). It is well documented that grasses differ in their competitivenessto clover, but a grass’s adaption to an area usually takes precedence over its suitability as a companion grass to clover. Within the perennial ryegrass species, the lower producing, less persistent varieties permit the best performance by white clover (Cowling and Lockyer, 1965; Green and Corrall, 1965; Wright and Faulkner, 1977; Camlin, 1981). Orchardgrass and tall fescue are highly competitive (Chestnutt and Lowe, 1970;Van Keuren and Hoveland, 1985). Differences in the relative within-season patterns of growth can aid the success of the grass/clover association (Turkington and Harper, 1979b; Haynes, 1980),although defoliation management can alter the normal seasonal production of the associated species (Harkess et al., 1970).The slower spring growth ofwhite clover, because of its inability to grow at low temperatures, renders it at a competitive disadvantage in mixture with grass but it should be possible to exploit the genetic variation in white clover and select for earlier spring growth (Ollerenshawand Baker, 1981;Ollerenshaw, 1983; Glendining et al., 1985). Summarizing a series of trials using modern high-yielding perennial ryegrass varieties, Frame (1985b) concluded that ear emergence date did not have a marked effect on white clover performance (Table IV). There are indications that white clover contribution and production are higher with a tetraploid ryegrass companion than with a diploid ryegrass (Frame, 1985b; Collins, 1985; Davies, 1985). Possibly the coadaptation concept whereby white clover and perennial ryegrass varieties are developed from already coexisting genotypes will lead to improved compatibility and performance of mixtures based on these varieties (Turkington, 1979; Turkington and Harper, 1979c; Burdon, 1983; Evans et al., 1983; Evans and Hill, 1984). Little compatibility work has been undertaken with mixtures of grasses, yet commercial seeds mixtures often contain several species of grass and sometimes more than one variety of a species. F. WEEDCONTROL Cultural methods of weed control have always been important in the sward establishment process. Preparation of a clean seedbed is an example of preemergence cultural control, and topping of tall annual weeds is a post-

30

J. FRAME AND P. NEWBOULD

Table IV Herbage Production from Perennial Ryegmss/White Clover Swards Cut under a Simulated Grazing Regimea DM (tons/ha per year)

Ear Experiment

Grassb

emergence type'

1

D D/T D and T D and T D T D D T

E E I L I and L I and L I I I

2 3

4 5 6

Total herbage

White clover

1.9

4.3 3.8 3.8 3.5 3.4 3.9 4.1 5.6 5.1

1.2

8.7 8.3 8.5 8.5 7.8 8.8 8.1

White clover

(%I 54

53 44 42 40 46 53 64 63

From Frame (1985b). D, Diploid or 2n; T, tetraploid on 4n. E, Early; I, intermediate; L, late.

emergence example. The sown clover is at its most vulnerable in the establishment phase when the open sward canopy can permit very rapid weed development. Apart from adversely affecting the establishment of sown species, weed ingress can adversely affect the subsequent yield potential of the sown sward (Haggar and Squires, 1979). Since grassland is not a crop with direct financial rewards from weed control, commercial development of herbicides has been slow and grassland farmers have been slow to integrate herbicide use into their management compared with arable farmers. The critical densities of weed species at various phases in the life of a sward have not been defined. Clover-safeselective herbicides such as MCPB and 2,4-DB have a long history of use for weed control in establishing or established grass/clover swards, but with limited efficacy; consequently, they have been boosted with 2,4-D or MCPA or have been superseded (Williams, 1984). Benazolin-, bentazone-, and linuron-based herbicides, effective against a wide range of speciesincluding the major weed, chickweed (Stellariamedia), were introduced recently into the United Kingdom for establishingor established grass/clover swards. Chickweed is a particular problem especially when it forms large smotheringclumps of up to 50 plants/m2(Haggar et al., 1982);dense infestation (up to 3000 plants/m2)of annual meadow grass (Poa annua) can also suppress sown species. The problem is exacerbated in mild wet autumns if the establishment of sown species is slow or poor. However, as efficiency against broad-leaved weeds is stepped up, the

AGRONOMY OF WHITE CLOVER

31

chances of damage to clover is also likely to increase. Timing of application becomes more critical, and the guidelines for application must be closely followed. Because clover seedlings do not always emerge uniformly, all the plants are not necessarily at the required growth stage (usually one to three trifoliate leaves depending on the herbicide) for tolerance to the herbicide, and a check to clover development or, indeed, plant death is not uncommon following the application of herbicides (Kirkham and Haggar, 1983; Kirkham el ul., 1984; Standell, 1985). If clovers are suffering from pest or disease attack, they can suffer a severe setback even when sprayed at a safe growth stage. There is not a great deal of published experimental evidence proving definitive clover safety of herbicides, and damage may go unnoticed simply because the grass/clover sward is a dynamic entity and the reasons for a changing balance in grass :clover ratio are not always recognized. Scorch, temporary suppression, and severe checking of clover are terms often mentioned on the manufacturers’ labels. Frame and Newbould (1984) suggested that incorrect use of herbicides was a major cause of lack of clover presence in sown grass/clover swards. Choice of herbicide will depend upon the weed spectrum present. Pernicious perennial weeds such as docks (Rumexspp.), thistles (Cirsiumspp.), or nettles (Urticu spp.) should be treated presowing. The undersown cereal crop poses a dilemma since the best herbicide for the cereal crop may not be the safest to the clover. Also, both cereal and clover may not reach a safe growth stage for tolerance to the herbicide simultaneously. Late application in relation to the cereal growth stage can result in reduced grain yield; overearly application results in death or retarded development of clover plants (Broughton et ul., 1982).In general, clover seedlingsare more susceptibleto herbicide damage than grass seedlings. Because of the practical difficultiesassociated with clover susceptibilityto herbicides when establishing a grass/clover sward, Haggar et u1. ( 1985a) proposed an “evasive” strategy. White clover would be established in monoculture in spring with or without a suitable cover crop. Then preemergence weeds would be controlled by EPTC and postemergence weeds by MCPB/ benazolin/bentazone mixtures, and clover would be well established by late summer and could then be slot seeded with its companion grass, such as perennial ryegrass. In the established grass/clover sward, invasion by perennial broad-leaved or grass weeds can lead to sward deterioration and low production from the sown species (Dibb and Haggar, 1979). Herbicide options to control certain problem weeds adequately have increased recently, but a penalty to clover of plant scorch, suppression, or even death has had to be accepted, e.g., ethofumesate for control of annual meadow grass, soft brome (Bromus mollis), or

32

J. FRAME AND P. NEWBOULD

chickweed, linuron for control of chickweed, 2,4-D for control of ragwort (Seneciojacobaea), and dicamba/mecoprop/MCPA,triclopyr, or triclopyrbased herbicides for the control of perennial broad-leaved weeds such as docks and thistles in established grassland. Comprehensive recommendations for weed control in grass/clover and clover swards have been listed by Fryer and Makepeace (1978), Anonymous (1983d), Williams (1984), and Lee ( 1985). It is important to note that after removal ofweedsby herbicide, a judicious management strategy is necessary to remove or ameliorate the conditions causing weed problems. Various individual factors or combinations have been identified;they include poor drainage, acidity, plant nutrient deficiencies, overgrazing, and undergrazing. G. DISEASES Investigations into clover diseases have been and still are limited in relation to other aspects of clover studied (Anonymous, 1977, 1983c; Lewis, 1984).However, it is becoming increasingly apparent that disease is a major cause of clover unpredictability and loss in swards (Chamblee et al., 1983; Gibson et al., 1983;Davies, 1984;Frame and Newbould, 1984).The various diseases affecting white clover have been listed for the United Kingdom by Speddingand Diekmahns ( 1972),Burdon ( 1983), and Williams ( 1984), for Ireland by O’Rourke (1976), for Canada by Turkington and Burdon (1983), and for the United States by Leath (1985) and Barnett and Diachun (1985). Clover rot (Sclerotinia trifoliorum) is a serious disease of white clover, especially in England and the United States, which adversely affectsproduction and persistency (Gibson and Hollowell, 1966; Aldrich and Doling, 1967; Aldrich, 1970a; Scott and Evans, 1980; Davies, 1982, 1984; Anonymous, 1985b; Carlson et al., 1985; Leath, 1985). Other synonyms for the disease are collar rot, black patch, Sclerotinia root and crown rot, Sclerotinia wilt, and clover canker. The fungus is soil- and seed-borne. Infection occurs mainly in autumn by wind-borne spores produced from sclerotia which form in the crown of the plants. Symptoms, mainly seen in early spring, include stolon and root rot, wilting, and plant death. The recommended list of clover varieties for England and Wales (Anonymous, 1985b)gives varietal production from Sclerotinia-infectedand Sclerotinia-freesites. Varietal resistance ratings, devised from the assessment of plants grown on naturally infected field plots, are given on a 1-9 scale (9 = good resistance). Varietal resistance ranges from 1 (Grasslands Huia) to 6 (Blancaand Sonja).In dense stands of clover in the south of the United States, another fungal root rot disease (Sclerotinia rolfssii) causes severe damage especially in humid conditions (Carlson et al., 1985).

AGRONOMY OF WHITE CLOVER

33

Clover root deterioration can also be caused by a range of soil-inhabiting fungi, either individually or in combination. The principal pathogenic fungi appear to be species of Fusarium, although other genera are implicated (Halpin et al., 1963; O’Rourke, 1970, 1976; Menzies, 1973a). The root rot complex has long been a recognized problem in the United States (Leath et al., 1971). The symptoms are progressive necrotic breakdown of tap and lateral roots. The Fusarium species do not appear to become active until the plants are exposed to stress, whether nutritional, climatic, managemental,or from pests or other diseases. Because of the complex nature of the problem, breeding for resistance is likely to be difficult (Leath et al., 1971; Menzies, 1973a; O’Rourke, 1976). A number of conspicuous foliar diseases are widespread in many temperate countries of the world, especiallywhere wet conditionsare common. The production and quality of the clover forage are adversely affected although little is known ofthe extent of the losses. Leafloss, lack ofvigor, stunting, and leaf yellowing can occur. The diseases, mainly leaf spots, and their causal pathogens include pepper spot (Leptosphaerulina trifolii), sooty or black blotch (Cymadothea trifolii), anthracnose (Colletotrichum trifolii), the leaf spots (Stemphylium sarcinaeforme, Pseudopeziza spp., and Curvularia trifolii), spring black stem (Aschochyta imperfecta), and summer blackstem (Cercospora zebrina) (Gibson and Hollowell, 1966; O’Rourke, 1976; Brougham et al., 1978; Carlson et al., 1985). It has been reported that sooty blotch can result in increased levels of coumestans (flavonoid estrogens) in clover forage which, following ingestion by livestock, can lead to reproductive disorders (Newton et al., 1970; Wong and Latch, 1971). Rusts (Uromyces spp.) can be serious in some countries, causing leaf loss, lowered forage quality, and impaired N fixation (Gibson and Hollowell, 1966; ORourke, 1970, 1976). Clover is sometimes affected adversely by powdery mildew (Erysiphe trvorii). In attempting to control seed-borne disease in clover seedlings, Lewis ( 1984) reported inconsistent benefits from routine fungicide use (e.g., benomyl, metalaxyl) at establishment. Many virus diseases, systemic in clover plants, and frequently insect transmitted (principally by aphids), have been recorded. The virus damage is often subclinical but the effect can also be debilitating and severe, causing reductions in clover production and persistency (Kreitlow et al., 1957; Fry, 1959; Gibson et al., 1981). Virus infection can also reduce flowering and seed yields (Barnett and Gibson, 1977). It was noted by Carr ( 1979) and Carlson et al. (1985) that white clover was commonly affected by multiple infections of three or four distinct viruses. Viruses affecting clover include white clover mosaic, clover yellow vein, red clover vein mosaic, red clover necrotic mosaic, alfalfa mosaic, arabis mosaic, pea mottle, aster yellow mosaic, and peanut stunt (Davies, 1964, 1970; Gibbs et al., 1966; O’Rourke,

34

J. FRAME AND P. NEWBOULD

1970; Barnett and Gibson, 1975; Brougham et al., 1978; Turkington and Burdon, 1983;Carlson et al., 1985).Rugose leaf curl caused severe effectson seedlings in Australia (Grylls and Day, 1966). Clover phyllody is a mycoplasma disease reported from several countries (Kilpatrick and Kreitlow, 1961;O’Rourke, 1970, 1976; Gibson and Hollowell, 1966;Carr, 1979;Turkington and Burdon, 1983).Infected plants show poor rhizobial nodulation (Joshi et al., 1967), reduced vegetative vigor, and increased winterkill compared with healthy plants. Infected plants usually fail to set seed so the disease can be a problem if present in breeding material (Carr and Large, 1963).English stolbur (clover red leaf) and clover witches’ broom are other mycoplasma diseases, transmitted by leafhoppers, which cause morphological abnormalities (Williams, 1984). It has been concluded that the most stable long-term solution to clover diseases generally is to produce varieties resistant to the commonest and most damaging pathogens; also the introduction of highly susceptiblegermplasm into plant breeding programs should be avoided (O’Rourke, 1970; Davies, 1974;Carr, 1979; Fisher and Hayes, 1982).However, because of the complexity of some of the diseases, progress in breeding programs may be slow (Davies, 1970; Leath et al., 1971). A greater effort is also required to quantify the effects of disease on clover production and persistence, and ascertain the best means of control. H. PESTS In grass/clover swards there is a large biomass of insectsand other invertebrates in and on the soil and on plants. Of this biomass only a small proportion may be regarded as pests, and of these veiy few are capable of damaging white clover. Damage can range from sporadic to frequent and from small patches to large areas. Complete destruction of the clover component in a sward is not common. Because of the often-covert nature of their activities, the pests and the damage they inflict are not always appreciated (Anonymous, 1977, 1983~; Burdon, 1983). Also, variability in clover production is clearly influenced by a complex of factors some of which, such as soil or climatic, are more obvious and have therefore received more study. The various pests affectingwhite clover in the United Kingdom have been tabulated by Spedding and Diekmahns (1972), Burdon (1983), and Williams (1984). Major pests of clover in the United States are listed by Gibson and Hollowell (1966), Carlson et al. (1983, and Manglitz (1 985). Cook and York (1979) referred to plant stunting, distortion, and drying out caused by stem eelworm (Ditylenchus dipsaci) in spaced white clover plants and speculatedthat such effectscould contribute to the unpredictabil-

AGRONOMY OF WHITE CLOVER

35

ity of white clover in grass/clover pastures. Williams and Barclay ( 1972) noted that stem eelworm infestation during sward establishment could inhibit clover and delay the development of a well-balancedgrass/clover sward in New Zealand. Varietal resistance to stem eelworm can vary. Williams ( 1972) ranked Ladino clover varieties as highly resistant, while Bingefors ( 197 1) also classed a number of Danish varieties as resistant. Aberystwyth S 184 was susceptible (Cook and York, 1979). The clover cyst nematode (Heterodera trifolii) is widespread in British agricultural soils (Cook and York, 1985). While its effects have not yet been quantified it was associated with the disappearance of white clover from a cutting trial and lack of clover vigor in a grazingtrial (Morrison, 198 1). It was also found at sites where clover production was low (Evans et a!., 1982). They recorded better clover establishment (three to six times) with nematicide (aldicarb)treatment relative to untreated on a site infected by stem (0. dipsaci)and cyst (H. trifolii)nematodes. Evans and Cook (1 983) obtained an improvement in herbage production of 2.5 times from aldicarb-treated plots. The adverse effect of clover cyst nematode on clover performance has been noted in the Netherlands (Ennik et al., 1965) and in New Zealand (Yeates et al., 1975). At sites in New Zealand with mixed populations of H. trifolii and the root knot nematode (Meloidogyne hapla), seedlings and established plants were damaged (Healy et al., 1972, 1973; Yeates et al., 1973). Damage to white clover by the root knot nematode is also widespread in the southern United States (Carlson et al., 1985). Free-living nematodes (Pratylenchus spp.) have been reported to delay clover growth and plant development in Canada and the United States through feeding on the roots and reducing root size (Minton, 1965; Willis and Thompson, 1969). In parts of New Zealand, the grass grub (Costelytrazealandica) is a major pest. The larvae feed on the roots of pasture plants and high infestations can cause reduced herbage production and the disappearance of white clover (Brougham et al., 1978; East et al., 1980). Grub populationsincrease to peak levels in reseeds and in permanent pastures and then, for unknown reasons, subsequently decline naturally to low levels (East and Willoughby, 1983). Knowledge of population dynamics can help in formulating a strategy of control. Wilson ( I 978) identified a number ofpotentially tolerant and resistant lines of white clover. The subterranean grass caterpillar, porina (Wiseana cervinata), is also a serious insect pest of New Zealand pastures (Pottinger, 1968; Harris, 1969), but quantitative measurements of its effects on sward productivity are scarce. Larval feedingcan destroy clover plants but there can be regeneration after defoliation by porina if there is a developed mat of clover stolons (Harris, 1969).

36

J. FRAME AND P. NEWBOULD

Sporadic reports of damage to white clover by slugs, particularly in autumn, have been made (Anonymous, 1982; 1985b). The extent of slug damage in commercial swards in the United Kingdom is unknown but swards are most vulnerable in the establishment phase. Deroceras reticulatum is the species of slug most implicated in the United Kingdom but Carlson et al. (1985) note L i m a spp. in the United States. Slugs are also a likely problem to seedlings in minimal cultivation techniques such as direct drilling (synonyms are slot seeding, sod seeding) into established swards (Edwards, 1975; Allen, 1981; Clements and Bentley, 1983). In a review of seed establishment in directly drilled sowings, Naylor et al. ( 1983)listed a set of guidelines for successwhich included the incorporation of crop protectant chemicals applied presowing or pre- or postemergence. The attractions of the clover varieties Sonja and Milkanova to slugs, and therefore their susceptibility to damage, has been noted (Shakeel and Mowat, 1984; Anonymous, 1985b). The chemical methiocarb is effective against slugs. Angseesing ( 1974)observed that slugs, snails, and small mammals preferred acyanogenic forms of clover. Leathejackets (Tipula spp.) feed on and damage clover, especially in spring (White and French, 1968). They can also be a problem on direct drilled land (Edwards, 1975). Effective means of predicting the onset of leatherjacket infestation and damage in swards are available (Newbold, 1981; Anonymous, 1985d). Several pesticides are effective, e.g., chlorpyrifos, triazophos, quinalphos, and y-HCH. Other pests of lesser importance generally, but which can be important locally in various countries, include weevils (Sitona spp.), clover leaf and alfalfa weevils (Hyperapunctata and H. postica, respectively),and the potato leafhopper (Empoasca fabae). Pests which affect seed crops in the United States include the lesser clover leaf and clover head weevils (Hypera nigriorostris and H. meles, respectively), clover seed weevil (Miccotrogus picirostris), Ladino clover seed midge (Dasineura gentneri), the lygus bugs (Lygus hesperus and L. elisus), and spider mites (Tetranyclues spp.) (Gibson and Hollowell, 1966;Carlson et al., 1985;Manglitz, 1985).In England, attack by the white clover seed weevil (Apion dichroum) can reduce seed production by one-quarter (Anonymous, 1979). I. SEED PRODUCTION Annual usage of white clover seed in the EEC is -3000 metric tons (Fritsen, 1985). Denmark is the major producer in the EEC but production has declined to a quarter (800tons) ofthat adecade ago. A similar pattern has

AGRONOMY OF WHITE CLOVER

37

occurred in the United Kingdom; annual production is now only 30-40 tons compared with 120- 160 tons formerly (Anonymous, 1983c). Italy produces a few tons of Ladino clover types, including the variety Expanso. The main reason for the decline in Denmark and the United Kingdom is the wide variation in seed production among individualcrops and years, leading to erratic economic returns for growers. Fritsen (1 985) quoted a “normal” mean production of 430 kg/ha k 30% but noted that production was only about 100 kg/ha in years with poor weather conditions. The average seed production in the United Kingdom of certain old-established varieties (S 184, S 100, Kersey) is only 100- 135 kg/ha (Anonymous, 1983~). Home-produced seed in the United Kingdom now consists of only 4% of the annual usage of 900- 1000 tons (Davies, 1982; Anonymous, 1983c). This reflects the climatic disadvantages of the United Kingdom for producing seed (Williams, 1970). Yet many varieties which are agronomically superior under United Kingdom conditions have been developed and are recommended (Anonymous, 1985b,e). In addition, breeders of these newer varieties have paid increasing attention to seed production potential (Evans and Davies, 1978; Davies, 1982). The problem of clover seed production for the United Kingdom is being approached from two angles. First, a consortium of commercial seed firms and the National Seed Development Organisation is seeking suitable areas abroad for seed multiplication, and seed is now being produced in Oregon and Idaho in the United States. Second, research has been stepped up to overcome the problem of low seed production under United Kingdom conditions. Hides et af.( 1984)have demonstrated that achieved seed production could be only 28% of the potential due to variation in flower ripeness at harvesting, low seed setting, and harvesting losses depending upon management. Haggar et d.(1963) achieved 25-75% potential in their trials. Vigorous stolon growth, which is encouraged by light penetration (Zaleski, 196I), is necessary during the seed harvest year since flowers arise principally from nodes developed that year (Thomas, 1981b). To encourage coincident stolon growth and floral development in a first-harvest-year sward, Roberts ( I 980) found that sowing clover in wide drills under a spring cereal crop in the establishment year was advantageous, while Hides et af. ( 1984)found thinning of an established clover sward beneficial; spring defoliation to remove the leaf canopy during floral developmentis also of benefit but timing depends on the variety and growing conditions. Haggar el al. ( 1963)noted that severedefoliation by continuous sheep grazing rather than rotational grazing or silage gave better clover seed production from the Kent Wild White variety, traditionally grown in perennial ryegrass/clover swards, because of better removal of the leaf canopy. Grazing these swards until late

38

J. FRAME AND P. NEWBOULD

May/early June to reduce grass competition gave better clover seed yields than earlier cessation of grazing (Evans and Knappet, 1985). Poor seed setting, which can result from reduced insect pollination and plant nectar release, can impair seed production (Hides et al., 1984); low temperatures and wet weather are adverse influences. Mechanical harvesting techniques need improvement (Anonymous, 1977),but an important factor is time of harvest, since clover has an extended flowering and ripening period and thus all the flowerheads do not ripen simultaneously.

V. NITROGEN FIXATION In addition to the high feed value and acceptability to stock of the herbage of white clover there is great interest in knowing how much N is fixed, what factors affect this, and how much of it is transferred to companion grasses. This knowledge is important so that agronomists can devise grazing strategies for pastures which will ensure optimum growth of herbage and N fixation, but also optimum intake and utilization by livestock.

A. AMOUNTS The literature is full of papers debating the precision of methods used to assess N fixation, and the subject is reviewed frequently. Three main methods, the difference method (Whitehead, 1970), acetylene reduction (Turner and Gibson, 1980), and isotopic dilution using I5N (Witty, 1983; Chalk, 1985), are used. Nutman ( 1971) published a summary table describing the amount of N fixed by forage legumes which included a top value of 682 kg N/ha per year from Sears et al. (1965) in New Zealand. The most recent ranges of figures are shown in Table V, from which it is seen that the top value is 350 kg N/ha per year. The wide ranges of values from all sources reflect large variations in the amount of white clover as well as the influence of a range of factors, such as Rhizobium strain effectiveness, amount of mineral N available,defoliation regime, and climatic influences. However, if a median value from all the sources shown in Table V is calculated, this would indicate fixation of 150 kg N/ha-year. Since only about 50%(range, 30- 80%)of fertilizer N added to pastures appears in the plants, the input of N equivalent to that fixed would require application of 300 kg N/ha.

-

39

AGRONOMY OF WHITE CLOVER Table V Amounts of Nitrogen Fixed by White Clover in Mixed Swards Country

Type of pasture

United Kingdom United Kingdom Netherlands New Zealand United States

Lowland Hill and upland Lowland various various

N fixed (kg/ha per yead

74-280

100- 150 65-200 85 - 265 50-350

Reference Cowling ( 1982) Newbould (1982) t’Mannetje (1985) Hoglund et al. (1979) Burton (1985)

B. FACTORS AFFECTING N FIXATION Assuming that effective rhizobia are present in sufficient numbers, the amount of N fixed depends primarily on photosynthates reaching the nodule, and thus on the amount of photosynthetically active leaf area in the sward. The amount of clover leaf is determined by the number of plants and/or branches with leaves that are present in full light in a sward. Climatic conditions, such as the availability of sunlight and the temperature of the leaves, affect the rate of photosynthesis as does the area of clover leaf which is affected by cutting or grazing. The conflict between keeping sufficient clover leaf for N fixation and yet providing sufficient feed for animals is apparent. There is much debate on the amount ofwhite clover that should be present in a grazed sward. Most animal nutritionists think an average of 30% or above of clover herbage over the season is a desirable target (Martin, 1960; Davies, 1974; Wright, 1975; Curll, 1982; Stewart, 1984). Attainment of this target would result in the amounts of fixed N shown in Table V. Low soil temperatures at the start and end of the growing season and midsummer drought, which all affect the appearance, growth, and functioning of leaves, also depress N fixation. In United Kingdom conditions, the peak nitrogen fixing activity occurs during the period of vigorous growth in late spring-early summer (Fig. 3; Marriott et al. 1984). Application of nitrogen reduces nodulation and N2fixation in pasture legumes, the response varying with species, cultivar, Rhizobium strain, form of N, amount of N, time and site of N application, age and size of host plant, and prevailing environmental conditions (Young, 1958; Sprent et al., 1983). For grass/white clover mixtures, applied N also enhances the growth of the grass component, and the resultant competition for light and nutrients may be more detrimental to the legume than to effects on nodule activity (Mulder et al., 1977). For perennial pasture legumes, mineral N application during

40

J. FRAME AND P. NEWBOULD

2

-50

I-

t c

-

-40 c

-

0

-30 L

3

f

-

-20

Temperaturec ---e 1 I

I

J

F

M

-.

c-c

* I

A

M

I

I

1

J

J

I

A

I

I

S

O

I

N

-

10 O

.L

0

"

D

FIG.3. Seasonal profile of (a) nitrogen fixing (acetylenereducing)activityand standing DM ofclover leaf and (b)soil moisture content and mean 100 mm soil temperature.(From Maniott and Rangeley, 1984.)

regrowth after defoliation can be particularly detrimental (Groat and Vance, 1981). Another effect of fertilizer N on fixation of N is that it reduces the proportion of white clover in the mixed sward (see Section VII).

C. NTRANSFER Atmospheric N fixed by white clover can be transferred to companion grasses via the grazing animal, via the atmosphere, or by death, decomposition, and mineralization of nodules, roots, and other plant parts (Haystead, 1983).

AGRONOMY OF WHITE CLOVER

41

I . Transfer via the Grazing Animal The consumption by livestock of clover shoots containing fixed N and the return of 80% of the N to the pasture as urine is the most rapid route for transfer, but it is also the route most subject to losses due to volatilization of ammonia (NH,), and possibly due to leaching too. Recent work by Ryden et al. ( 1984) and by Thomas et al. ( 1985) has examined aspects of this transfer mechanism.

2. Transfer via the Atmosphere The magnitude of transfer as atmospheric NH, is thought to be small (Denmead et al., 1976; Mamott and Rangeley, 1984), but recent work indicates it is possible and that care must be taken to allow for this route in both laboratory growth room and field-scale trials. 3. Transfer via Death, Decomposition, and Mineralization The route through decomposition is thought to be the most important for the buildup of long-term soil fertility, but until recently the amounts involved have been difficult to measure (Haystead and Marriott, 1978). Haystead and Marriott (1978) concluded that significant N transfer only occurred when the plants reached maturity or when the plant was subject to stress through shading or defoliation. They found significant transfer from clover to ryegrass ( 12 - 27% of grass N) only after the sward had been defoliated on four occasions during about 20 weeks of growth, and transfer was not detected using the I5N dilution technique until after the fourth harvest. The publication of more sophisticated techniques involvingthe direct labeling of legume foliage with 15Nis likely to lead to the accumulation of much more data on this important topic (Ledgard et al., 1985).

VI. QUALITY Increasing interest in the potential of white clover for animal production has been reflected in the number of recent reviews (Ulyatt, 1973; Thomson, 1979, 1982, 1984; Greenhalgh, 198 1). Thomson ( 1984) noted that in contrast to earlier studies which were conducted primarily with sheep, there is now more emphasis on cattle and dairy cows. A notable feature has been the comparative element with grass, principally perennial ryegrass.

42

J.

FRAME AND P. NEWBOULD

A. CHEMICAL COMPOSITION There is less information on the chemical composition of white clover than on grasses. A number of reviews on the general composition (Bland, 1968; Speddingand Diekmahns, 1972) and on mineral composition (Whitehead, 1966; Fleming, 1973; Jones and Moseley, 1984) are available for grassland forages including legumes. Essig ( 1985) recently discussed the beneficial and detrimental components of clovers, including white clover. It is well documented that the chemical composition of forage from grassland species can vary according to stage of growth, plant part sampled, season of year, soil pH, and soil nutrient status. Lime and fertilizer application can influence major element uptake more than the parent soil type, while trace element uptake is strongly influenced by soil pH (Mengel and Kirby, 1982). Mineral nutrition and its implications on animal health and production were reviewed recently (Reid and Horvath, 1980; Grace, 1983; Suttle et al., 1983). It is obviously important to be aware of potential mineral problems, especially where grassland forage is the major food for livestock production. Grazing strategy can have a major effect on chemical composition by controlling stage of growth of forage, by altering the botanical composition, particularly the content of white clover in mixed swards (see Section VIII,E), and by cycling of plant nutrients. The richness of protein, the high energy content, and high content of many minerals are major advantages of white clover forage. To illustrate typical chemical composition data from clover, ranges abstracted mainly from Spedding and Diekmahns (1972) are presented in Table VI. Most of the elements listed are necessary for plant nutrition, but some which are not (iodine, selenium) are essential for animal nutrition and are absorbed by the plant passively. There is little information on the content in clover of several other minor elements believed to be essential for animal nutrition. In relation to grasses, white clover is usually lower in cellulose, hemicellulose, water-soluble carbohydrates, lignin, sodium, manganese, and silicon contents. It is higher in water, pectin, total nitrogen compounds, organic acids, carotene, estrogeniccompounds, calcium, magnesium, iron, copper, cobalt, molybdenum, boron, and selenium. It is similar in lipids, phosphorus, potassium, sulfur, chlorine, zinc, and iodine. Because of a different chemical composition than grass, clover has been used in simulation and modeling work in animal nutrition (Black et al., 1976, 198 1). Many of the compositional differences between white clover and grasses, and indeed other forage legumes, are due to its stoloniferous habit of growth whereby the harvested portion is mainly youthful leaf and petiole, together with flowerheads and peduncles when flowering;stolon may be partially harvested particularly by sheep (Clark ef al., 1984a). In contrast, there is a decline in the leaf to stem ratio in the other foragesas they mature; this increasing maturity is accompa-

AGRONOMY OF WHITE CLOVER

43

Table VI Chemical Composition of White Clover" Constituent

Content range (% in DM)

Cellulose Hemicellulose Pectin Water-soluble carbohydrates Lignin Nitrogen compounds Lipids Organic acids Phosphorus Potassium Calcium Magnesium Sulfur Sodium Chlorine

(P) (K) (Ca)

(Mg)

(S) (Na) (Cl)

I .8 -26.6 3.6-12.0 3.5-7.7 5.2- 14.6 2.0-2.6 2.7-5.3 4.0-7.0 6.0-9.5 0.19-0.47 1.54- 3.80 1.20-2.31 0.15-0.29 0.24-0.36 0.05 -0.20 0.34- 1.56

(ppm in DM) Iron (Fe) Manganese (Mn) Zinc (Zn) Copper (CU) Cobalt (CO) (1) Iodine Molybdenum (Mo) Boron (B) Selenium (Se)

102-448 40-87 22 - 32 5.4-9.7 0.10-0.38 0.14-0.44 1.3-14.2 26 50 0.005-153

-

AFter Whitehead (1970); Spcdding and Diekmahns (1972); Osbourn (1980); Paynter (1985).

nied by an increase in structural carbohydrates and lignin and a decrease in nitrogen compounds and many minerals. The physical attributes of the chemical constituents in white clover may also be distinct from those in grasses (Van Soest, 1982).

B. INTAKEAND DIGESTIBILITY Thomson ( 1984) summarized the results from voluntary intake studies comparing white clover with perennial ryegrass. A range of forage types

J. FRAME A N D P. NEWBOULD

44

(fresh, dried, hay, silage) and animals (sheep, young cattle, lactating dairy cows) were used. Consistently, there was a higher voluntary intake of dry matter, 20%, of white clover compared with grass. The nutritive value of a forage to animals depends on the proportion of nutrients digested (apparent digestibility) and on the efficiency with which the digested nutrients are absorbed and utilized by the animals. The factors which affect the digestibility of temperate grassland specieswere reviewed by Raymond ( 1969). Digestibility of the organic matter (OMD) is a widely used expression in the United Kingdom. For many routine experimental purposes, it is usually derived by an in vitro digestion technique (Tilley and Terry, 1963; Alexander and McGowan, 1966). A more precise measure of energy value, the metabolizable energy (ME) of a forage, is used in animal nutrition work, and can be derived from the digestibility data by appropriate formulas (Osbourn, 1980). For most grassland species, digestibility is clearly related to stage of maturity. With advancing maturity, the ratio of stem to leaf increases, and the digestibility of the stem declines compared to that of the leaves (Harkess, 1963; Terry and Tilley, 1964; Aldrich and Dent, 1967; Harkess and Alexander, 1969). Unlike grasses and other legumes such as lucerne or red clover, white clover maintains a high digestibility even as it matures since there is a continual replacement of old leaves and petioles by new ones. This has two effects (Fig. 4); clover’s digestibility is maintained at a high level and the rate of decline is slow (Harkess, 1963, 1969; Davies et al., 1966; Harkess and Alexander, 1969). White clover regrowths during the season also maintain high digestibility relative to grass species because of clover’s habit of growth. Clearly the proportion ofwhite clover in a grass/clover sward can have a marked positive effect on the digestibility of the sward‘s forage. There has been little work carried out on the digestibility of clover plant fractions, but recently Gibb and Treacher (1983) found that the digestibilities of the flowers and flowering stems of white clover in a mixture of cvs. Blanca and Pronitro were low. It is not the purpose of this review to discuss animal production. Suffice it to say that many reviews have confirmed the superior feeding and nutritive value of white clover compared with grass for ruminant animal production (Ulyatt, 1973; Thomson, 1979, 1982, 1984; Greenhalgh, 1981; Stewart, 1984). The high voluntary intake characteristics,the high net supply of apparently absorbed amino acids per megajoule of metabolizable energy, and high mineral contents of white clover were major reasons given for its superiority.The main drawback ofwhite clover is its potential to cause bloat in grazing stock, especially cattle, but a number of preventative measures exist (Austin, 1982; Essig, 1985); cyanogenicglycosidesare not believed to be of importance in the grazing ruminant.

-

45

AGRONOMY OF WHITE CLOVER

0 .... . , ' o '......o.

\ \

\

...

'0

u,

1

2915

I

I

l2/6

I

1

26/6

I

I

10/7

0

I

I

I

24/7

C u t t i n g Date

FIG. 4. Changes in the percentage in vitro organic matter digestibility (96 OMD) with increasingmaturity. (0)Italian ryegrass;(0)white clover;(A) red clover. (From Harkess, 1969.)

VII. GRASS/WHlTE CLOVER DYNAMICS The establishment ofwhite clover plants in a mixed sward is relatively easy once the appropriate soil and microbial requirements have been met and climatic conditions are suitable. However, the attainment of a target proportion of white clover, and maintaining it over a period of time so that it provides a consistent and predictable contribution to the sward, remain very difficult tasks for the agronomist. Lack of such knowledge and the generally unpredictable behavior of white clover within and between years has not encouraged farmersto move from use of fertilizer N with grass swardsto sole use of mixed grass/clover swards (Frame and Newbould, 1984). The former are relatively easy to manage, and shortagesof herbage which are anticipated or arise within the growing season can often be overcome by use of fertilizer N.

46

J. FRAME AND P. NEWBOULD

l o90 o

t

a

Nil fertilizer N

nay

June

July

August

%?[ember

October

Growing season

FIG.5. Average daily growth rate of grass and white clover in grass/clover swards at two annual fertilizer N rates: (a) no fertilizer N, (b) 200 kg N/ha per year. Means of three clover varieties,Aberystwyth S 184, AberystwythS 100, and Kersey, over three harvest years and three Scottish National List centers during 1975- 1984; the companion grass was perennial ryegrass cv. Premo. (From M. Talbot, personal communication.)

Plants growing in mixtures compete above ground for space and light and below ground for space, water, and nutrients. When such mixtures are grazed, the type of animal, its treading and excretal patterns, and its preference or rejection of one or other partners can markedly influencethe balance of species (Section VII1,E). Man can influence the dynamics of grass/white clover swards by choice of grazing severity and pattern, and of lime and fertilizer (especiallynitrogen) additions. The situation is extremely complex and it is not surprising that there is no general agreement on the primary influences or on precise management guidelines for mixed swards containing clover. Competitive aspects of the grass/legume association were comprehensively reviewed by Haynes (1980) and the influence of nitrogen on

AGRONOMY OF WHITE CLOVER

47

100

90

80

-

b

-m

200 k g “/ha

per y e a r )

57c-60 n

m

2

I

m

r

j 5C’ m z

Z40 m

-

0 0

m

230>

a

2c-

1c-

d

I

I

nay

June

I July

1

I August

September

I October

Growing season

FIG.5b.

this by Vallis ( 1978) and Ennik ( 198 1). Aspects of the physiological basis of variation in production from grass/clover mixtures were described by Rhodes (198 I), and other factors affecting interactions between white clover and grasses were discussed by Snaydon and Baines ( 198 1). The ecology and management of white clover-based pastures in New Zealand were described by Brougham et al. (1978), in a discussion which covered many of the generally relevant aspects. There is general agreement that white clover is at a competitive disadvantage when grown with most pasture grasses. The latter are taller, have a greater mass of fine roots, and have less precise requirements of climate and soil nutrition for growth. Thus, unless management concentrateson providing a favorable environment for white clover, it is bound to be difficult to maintain a useful proportion of clover plants in a mixture with grasses. A further disadvantage of clover is that if plants die they are replaced by grasses or weeds but rarely by white clover (Wright, 1975).

48

J. FRAME AND P. NEWBOULD

Studies of plant demography in permanent pasture suggest that soil N levels and aggressiveness of grass species competing with white clover are dominant influences. Turkington and Harper ( 1979a)postulated a regeneration cycle for micrositesin the ecosystem which helps to explain the behavior of sown grass/clover swards. Initially, clover and perennial ryegrass coexist, mainly due to dissimilar growth cycles. As soil N builds up following inputs from clover N fixation, the grass increases its dominance; due to competition, clover declines and adopts dwarflike forms, and eventually disappears. The loss of fixed N inputs leads to the replacement of perennial ryegrass by short forms of Agrustis and other species, which then leaves the possibility that white clover can establish again if seeds are present at the site, so starting the cycle again. The growth cycles shown in Fig. 5 (M. Talbot, personal communication) were derived from the grass/clover swards used at three sites in Scotland during 1975- 1984 as standards in clover varietal evaluation for National Listing purposes (see Section IV,A). They are similar to those found in New Zealand (Brougham, 1959b; Haynes, 1980).Other examples of time course effects in competition between perennial ryegrass and white clover come from short-term box experimentsby Martin and Field (1984). They showed that perennial ryegrass was more competitivethan white clover throughout a 20-week experiment but its overall competitive ability increased with time; ryegrass roots dominated at first harvest, ryegrass roots and shoots dominated at second harvest, and ryegrass shoots dominated clover shoots at the third and fourth harvests. The applicationof fertilizerN increased the overall competitive ability of perennial ryegrass relative to white clover, due mainly to the increase of shoot competitive ability. The operation of long-term cycles of these types in experimental swards is often missed because experiments frequently last for the length of a research project (e.g., 3 years), so missing the clover decline phase. The occurrenceof unexpected “crashes” or declines in clover performance has been noted by Stewart et al. ( I970), Garwood and Tyson ( 1979),Davies ( 1982), and Stewart and Haycock (1984) (Fig. 6). Frame and Newbould (1984) gathered a collection of the causes given for clover inconsistency, and the wide extent and variability of the list (Table VII) illustrate the lack of precision of our knowledge. Support for the idea that compatible grasses and clovers can be found and must be used in mixtures comes from recent observations on coadaptability by Evans et al. (1983). The use of clover and grass selected from the same site of origin can result in higher annual production of total herbage (10.3 tons DM/ha) than if plants from different sites were used (7.4 tons DM/ha) without change in the percentage of clover which was 69% in their trial.

AGRONOMY OF WHITE CLOVER

1

2

3

4

5

6

49

8

7

Year

FIG.6. Example of variation and sudden “crashes” in white clover content (September data) in grazed grasslclover sward over an 8-year period, 1978- 1985. (From Stewart and Haycock, 1984; T. A. Stewart, personal communication.)

VIII. PRODUCTION AND MANAGEMENT There are many published production results from cutting experiments usually at research institutes and under conditions where many possible limiting factors are removed. The volume of data is much greater from grass/clover than clover swards. Traditionally, grass is included in seed mixtures with white clover since the production potential of the mixture is greater. Weed ingress is less of a problem in the mixed sward. The grass component also exploits the nitrogen fixed by the clover. Most data are derived from cutting regimes designed to simulate grazing, i.e., five to seven harvests per year. However, a limited amount of work has been done on the production from silage cutting systems or simulated grazing systems with silage cutting interposed at various times during the season. There is a relaTable VII

Major Causes of White Clover Inconsistencya Climate Soil Sward Animal Management

Radiation, temperature, rainfall Compaction, wet vs dry,acidity, nutrients Grass-toclover balance, clover longevity, pests and diseases, weeds Treading, excreta, selectiveness, species of animal Maintenance lime and fertilizer, fertilizer nitrogen, method of utilization, system of production ~

After Frame and Newbould ( 1984).

~~

50

J. FRAME AND P. NEWBOULD

tive paucity of data from grazing experiments and also of the performance of swards in farm practice. A. WHITECLOVERMONOCULTURES The highest recorded production from an imgated white clover monoculture in the United Kingdom was 11.80 metric tons DM/ha, but it included a grass weed component (Spedding and Diekmahns, 1972). On a white clover sward kept pure by carbetamide application in late winter, 10.25 tons DM/ha (100%white clover) was obtained without irrigation (Reid, 1983a). Under differing cutting regimes, Spedding and Diekmahns (1 972) cited ranges of 3.90 to 8.60 tons DM for unirrigated swards and 6.1 1 to 1 1.80 tons DM/ha for imgated swards while Davies (1969) cited 8.03 to 9.48 tons DM/ha for unirrigated swards. Suckling ( 1960)recorded 10.55 tons DM/ha from unirrigated swards in New Zealand, which is close to the 10 tons/ha forecast by Smetham (1973). From specific silage regimes, production levels of6.76 - 8.06 tons DM/ha with 75 - 78% white clover were reported by Castle et al. ( 1983)and Frame (1 985a), while under a range of cutting systems, i.e., 3- to 5-week intervals, D. Reid (personal communication) obtained 8-9 tons DM/ha. White clover swards are not responsive to fertilizer N application (Cowling, 196la; Armitage and Templeman, 1964). Recently, Hughes (1981) and Reid (1983a), using levels of N up to 750- 1000 kg/ha per year, obtained only small responses to N from pure white clover swards: 8.1 3 8.56 tons DM/ha with no N and means of 9.13 - 9.23 tons DM/ha over all the N application rates. B. GRASS/WHITE CLOVERMIXTURES

It is pertinent to examine production data from the 11 centers in the United Kingdom involved in evaluating white clover varieties for National Listing purposes (see Section IV,A). A total of 460 sets of data for perennial ryegrass cv. Premo/white clover swards exist for harvest years 1979- 1983 (Table VIII). The clover variety standards in the varietal evaluation work, mainly Aberystwyth S 184, Aberystwyth S 100, Kersey, and Milkanova, were sown annually during the period and herbage production measured for two successive harvest years (second and third after sowing). The highest recorded value in the United Kingdom of 15.53 tons DM/ha (with no applied N) obtained at Cambridge is noteworthy, although it is still only three-quarters of the theoretical potential (Frame and Newbould, 1984).The production ranges cited in Table VIII for both rates of fertilizer N

51

AGRONOMY OF WHITE CLOVER Table VIII Range of Annual DM Production and Clover Content of Grass/ Clover Swards from United Kingdom National List Trials, 1975-1983"

DM (tons/ha) Fertilizer N (kg/ha per year)

Total herbage

White clover

White clover

(96)

Highest Lowest Mean

0 0 0

15.53 2.03 8.31

12.12 0.49 4.16

24 50

Highest Lowest Mean

200 200 200

19.94 3.13 9.8 I

2.19 0.75 2.46

11 21 25

78

application (0 and 200 kg/ha per year) encompass achieved levels from many other types of cutting experiment involving grass/clover swards (Reid, 1970, 1972, 1983a; Spedding and Diekmahns, 1972; Morrison, 1981;Morrison et al., 1983;Frame, 1982a, 1985a;Frame and Newbould, 1984).However, most herbage production levelswithout applied N tend to be within the 6 - 10 tons DM/ha range. Very low levels have invariably been the result of pests and/or diseases (Aldrich, 1970a; Momson, 1981; Cook et al., 1984). Over nine sites scattered throughout New Zealand, and without fertilizer N, annual total herbage DM ranged from 6.7 1 to 14.92 tons/ha (Hoglund et al., 1979); the mean DM production was 8.40 tons/ha of which clover contributed 3.13 tons/ha (37%). Vartha ( 1972)cited a DM range of 10.031 1.94 tons/ha. From small paddock evaluations under grazing, Brougham (1977) listed highest production levels achieved over a range of environments at research stations as 12.0 tons DM/ha for hillsides, 16.2 tons/ha for nonirrigated dryland, and 22.8 tons/ha for a temperate site. The levels were about a third higher than the amounts of herbage utilized in farming system studies. Sward production, and indeed the clover component, is generally enhanced by infrequent defoliation compared with frequent defoliation and by close defoliation relative to lax (see Section VII1,D). Hill and upland areas often provide a harsh, variable, climatic environment for white clover growth (Munro and Davies, 1973; Francis, 1978; Newbould and Floate, 1979). In the United Kingdom the total herbage production of sown grass/clover swards rarely exceeds 5 -6 tons DM/ha in these situations and is often lower (Newbould, 1974, 1975, 1982).At a range

52

J. FRAME AND P. NEWBOULD

of Scottish sites, DM production levels from grass/clover swards given no applied N were 2.52-4.80 tons/ha (Burnham et al., 1970; Sandford, 1979a,b; Tiley, 198 1a; Newbould, 1982; Tiley and Frame, 1984). The establishment in the hills and uplands of grass/white clover swards is a desideratum since their production usually exceeds that of the indigenous swards (Frame et al., 1985) and the herbage is superior in feeding value (Thomson, 1984). The imposition of controlled grazing on indigenous grassy swards, allied to the use of sown grass/clover swards at key times in the grazing animal’s nutritional requirement cycle, led to the strategy of the “two-pasture” system of hill sheep farming (Eadie, 1970; Eadie et al., 1979; Armstrong et al., 1986). There is a lack of data from farm grass/clover swards, although Pflimlin ( 1984) reported that the use of grass/white clover swards on dairy farms in Brittany, France, was a viable alternative to high N use on grass swards. The variability of production on farm swards will be greater than that from experimental swards where more precise management control is exercised. Surveys have shown that the clover content in United Kingdom grassland is very low (Younie and Black, 1979; Forbes et al., 1980; Green, 1982; McAdam, 1983a,b; Swift et al., 1983), so total herbage production from these swards will be very low unless fertilizer N application rates are adequate. For example, in surveying450 farms, Forbes et al. ( 1980) recorded a mean of 2% clover ground cover on dairy farms; only 10% of the farms had enough to make an impact on production, compared with 25% on beef fatteningfarms and 30% on beef breeding farms. For these farm types in order, 50,30, and 25% of all swards had no sown legumes remaining, regardlessofage of sward. Corresponding mean fertilizer N rates applied per year were 154,65, and 42 kg/ha. Thus, either a vigorous clover content was not an objective, even on the beef farms with low N use, or else it was a failed objective, even though Doyle et al. ( 1984) have shown that a good economiccase exists for the use of grass/clover swards in sheep and beef enterprises with low fertilizer N usage. C. EFFECTOF FERTILIZER NITROGEN 1. Repetitive Application

Nitrogen is the main nutrient affecting herbage production from grassland. Accordingly, the production response of grass/clover swards to fertilizer N applied regularly throughout the season has been a subject of frequent study. Many response data (in kg DM/kg N applied) have accumulated. Variation in response is considerable from year to year, and from site to site.

53

AGRONOMY OF WHITE CLOVER

Chestnutt and Lowe ( 1970),summarizingthe responses obtained mainly in cutting experiments by experimenters, cited a range of 3.7 to 17.4 (mean, 8.8) from various annual N application rates up to 390 kg/ha; in six experiments where grass swards were included, the grass response range was 22.5 57.6 (mean, 32.4) for annual N rates up to 350 kg/ha. Perusal of more recent work (Bastiman, 1969; Reid, 1970, 1972, 1983a; Frame, 1973, 1982c; Orr and Laidlaw, 1978; Hood, 1982; Wilman and Asiegbu, 1982a; Boyd and Frame, 1983; Momson et al., 1983; McEwen and Johnston, 1985; Wilman and Hollington, 1985)together with the body of results from UK National List trials (see Section VIII,B) over the annual N application rates, 0 to 400 kg/ha, shows a somewhat wider response range, -6 to 20; however, the mean was - 8. The occasional negative responses occurred at low or moderate N rates (Cowling, 1961b; Frame, 1973; McEwen and Johnston, 1985). The size of response is related to the clover content of the sward and increases as the clover content decreases. The response also increases as cutting frequencydecreases. When cutting frequency is varied, the decline in response to N at higher N rates is less marked with frequent than infrequent harvesting (Holliday and Wilman, 1965; Cowling, 1966a; Frame, 1973). The effects of annual fertilizer N rates up to 900 kg/ha were examined on grass/clover and grass swards (Reid, 1970, 1972, 1983a). Production from grass/clover swards was superior to that from grass swards at N rates up to 300 to 350 kg/ha but not significantly different from grass at the higher N rates. Dobson and Beaty (1 977), using fewer N rates, obtained comparable production from grass/clover and grass fertilized with 336 kg N/ha per year in Georgia, United States. In Reid’s work the proportion of clover-fixed N transferred to the grass in the grass/clover sward decreased as N rate increased, a finding in agreement with Hemott and Wells (1 960), Reid and Castle (1 965), and Shaw et al. ( 1966). The use of large-leaved white clover cv. Blanca (Reid, 1983a)compared with a medium-leaved clover cv. Aberystwyth S 100 (Reid, 1970) resulted in better white clover performance, but in more recent work (D. Reid, personal communication), there was little difference in production between medium- and large-leaved clovers. In Reid’s swards, the amount of fertilizer N required annually on grass swards to produce the same total herbage DM as from the grass/clover without added N (the so-called fertilizer N equivalent of the gross effect of clover on production) was 165kg/ha for the grass/S 100sward and 265 kg/ha for the grass/Blanca sward when periods of 3 harvest years were compared. The grass/S 100 experiment was continued for a further 3 years and its nitrogen equivalent over the 6 years then averaged 140 kg/ha (Reid, 1972). In other recent work from 2 1 sites over 4 years (Morrison, 1981 ), fertilizerN equivalents of 180 and 200 kg/ha were obtained for perennial ryegrass/S 100

-

54

J. FRAME AND P. NEWBOULD

and ryegrass/Blanca swards, respectively (see Fig. 7). However, data for the United Kingdom summarized in Anonymous (1983d) suggest a wider N equivalent range, namely, experimental means of 124 - 275 kg/ha; the range of mean annual values was 28 - 302 kg N/ha. The decline in clover contribution of mixed swardswith increasing rates of fertilizer N is well documented. In Table IX, mean data from Frame ( 1985a), who used white clover varieties of differing leaf sizes, illustrate the effects. There has been an increase in the use of animal slurry (dilute mixture of urine plus dung) to fertilizeswardsin regionswhere winter housing of stock is practiced. Considerable savings in purchased fertilizers can be made if a rational program of manuring with slurry and fertilizers is instituted. However, there has not been a concomitant increase in research on the effects of slurry on swards. In comparison with fertilizer N, slurry and dilute urine application at equivalent N rates resulted in a greater content of white clover

-

12

-

S 23/Blanca S 23/S 100 c--..S 23

0 .

0

I

100

I

200

I

300

1

400

I

500

I

600

N (kg/ha per year) FIG.7. Mean total herbage production from grass(perennial ryegrass cv. Aberystwyth S 23) and grass (cv. S 23)/white clover (cv. Blanca or Aberystwyth S 100) swards; 2 1 sites X 4 years (Morrison, 198 I). Fertilizer

55

AGRONOMY OF WHITE CLOVER

Table IX Effect of Fertilizer Nitrogen Application Rates on White Clover Performance in Perennial Ryegmss/Clover Swards’

DM (tons/ha per year) Fertilizer N (kg/ha per year)

Total herbage

White clover

0 120 240 360

7.83 8.7 I 9.98 11.70

4.14 2.43 I .07 0.5 I

White clover (%) 53 28 I1 4

a Three-year means, two cutting heights, four white clover varieties. Afier Frame (1985a).

in grass/clover swards (Castle and Drysdale, 1962;Herriott and Wells, 1962; Drysdale, 1965).

2. Strategic Nitrogen Use The poor growth of white clover at low temperatures (springand autumn in the United Kingdom) is well documented. Thus, the use of fertilizer N to improve the grass/clover sward’s herbage production at these times has been advocated. Following spring application of N, white clover content and production were reduced, especially in midseason, and reductions were more marked as the rate of N application was increased (Cowling, 1966a; Heddle, 1966; Denehy and Morrison, 1979; Laidlaw, 1980, 1984; Laissus, 1983; Morrison et al., 1983; Frame, 1985a). Over differing rates of N application, but mainly in the range 0- 100 kg/ha, herbage DM responses vaned from 3 to 30 kg per kg N applied; responses were generally best at the low to moderate N rates (30 - 60 kg/ha) and where defoliation intervals were infrequent. Comparing first-harvest (May) responses at 15 sites from spring N rates of 0 and 67 kg/ha, Morrison et al. (1 983) obtained a mean response of 17 (range 6 - 35) kg DM/kg N. These trials were all conducted under cutting and white clover made up 35 - 65% of the total herbage in the treatments without N. Thus, even with reductions in clover caused by fertilizer N, clover contribution still remained at substantial levels. Laidlaw (1980) concluded that moderate spring N (up to 60 kg/ha) did not seriously reduce clover persistence, and later (Laidlaw, 1984) developed a method for predicting the effect of spring N on clover content. He found that medium-leaved and large-leaved

56

J. FRAME AND P. NEWBOULD

clover types responded similarly to spring N, but Aldrich ( 1970b)and Morrison et al. (1983) has suggested that large-leaved clovers are suppressed less than small- and medium-leaved types. There is a paucity ofwork on response to spring N ofgrass/clover swards in which white clover content is low, or on grazed swards (Brockman and Wolton, 1963; Wolton and Brockman, 1970) in which clover is under greater competitive pressure than in cut swards (see Section VII1,E). Frame (1985a) examined the effect of spring N only, autumn N only, or spring N and autumn N. He found that total herbage DM was increased by N in all systems, but white clover content and production were reduced. At equivalent N rates (25 - 75 kg/ha), autumn N suppressed clover less than spring N; similarly, with spring N plus autumn N combinations (up to 125 kg/ha per year) clover was most suppressed when a high proportion of N was applied in the spring. Annual herbage responses (in kg DM/kg N) were 1 1.5 for spring N increments, 7.2 for autumn N, 5.7 for spring N plus autumn N increments, and 7.1 over all N treatments. Armitage and Templeman ( 1964) obtained a herbage response of 7 kg DM/kg N applied from spring plus autumn N applications. Purvis and Younger (1984) noted that clover was able to make a greater contribution to total production when fertilizer N was applied in midseason compared to early or late season application. Clearly the value of increased herbage production must be weighed against the depressive effects of fertilizer N on clover performance. Clover is likely to persist if low to moderate N rates are used strategically and provided a grass/clover sward with a high clover content is used. However, it must be realized that all applied N is boosting the soil N pool, and must therefore be helping to tip the balance against clover in the successional cycle of the grass/clover sward (see Section VII). D. EFFECTOF CUTTINGDEFOLIATION 1. Frequency of Cutting

Many cutting studies have shown that total herbage production from grass/white clover swardsgenerally increases as the interval between defoliations is lengthened. This finding has been obtained as the fixed rest interval between defoliations was lengthened, or as the sward canopy height at cutting was increased. With annual N levels of 0-400 kg/ha, white clover production was increased, especially in early season, as the interval between defoliations lengthened (3- to 6-week intervals, or from the “simulated grazing” to the “silage” stage) but the proportion of white clover in the total herbage decreased (Holliday and Wilman, 1965; Frame, 1973; Orr and

AGRONOMY OF WHITE CLOVER

57

Laidlaw, 1978) or was little affected (Burger et al.,1958; Reid, 1959, 1962; Bland, 1967; Wolton et al., 1970; Brereton and Carton, 1985). However, Hollidayand Wilman ( 1965)noted that increasingthe defoliation interval to 14 weeks reduced both the amount and proportion of white clover in the total herbage. Using differingleaf-size clover types in association with perennial ryegrass, Wilman and Asiegbu ( 1982a)obtained enhanced white clover production with defoliation intervals of 3 to 12 weeks but the proportion of white clover was not markedly affected; their results supported the view of Spedding and Diekmahns ( 1972) that medium- to large-leaved varieties respond better to less frequent defoliation than smaller-leaved varieties. Tesar and Ahlgren (1950) and Chamblee et al. (1983) noted that Ladino clover benefited from infrequent compared with frequent cutting. The above results do not support the former, typically held view that frequent defoliation tends to offset the adverse effect of applied N on white clover by reducing the effects of shading by the grass (Chestnutt and Lowe, 1970; Whitehead, 1970). Increasing the defoliation interval can increase petiole length, leaf weight, stolon length, and stolon diameter (Wilman and Asiegbu, 1982b). Petiole elongation has also been noted by Wolton et al. ( 1970). Dennis and Woledge ( 1983)and Dennis et al. ( 1984) have observed that white clover can avoid the damaging effect of shade since petiole extension raises the developing lamina into bright light before its photosynthetic capacity is determined. Conversely to infrequent defoliation, severe and repeated defoliation, as with continuous and heavy sheep stocking, can result in reduced size of aboveground leaflets, petioles, and stolons (Briseiio de la Hoz and Wilman, 1981; Curll, 1982). It has been noted that relaxation of intense defoliation, for example, a silage cut in a grazing system (Wolton et al., 1970; Wilman and Asiegbu, 1982a;Curll and Wilkins, 1985) or a switch from high to low stockingdensity (Curll, 1982)or a change from continuous sheep stockingto rotational grazing (Marsh and Laidlaw, 1978; Laidlaw and McBratney, 1983; Newton et al., 1984), can result in increased aboveground parts of white clover and therefore an increase in vigor and production. Thus, while white clover is essentially a speciesfor grazing, it is more suited for conservation than previously thought. White clover has potential for greater persistence than the recognized “conservation” legumes, red clover and alfalfa. On account of its mode of growth (initiation of new petioles/ leaves), its primary growth and its regrowths have a higher level of digestibility than the “stemmier” conservation legumes (Harkess, 1963;Harkess and Alexander, 1969; Frame et al., 1983). Its disadvantage of a shorter growing season relative to grass is of less significance in a conservation context. If grass/white clover is to compete against grass plus fertilizer N systems in practice, it must be able to tolerate flexible grazing and cutting management.

58

J. FRAME AND P. NEWBOULD

In an era when rates of application of fertilizer N to grassland were much lower than now, Green and Cowling (1961) had suggested that the most effective exploitation of the N-fixing ability of white clover might be achieved by cut-forage feeding. Shaw et al. (1966) also concluded that the grass/white clover sward had considerable potential for cut-forage feeding and that there was little justification for the use of fertilizer N on grass/clover swards for cutting. Forage drying systems were suggested as a means of utilization by Lowe (1970). Pure-sown white clover gave satisfactory production and quality in specific silage cutting systems (Spedding and Diekmahns, 1972; Frame, 1982a, 1985a;Boyd and Frame, 1983;Castle and Watson, 1983; Frame et al., 1983) and with infrequent cutting regimes (Hughes, 1981; Reid, 1983a,b). Ingress of unsown weed species is a problem with pure-sown white clover swards, but weed grasses can be selectively suppressed by chemicals such as carbetamide (Wasmuth and Miles, 1972; Soper and Hutchinson, 1974; Reid, 1983a), propyzamide (Haggar and Bastion, 1980; Standell and Haggar, 1982), and paraquat (Blood, 1962). The potential ofgrass/white clover swards cut for silage has been shown in specific silage systems (Momson, 1981; Frame, 1982a, 1985a; Boyd and Frame, 1983; Frame et al., 1983) by the interposition of silage or hay into grazing or simulated grazing systems (Wolton et al., 1970; Chamblee et al., 1983; Curl1 and Wilkins, 1985; Frame, 1985a) and in fixed defoliation frequency work where infrequent cutting was among the treatments (Wolton et d.,1970;Frame, 1973;Orr and Laidlaw, 1978; Wilman and Asiegbu, 1982a). To ensure sustained white clover performance and adequate proportions in the harvested material, minimal fertilizer N should be applied (see Section VII1,C).

2. Closeness of Cutting Past work on grass/clover and grass swards generally showed that close cutting(- 25 - 50 mm from ground level) increased total herbage production considerably (up to 44%) compared with lax cutting (- 60 - 100 mm), provided adequate recovery periods were allowed between defoliations (Burger et al., 1958; Reid, 1959, 1962; Hunt and Wagner, 1963; Appadurai and Holmes, 1964; Frame, 1966a-c; Bryant and Blaser, 1968; Dobson et al., 1976).White clover production and proportion were either little affected by closeness or else were enhanced by close cutting. More recently, Boyd and Frame (1983) and Frame (1985a) obtained enhanced total herbage and clover performance from close (40 mm) relative to lax (80 mm) cutting from perennial ryegrass/white clover swards using white clover varieties with

AGRONOMY OF WHITE CLOVER

59

varying leaf sizes: the beneficial effect of close cutting on clover was most marked at high rates of applied N (240 and 360 kg/ha-year) and in later harvest years. Likewise, Brereton and Carton (1985) obtained improved total and clover production from a 50-mm cutting height compared with 75 mm. The stimulus to total herbage production from close cutting has been attributed largely to more efficient utilization, since a high proportion of a sward’s production is in the lower layers (Anslow, 1967). Close defoliation will also stimulate grass tillering and utilization of regrowth by removing floweringgrass shoots and by permitting better light conditions at the base of the sward. The stimulus to the clover production will include better utilization, while it may also benefit from reduced grass competitiveness. Better light conditions at the sward base will also encourage the number of clover growing points and promotion of photosynthetically efficient leaves (Woledge and Dennis, 1982).

E. EFFECTOF GRAZING The effects of grazing animals on pastures can be both harmful and beneficial. In addition to the defoliation, animals selectively graze, trample, and deposit dung and urine and their actions can distribute seeds (Watkin and Clements, 1978). The latter process, plus the marked heterogeneity arising from excretal return, helps to maintain species diversity in natural pastures but can make the management of sown grass/white clover pastures extremely difficult to regulate. Moreover, the species of animal can interact with one or several of the above processes, so adding to the complexity of studying the sward/animal interface. Few studies have tried to investigate the relative magnitude of the individual grazing effects on the proportion of white clover in a sward and on total herbage production. This follows because the effects occur simultaneously and are interdependent, and at any time one may be dominant; moreover, there are great practical difficulties in carrying out the type of research required under actual grazing conditions. Attempts to simulate some of the separate effects in cutting trials have not always been very successful. Many workers have shown that the proportion of white clover in a mixed sward is lowered by sheep grazing compared to cutting, especially when fertilizer N is applied (Table X).Cattle grazing depresses clover less than sheep grazing (Boswell, 1976; Briseiio de la Hoz and Wilman, I98 1), but more than cutting (Taylor et al., 1960; Bryant and Blaser, 1968;Garwood et al., 1982). In the assessment of existing clover varieties and potential varieties under

J. FRAME AND P. NEWBOULD

60

Table X Effect of Defoliation Method and Level of Nitrogen Fertilizer on Mean S e a ~ o White ~l Clover Content in Crass/White Clover Swards ~~

Clover content (%) ~~

N level

(kg N/ha per year)

Clover variety

0 I20

s 100

120 I20

S 100, Huia

0 68 I38 204

s 100

0 150 260 - 300

Huia, Blanca

Cutting

Sheep grazing

38 21

2I 5

24 11-19”

7 7

45 34 22 12

35 18

48 29 14

40 19 6

Reference Frame ( 1966a) Wolton et 01. (1970) Shaw et al. (1966)

LO 5

MacKenzie and Daly (1982)

Silage cut interposed once at various times in grazing regime.

a high fertilizer N regime, Evans (1 979) noted a mean 48% reduction of clover production in grass/clover swards with rotational sheep grazing compared with cutting. With a similar technique, but with no applied N, Evans and Williams (1982) found that clover growth rate in a grass/clover sward was reduced under grazing to the extent of a mean 50%reduction in annual clover DM. Since grass and clover respond differentially to defoliation, manipulation of grazing practices can effect botanical change markedly. 1. Treading

Edmond ( 1964)rated white clover as more susceptibleto treading damage than perennial or hybrid ryegrass but more resistant than timothy or cocksfoot. However, he also noted that clover was more tolerant of treading when grown in mixtures with grass than when grown in monoculture. Treading can directly damage plant parts, particularly under repeated treading in wet conditionswhich leads to poaching of the sward. Clover (and grass) can also be adversely affected by treading should it lead to soil compaction, soil puddling, lack of water infiltration, and reduced soil aeration (Brown and Evans, 1973). Curl1 (1980) assessed damage to marked plants in mixed ryegrass/white clover swards and recorded a 20% greater frequency of tread-

AGRONOMY OF WHITE CLOVER

61

ing on clover compared to grass. However, he also observed that defoliation occurred three times more frequently than treading. Curll and Wilkins (1983a,b), working with sheep set stocked at 25 and 50 wethers/ha on ryegrass/white clover swards, attempted to separate the effects of defoliation from defoliation and treading by use of “graze-through” cages. They observed that treading by sheep at 25 wethers/ha reduced the clover content of the sward only at the end of the grazing season, but the higher stocking rate had a greater depressive effect on clover. Curll (1982) concluded that the sensitivity of grass/clover pastures to treading by sheep was less than assessed for pastures by the short-term mob-stocked nongrazing methods used by Edmond (1964). Little information on the effects of treading by cattle on white clover has been reported. However, Monteath et al. (1 977) found that the top 7.6 cm of soil was more compacted under cattle than under sheep grazing; such treading effects may also indirectly affect the growth and persistence of white clover via effects on roots (F. X.de Montard, personal communication, 1985).

2. Selective Grazing Selectivity of grazing depends upon stocking density in relation to available herbage mass and sward growth rate. Sward species respond differentially to defoliation and have differing seasonal growth patterns. Type of animal influences selection and amount of utilization. Grazing management can thus effect botanical change by manipulation of frequency, intensity, and timing of grazing, and by changing the species of animal. White clover can survive selection during grazing by its plasticity, since it can develop a prostrate, small-leaved, short-petioled habit of growth (Curll, 1982). Animal species vary in their grazing behavior. Sheep graze more fastidiously than cattle and can readily select individual leaves and stems. Cattle tend to prehend bunches of foliage and will graze longer material. Sheep are said to graze white clover selectively in a grass/clover sward unless the stocking density is high enough to preclude selectiveness (e.g., Laidlaw and McBratney, 1983). Both Milne et al. ( 1982) and Curll et al. (1985) found that the proportion of clover in the diet consumed from a mixed grass/clover sward was higher than that in the total herbage on offer. However, Milne et al. ( 1982) and Woledge et al. (1984) noted that a higher proportion of clover than grass was found in the upper zones ofthe sward, the zone first grazed by sheep. Thus, a high proportion of clover in the diet may reflect the fact that grazing takes place mainly in the surface layers of the sward rather than active selection. If the depressive effect of sheep grazing on clover is not due

62

J. FRAME AND P. NEWBOULD

to selective grazing or clover’s accessibility, it follows that treading and pattern of excretal return must be implicated. There is much less information for cattle grazing than for sheep grazing. Briseiio de la Hoz and Wilman (198 1) studied the effects on clover of sheep grazing, cattle grazing, and cutting in a grass/clover sward by defoliation involving set stocking, frequent grazing, or cutting techniques aimed at keeping sward heights 40,60, or 80 mm above ground level. Sheep grazed clover preferentially and, relative to cattle grazing or cutting, the plant responded by reducing the amount of stolon, length of internodes, petioles, and leaflets; also stolons were thinner and closer to the ground. Increasing seventy of defoliation from 80 to 40 mm also reduced clover dimensions. The effect of cattle grazing on clover was similar to cutting. Following a respite from severe sheep grazing, the sizes of aboveground parts of white clover recovered to dimensions similar to those of clover which was cattlegrazed or cut; considerable phenotypic plasticity was thus exhibited. In farmlet studies in New Zealand (Monteath et al., 1977) clover comprised 13% of the sward under rotational sheep grazing and 16% under rotational beef cattle grazing. More recently, Morrison et al. (1986) compared the effects of continuous stocking by cattle over 2 years on swards maintained at 40,60, and 80 mm high. Clover survival was good in all treatments and there were only small effects on clover content. Purvis and Younger (1 984) obtained better clover performance under continuous stocking with young cattle than under monthly cutting over a range of fertilizer N rates from 0 to 300 kg/ha per year. Very few direct comparisons of the effects of sheep and cattle grazing have been carried out, but Calder ( 1970) noted that sheep grazed more severely than cattle and defoliated leaf portions of plants more than cattle. Sheep pastures produce more herbage dry matter than cattle pastures (Monteath et al., 1977). A recent study by Arosteguy et al. ( 1982)provided some explanation. Sheep and cattle grazed together gave tiller populations, rates of herbage growth, and net production values closer to swards grazed by sheep only than cattle only. Curl1( 1982)in a review of sheep grazing stated that changes in the grass and clover contents in response to grazing management relate to variation in the relative importance ofthe grazing processes and the ability of the clover or grass components to adapt their growth habit to survive these processes. The plasticity ofwhite clover grown in competition with perennial ryegrass has been demonstrated by Hill (1977) who found the expression of genotype, e g , petiole length, depended upon N source, the nature of the grass competitor, and, presumably, defoliation. Evidence has accumulated that continuous sheep stocking militates against white clover production and persistency in mixed swards compared

AGRONOMY OF WHITE CLOVER

63

with rotational grazing (Calder, 1970; Raguse er af., 1971; Laidlaw and McBratney, 1983; Newton et af.,1984; Curll et af., 1985). Removal of clover photosynthetic tissue is a factor at high stocking rates (Korte and Parsons, 1984) while selective defoliation (Leigh and Holgate, 1978;Stevens, 1978)and shading of stolon growing points by grass (Woledge and Dennis, 1982) may be factors at low stocking rates. Under rotational grazing, clover performance was better as the interval between grazings increased (Brock, 1971, 1974; Lancashire, 1974; Widdup and Turner, 1983), a finding which equates with cutting work (see Section VII1,D). Incorporating a rest interval from grazing and then taking a silage cut has been found to stimulate the clover component of swards continuously stocked or rotationally grazed by sheep (Wolton et al., 1970; Curll and Wilkins, 1985). In New Zealand, Hay and Baxter (1986) found that continuous stocking in spring increased the number of growing points compared with rotational grazing but the growth potential of those growing was realized better later in the season by rotational grazing rather than continuous stocking. Clearly, it is emerging that flexibility of grazing and cutting management, with the aim always offavoring the needs ofthe clover component, is the key to satisfactory clover performance in mixed swards. It has been shown recently (Batten, 1979; Clark et al., 1984b) that goats avoid white clover shoots when grazing and, ifgiven no alternative, prefer to eat white clover roots rather than shoots. Attempts to utilize this aversive grazing behavior as a way of removing the tall grasses and weeds from degraded, potentially clover-rich swards have been reported (Grant er al., 1984). It is worth noting that migrant wild geese preferentially select white clover (Owen, 1973;Owen er al., 1977).Preferential grazing, stolon removal, and scratching were given as possible reasons for white clover decline under poultry grazing (Cowlishaw, 1960).

3. Return of Excreta Excretal nutrient return vanes according to herbage intake and its composition. Excretal N operates against white clover in grass/clover swards by stimulating grass growth, thus increasing its competitiveness. Frame ( 1966a, 1976b) recorded increases in herbage production from rotational sheep grazing relative to cutting with equivalent defoliation regimes. The result was mainly attributed to recycled excreta N. White clover content and production were markedly reduced under grazing because of the boost to grass by excretal N and, to a lesser extent, because of the depressive effect of treading and selective defoliation on white clover. The results accord with

64

J. FRAME AND P. NEWBOULD

Shaw et al. ( 1966), Wolton et al. (1970), and MacKenzie and Daly (1 982) (see Table X). Laidlaw and McBratney ( 1 983) noted a reduction in clover leaf size from continuous high sheep stocking, but concluded that the effect could be partially alleviated by rotational grazing which would improve the ability of clover to compete with increased grass growth resulting from excretal N. Excreta can also influence the botanical composition of pasture by altering nutrient concentration in the soil, by “burning” plants, and by influencing grazing patterns. Urine and feces together and urine alone increase the proportion of grass in grass/clover swards (Ball ef al., 1979), while feces may give a boost to white clover (Weeda, 1967). The K in urine may also stimulate white clover (Scott, 1976). While the excretal return pattern from sheep grazing cannot be described as uniform, it is more evenly distributed than from cattle grazing (Monteath et al., 1977). Reduction in clover growth by excreta may not be simply due to N increasing the competitive ability ofgrass for light, moisture, and nutrients, but it may reflect the greater sensitivity of clover to urine bum, to the presence of free NH3 (Curll, 1982) and the temporarily increased pH of urine-affected soil (Thomas et al., 1985). To summarize, it appears that sheep grazing militates against clover whether by preferential grazing or by the net effect of trampling and excretal N, and this can lead to a reduction in the clover content of a sward. Cattle grazing is more equivalent to cutting than to sheep grazing. Clover is more susceptible to treading than grass. The return of excreta encourages grass growth which can cause suppression of clover. The composition of mixed grass/clover swards is sensitive to grazing management (Watkin and Clements, 1978) and to the ability of the particular grass or clover varieties to adapt their growth habits. There is a general consensus that rotational grazing encourages clover performance more than continuous stocking; it relieves clover from selective defoliation and allows the plant to elongate petioles and place leaves in the top of the canopy (Betts et al., 1978; Curll, 1982; Frame and Newbould, 1984; Newton et al., 1985).

F. RECOVERY MANAGEMENT Recent surveys of pastures in the United Kingdom (Forbes et al., 1980; Swift et al., 1983; McAdam, 1983a,b) confirm that, though sown originally with white clover, many now lack sufficient white clover to contribute significantly to herbage quality (see Section VI) or to fix N. Full reseeding is possible but expensive, and agronomists are often asked how the proportion of white clover can be restored by less expensive treatments. A range of methods including soil amelioration (lime and nutrients), change of defolia-

AGRONOMY OF WHITE CLOVER

65

tion practice, manipulation with herbicides, and slot seeding or oversowing is available.

I . Soil Amelioration The disappearance of white clover from pastures in many upland wetter regions of the United Kingdom is hastened by increases in soil acidity and a decline in the amount of available P and K. The ideal soil conditions described in Section III,B must be restored if white clover is to increase in the sward. In the United Kingdom this usually means applying 2 - 5 tons lime/ ha depending on the soil type and 20-40 kg P plus 40-80 kg K/ha (Newbould, 1986).Reduction in the total quantity of fertilizer N and/or alteration of the seasonal pattern of application may also help to reverse the decline in white clover content (see Section VII1,C).

2. Change of Defoliation Practice Relaxation of grazing pressure, change of animal species, the use of conservation cuts, and alternation of rest periods and grazing can often restore white clover to a sward (Frame and Newbould, 1984).The impact of change of animal species is shown by the use of goats, which eat tall grasses and weeds but ignore white clover (Clark et al., 1984b), so helping to restore white clover to a sward (see Section VII1,E). 3. Manipulation with Herbicide In the established grass/clover sward, invasion by perennial broad-leaved or grass weeds can lead to sward deterioration and low production from the sown species (Dibb and Haggar, 1979).Herbicide options to control certain problem weeds adequately have increased recently, but a penalty to clover of plant scorch/suppression or even death has had to be accepted (see Section IV,F). Haggar and Squires (1979) reviewed the history of manipulating the sown-species composition of grassland by herbicides. The important point was that the suppression of unwanted speciesset the stage for colonizationby existing sown species, but the success of this depended upon (1) an adequate presence and distribution of the species intended to be encouraged and (2) management, especially fertilizer use and stocking density to ensure the induced botanical change persisted. Stoloniferous white clover offers advantages as a colonizer if there is a

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sufficient nucleus of material and conditions are suitable. Hagger and Bastian ( 1980) evaluated a range of grass-suppressing herbicides (carbetamide, propyzamide, paraquat, asulam, glyphosate, and dalapon) to which clover is relatively tolerant. Their use in late winter led to a marked increase in white clover content subsequently. Initial loss of spring grass growth was high but there was compensation with extra growth in midsummer, so annual total production was unaffected. Standell and Haggar (1982) then assessed the effect of autumn application of carbetamide, paraquat, and propyzamide on a clover-depleted grass sward. All encouraged clover growth the following year without unduly reducing grass growth. Propyzamide was the best in terms of clover increase and avoidance of grass suppression. A method to increase clover content in mixed swards by spraying with mefluidide, a plant growth regulator which inhibits seed-head formation in temperate grasses (Glenn et al., 1980), is under investigation in New Zealand (Leonard et al., 1985) and the United Kingdom (Haggar et al., 1985b). The latter workers found that although the chemical caused a short-term check in the growth of white clover through a shortening of the stolons, it provided opportunity for increased colonization by “flattening” the seasonal growth peaks of the companion grass. Development work is required to find out how the use of grass suppressants can be integrated with animal production systems. Since some of the chemicals are expensive there is also a need to assess the costbenefit of these procedures against the cost of a full replacement by reseeding of both grass and clover. 4. Oversowing/Slot Seeding of White Clover In cases where few or no propagules of white clover are present but where good quality productive grasses still remain and where soil and climatic conditions are favorable to the growth of white clover, it is theoretically attractive to consider the introduction of white clover seeds. Various methods of minimal cultivation, with and without chemical suppression of grasses, have been devised to introduce clover into existing swards but consistent success has not been achieved (Tiley and Frame, 1980;Tiley, 1981b). The use of coated or pelleted seed has proved successful in New Zealand where grass swards are open and soils are low in organic matter (Lowther, 1978, 1980), but attempts to apply similar methods in the United Kingdom have not been successful. The presence of a very closed competitive sward, together with a spongy organic mat, and often the need for more lime than can be added in a thin coat or small pellet are thought to be reasons for the poor showing of these methods in the United Kingdom (Newbould, 1974, 1975).

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A number of machines have been devised to introduce white clover (and sometimesclover and grass) into existing grassy swards (Squireset al., 1979; Charles and Daniel, 1983; Sheppard et ul., 1985). In some cases (Charlesand Daniel, 1983) this has been successful, but in others only small percentages of clover have become established 2 or 3 years after drilling (Newbould, 1986). Problems arise due to the slowness of the clover in spreading from the slot; marked selection of the young clover plants takes place by sheep when grazing and from slugs and insects present in the existing sward (Naylor et al., 1983). In hill land with acid grassland (Agrostis/Festucu) where the introduction of white clover afier soil amelioration would be desirable, access and the presence of the organic mat limit the usefulness of the slot-seeding machines (Tiley, 198 1 b).

IX. POTENTIAL PRODUCTION It is of theoretical interest to consider the levels of production that might ideally be possible from either pure white clover or mixed grass/white clover swards. However, because of the many factors reviewed in this article, and variable individual farmer management, which influence the production and utilization of grazed swards, it is extremely unlikely that the attainment of any predicted potential set by climate will ever be approached in practice. Several workers have attempted to calculate the theoretical potential of grass production but few, with the possible exception of Frame and Newbould (1984), have tried to do the same for white clover. This is partly because the plant has to be grown in mixture with grass and partly because it is mainly used in the grazed sward with all the uncertaintiesthat can occur in this situation (see Sections VII and VII1,E). It is also because both the composition of mixed grass/clover swards and the performance of white clover are so markedly influenced by the supply of available N in the soil, whether from mineralization of organic matter, nitrogen fixation or from fertilizer N additions. Despite the uncertaintiesinherent in the calculationsit is relevant to consider what levels of production are theoretically possible in pure stands of white clover and in grass/white clover mixtures. A. WHITECLOVER MONOCULTURES As previously described,pure stands of white clover are not used in practical agriculture because of the difficulty of maintaining them weed free, the

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relatively low level of DM production, and practical difficulties in their utilization, especially by cattle (e.g., bloat). Calculations of the potential harvestable production from monoculture grass swards in Europe give values in the range of 20 - 30 metric tons DM/ha (Alberda, 197 1;Cooper and Breese, 197 1 ;Leafe, 1978); for monoculture white clover without added N, calculations suggest an annual herbage production of 16 tons DM/ha (Frame and Newbould, 1984). The highest recorded production from an irrigated monoculture white clover was 11.8 tons DM/ha (Spedding and Diekmahns, 1972).

B. GRASS/WHITE CLOVER MIXTURES For United Kingdom conditions, Frame and Newbould (1984) calculated that, given a 50 :50 mixture ofgrass and clover which each performed to half its theoretical monoculture potential, and with suitable utilization and fertilizer N regimes, total herbage of 18.5-22.5 tons DM/ha per year should be possible. Thus, a grass/clover sward with no applied N and with a soil-mineralizing 100 kg N (Hobson and Richards, 1978) would give 18 - 22 tons DM/ha per year. For New Zealand, where the growing season and soil conditions are said to be ideal for white clover, Smetham (1973) has described a production ceiling set by genotype and climate for a mixed perennial ryegrass/white clover sward at 22-28 compared with 10 tons DM/ha per year for white clover monocultures. However, calculations by Brougham ( 1959a) predicted annual herbage production from irrigated ryegrass/clover swards of 24.7 tons DM/ha; assumingmaximum white clover proportions in New Zealand pastures of 68% (Harrisand Thomas, 1973) the highest annual production of clover would be in the order of 16.8 tons/ha. This is surprisingly close to the calculated theoretical potential of white clover for the United Kingdom. The highest recorded production from a cut, perennial ryegrass (cv. Premo)/white clover (cv. Milkanova) sward for a season in the United Kingdom came from Cambridge,England, and was 15.53 tons DM/ha with white clover contributing 12.12 tons of the total herbage (i.e., 78%) (Table VIII, and Section VII1,B). Nitrogen was not applied and the sward was cut six times. With 200 kg N/ha the total production was 19.94 tons DM/ha per year but white clover contributed only 2.19 tons DM ( 1 1%) of the total. The highest annual pasture production levels achieved in New Zealand under grazing and with no applied N ranged from 16.2 to 22.8 tons DM/ha, depending upon site conditions (Brougham, 1977).

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X. MANAGEMENT GUIDELINES The preparation of a single blueprint for the exploitation of white clover applicable to all situations is not possible. White clover is grown in a wide variety of soils and climatic conditions, and is utilized for a range oflivestock enterprises. As is evident from the above, a host of interacting factors and relatimships can influence clover’s performance and persistence, and as yet they are not all fully understood. Nevertheless, not surprisingly, a number of authors have given sets of management guidelines on how best to exploit white clover in the mixed sward (Chestnutt and Lowe, 1970; Leffel and Gibson, 1973; Frame, 1982a; Anonymous, 1983b; Frame and Newbould, 1984; Tedstone, 1984; Stewart, 1985; Van Keuren and Hoveland, 1985). A single dominant guideline in any situation is to seek and remedy the limiting factor(s). However, the development of a well-distributed amount of white clover, inoculated with rhizobia in the stand‘s early life, clearly must be an essential target, implying attention to the initial stages of establishment and subsequent management. As a general rule, it can be stated that the use of fertilizer N on the grass/clover sward must be minimal. Any addition of N to the soil pool contributes to “loading the scales” against clover in the mixed sward. The key to utilization is flexibility of management system, so that periods of rest for clover to recover its vigor are integral parts of any system. In summary, at all stages in the management of the grass/clover stand, the needs of the white clover component should be paramount. Key guidelines are summarized in Table XI. Table XI Management Guidelines for White Clover Seeds Establishment

Production

Utilization

Choose compatible companion grasses Sow 3-4 kg clover seed/ha in seed mixture Use a blend of white clover leaf types Sow shallow (10- 15 mm) Soil pH a minimum of 5.5 Adequate basic soil fertility Preferable to sow directly in spring Make minimal strategic use of fertilizer N Maintain soil pH and P and K status Choose and use herbicides carefully Imgation is advantageous in dry areas Avoid continuous severe grazing Provide clover recovery periods, e.g., use rotational grazing, insert a silage cut in a grazing system Always favor the needs of white clover

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XI. CONCLUSIONS Most reviewers predict that sooner or later world agriculture will have to expand the use of legumes, and this applies particularly to forage legumes such as white clover in temperate regions ofthe world. A role for white clover is also foreseen in the subtropics and possibly the tropics, provided the necessary research is undertaken to widen its climaticand edaphic adaptability (Brougham et al., 1978).A major eulogistic reason for the above predictions was cited by Gladstones (1975): “Legumes make possible an ecologically sound nonexploitive and yet productive agricultural system, with which a hopefully stabilized (human) population can live in permanent balance or better.” Study of the relative role of legumes in the N economies of three temperate countries where grazing livestock are used indicates that they dominate in New Zealand, are little used in the Netherlands, and that low to moderate reliance is placed on them in the United Kingdom (Table XII). The types of livestock enterprise, suitability of the land for arable or forage cropping, and economic evaluations largely determine the national fertilizer N strategy. Taking the situation in the United Kingdom as an example, little reliance is placed on the use of white clover in lowland dairy systems;the main source of N is fertilizer N at annual rates up to 400 kg/ha. However, the average annual use of fertilizer N on United Kingdom grassland, excluding rough grazing, is now 105 kg/ha (Jollans, 1985);usage has risen steadily in recent years. In upland and marginal farming regions, both white clover and fertilizer N are used for beef cow and cross-bred sheep systems. The fertilizer N is used strategically mainly in spring and autumn and with annual amounts up to 150 kg/ha. In the more remote hill sheep farms where only small percentages of the total land area are upgraded to sown grass/white clover swards, economics determine that fertilizer N is used only in the establishment of

-

Table XI1 The Relative Sources of Nitrogen in the Agriculture of Three Contrasted Countries‘ New Zealand

Source Legumes Free living microorganisms Fertilizer Total N (million tons) in fertilizers (FAO, 1983)

United Kingdom

94 5 1

0.02

~

After Nutman (1971); Food and Agriculture Organization (1983).

25 2 73 1.4

Netherlands

5 1

94 0.5

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sown swards or in small amounts (20-40 kg/ha) to promote early growth in subsequent springs; white clover is regarded as the main source of N. As indicated previously the white clover content in pastures in the United Kingdom is usually low (e.g., Forbes et al., 1980),and this is despite the use of about 1000 tons of clover seed per year. The use of fertilizer N by British farmers undoubtedly hastens the demise of white clover in swards. In the summary of a recent symposium on forage legumes in the United Kingdom, Wilkinson (1984) gave good reasons why legumes were not exploited: “It is easier to grow grass than legumes and with grass/white clover swards the contribution made by clover can vary markedly from year to year. Bloat, oestrogens, diseases, lack of seedling vigour, combine to make legumes less flexible than other forage crops.” The role of pasture legumes may lie in the early history of pasture development (Greenhalgh, 1981; Newbould, 1982). Once a threshold quantity of organic matter has been built up which mineralizes sufficient nitrogen to support moderate levels ofgrass growth, it is extremely difficult to sustain the presence of white clover (Section VII). The farmer should then concentrate on the use of high-yielding grasses using additional fertilizer N, while this is available and its use is economically justified. This strategy might have to change should supplies of fertilizer N become limited and expensive because of shortage of fossil fuel energy. However, it is not outside the bounds of possibility that alternative energy sources to fossil fuel energy may be developed in the future which will ensure the continued manufacture of nitrogenous fertilizer. There is not a great deal of scope left to reduce further the amount of total energy required to manufacture fertilizer N (Jollans, 1985). Our review of the literature on white clover has caused us to question if white clover is the perfect pasture legume, if perennial ryegrass, and indeed other typically used cool-season perennial grasses (Van Keuren and Hoveland, 1985), are the ideal companions for it, and if the present agronomic practice ofgrowing and grazing mixed pastures ofgrass and clover is the most sensible (see summary in Table XIII). Ifthe attainment ofat least 30%white clover DM in total herbage for the growing season is a desideratum (Martin, 1960; Wright, 1975; Curll, 1982; Stewart, 1984), then most published evidence suggests that this clover content cannot be achieved or sustained consistently with the present varieties of clover and most of the typical companion grasses, and with present systems of grazing management. Plant demographic studies (Turkington and Harper, 1979a; Bradshaw et al., 1982)suggest that plants have preferred niches so that iftwo plants are to share the same space they must have complementary light, spatial, and nutrient requirements, or share these in a dynamically balanced temporal manner. The evidence reviewed briefly in Section VII indicates that on most of these grounds clover and its usual companion (perennial ryegrass) in the

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J. FRAME AND P. NEWBOULD Table XI11 Balance Sheet for and against White Clover Credit

Debit

Persistent under infrequent cutting

Not competitiveto perennial ryegrass(and many other grasses) Adversely affected by fertilizer N and urinary N Needs “good” climate: warmth, wetness Needs moderate soil fertility Within and between year variation in performance Bloat (mainly cattle)

Moderately persistent under grazing High nutritional value Fixes nitrogen Builds up soil fertility Enhanced animal performance Environmentally acceptable

United Kingdom and New Zealand are in direct competition. Moreover, if white clover is stressed by climatic (cold, drought), nutritional (pH, low P and K, high N), pest/disease, or management (sustained close grazing) factors, the likelihood is that it will be replaced by weed grasses or other weeds (Wright, 1975). Published literature includes examples of inexplicable and sudden white clover decline in both cutting and grazing trials in spite ofgood experimental technique and management (Garwood and Tyson, 1979; Davies, 1982; Stewart and Haycock, 1983, 1984; Frame and Boyd, 1984; Brereton and Carton, 1985). Clearly, marked production variability and clover inconsistency are even more likely in farming practice where a complex of factors, including soil, sward, animal, and management, many of them interactive, operate. Hence, in countries such as the Netherlands where fertilizer N is obtainable and its use is economically viable, farmers opt for animal production systems based on more easily and flexibly achieved herbage production from grass swards fertilized with mineral N. The management guidelines described in Section X stress that at all times agronomy and management of mixed grass/clover swards should favor white clover, but with few exceptions, the proportions of white clover found in farm pastures (Forbes et al., 1980; Swift et al., 1983;McAdam, 1983a)fall well short of the target needed to optimize its impact on animal nutrition or nitrogen fixation. Even in New Zealand, a country which relies almost entirely on white clover for its soil N supply, it is unusual to find in practice more than about 10-20% of the season’s total herbage production as white clover (D. A. Clark, personal communication). Indeed, Brougham ( 1977) stated that white clover made up 2 - 35% of the total annual herbage production, depending on the environment and region being assessed. Values of 20- 30% are typical in research station trials (Hoglund et al., 1979). In spite of the burgeoning volume of research on white clover in the last 50 years,

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Brougham et al. ( 1978) still concluded that more intensive research efforts are needed into all aspects of white clover’s use in temperate agriculture. To overcome all the uncertainties associated with the growth and utilization of white clover, increased research into its basic physiology, especially in association with grass, is required. There is inadequate information on growth analysis or on the seasonal behavior of the individual white clover plants; such information would help an understanding of its optimal growth requirements and management needs. While its potential role as a conservation legume should not be ignored, white clover is essentially a grazing legume; hence the greater proportion of experimentation should be under grazing conditions. Ideally, this should commence as early as the varietal evaluation stage. Thereafter, experiments, preferably long-term, should test varieties available in commerce rather than those for which little seed is available. Nitrogen from organic manures such as livestock slurry apparently inhibits white clover in mixed swards to a lesser degree than mineral N; this requires further investigation. Such studies also need extension to other forms of N, for example, slow-releaseN. Indeed, the total N cycle in grazed pastures requires study with long-needed emphasis on the N accumulation and release mechanisms in the soil. This knowledge is required particularly in countries such as those in southern Latin America which are almost totally reliant on a legume N economy (Frame, 1982b). Moreover, in his review on the flow of nitrogen in grassland, Ryden (1984) concluded that “It is important that the efficiency of the flow of N in fertilizer-based systems is compared with that in systems based on grass/clover swards.” Ryden also suggested “that the flow of N in grazed grass/clover swards is more precisely regulated than in ryegrass swards receiving fertilizer N.” If so, the grass/clover sward would be environmentally “safer” than the N-fertilized grass sward from which considerable N leaching can take place. In any further work on white clover and the N cycle there is a need to standardize methods of conducting grazing experiments among countries and among research groups within countries. There is also a need to define clearly what researchers mean by the proportion of white clover in a mixed sward; a wide range of methods, times of measurement, and methods of expression were evident in the literature. In our opinion, clover should be assessed in herbage production terms rather than ground cover estimates. Most importantly, all experimental work should be multidisciplinary (Le., a task force approach). All too often in the past, the single-disciplineexperiments performed have proved inadequate. More practical livestock systems work is needed and it should undergo simultaneous economic measurement and analysis. Ideally, seasonal target levels for clover contribution to differing animal production systems need quantification from both the viewpoints of animal

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nutrition and N fixation. Experimental work with white clover should be of sufficient duration to cover the inevitable grass/clover balance changes which occur insidiously, or suddenly. In the long term, other possibilities must be considered. Can white clovers be found with good growth in spring, an attribute not possessed by present varieties? Can more productive and persistent varieties of white clover or other pasture legume species be found or bred? Can low-fertility-tolerant varieties be developed?Admittedly, the history of breeding in forage legumes is short by comparison with the effort put into cereals. Can companion grass species be found that are as productive and nutritious as perennial ryegrass but are less aggressive to white clover? Somatic cell fusion techniques (Dale et al., 1984)might hasten the discovery of new legume plants which are easy to establish, high yielding, disease resistant (and therefore persistent), and bloat free. The use of coadaptation philosophies (Sections IV,E and VII) to match more closely legume and grass to give stable and productive mixed pastures with a high proportion of clover merits further high-priority research. The extension of this principle to encompass selection of appropriate microorganisms may also be advantageous. The use of farm systems with sole reliance on mixed grass/clover pastures warrants experimental consideration and evaluation, with complementary use of other legumes such as red clover or alfalfa (lucerne) where suitable. The use of some form of ‘buffer’ supplement (e.g., silage, hay, concentrates) at grazing, to even out feed supply at times when herbage production from grass/white clover is limited, merits investigation. Again various ways could be devised to use areas of monoculture white clover with areas of grass fertilized with mineral N. Both plants could then be grown in the manner that suits them best. The grazing of grass and red clover in separate but contiguous blocks by dairy cows was moderately successful (Greenhalgh, 1975). It is also possible that the principles of the two-pasture approach applied so successfullyto hill land in the United Kingdom (Eadie et al., 1979; Armstrong ef al., 1986) could be applied to lowland pastoral farms. One or two fields well suited to the growth of forage legumes could be scheduled for mixed swards while the remainder would be farmed conventionally with reliance on grass and the application of fertilizer N. However, little research has been instigated on this concept. Although the strategies just described might be described by some as pessimistic, it appears to us as eminently realistic. Efforts should also be turned to other “lateral thinking” strategies, some of which have been described by Greenhalgh (1982). Why not regard white clover rather than grass as the continuing element in a mixed sward and insert short-term grasses such as Italian ryegrass when required? If such a grass grows tall it can be mechanically harvested at a height which will enable the underlying

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clover-rich stubble to thrive and be available for utilization by grazing livestock. Clearly animal nutritionists should try to understand further why clover herbage is so nutritious to the ruminant. This knowledge could then be used to select and breed grasses so that they are digested and metabolized by livestock as efficiently as legumes. The growth of such grasses, possibly in rotation with white clover grown as a nitrogen-fixing monoculture, might avoid the complexities of trying to maintain clover in mixed swards on relatively fertile soils, a situation to which it is clearly not adapted. The technology of using pasture legumes should continue to be developed for use in improving existing grasslands, and for use in rotations on fertile arable land should fertilizer N become unavailable or too expensive. Such research would seem especially warranted in view of the fact that uniform land of good quality is of limited supply in the world and yet plays a crucial role in agriculture. Pasture legumes could also be used extensively as amenity plants to maintain land in reasonable fertility, and especially land which has to be taken out of agriculture because of surplus food production. Additionally, in many parts of the developing world, where land is at the earliest stage of plant succession, the role of legumes in improving soil fertility remains of vital importance. Despite the many difficulties outlined in this review, grass/white clover swards make an enormous contribution to temperate pastoral agriculture. They have the potential to produce large amounts ( 18 - 25 tons DM/ha per year) of high-quality feed for livestock and to fix up to 250 kg N/ha per year. Sadly, this potential is rarely approached. Challenging research is needed and is justified to solve the paradoxes of the agronomy of white clover.

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Westbrooks, F. E., and Tesar, M. B. 1955. Agron. J. 47,403-410. Wheeler, W. A. 1950. “Forage and Pasture Crops.” Van Nostrand-Reinhold, Princeton, New Jersey. White, J. H., and French, N. 1968. J. Br. Grassl. Soc. 23, 326-329. Whitehead, D. C. 1966. “Nutrient Minerals in Grassland Herbage.” Commonw. Bur. Pastures Field Crops, Hurley, England. Whitehead, D. C. 1970. “The Role of Nitrogen in Grassland Productivity.” Commonw. Bur. Pastures Field Crops, Hurley, England. Whitehead, D. C. 1982. J. Sci. FoodAgric. 33, 1227- 1235. Whytock, G. P., and Frame, J. 1985. Tech. Note- West Scotl. Agric. Coll. 245. Widdup, K. H., and Turner, J. D. 1983. N. Z. J. Exp. Agric. 11,27-31. Wilkinson, J. M. 1984. Occas. Symp. Br. Grassl. SOC.16,230-234. Williams, R. D. 1984. “Crop Protection Handbook-Grass and Clover Swards.” Br. Counc. Crop Prot., Croydon, England. Williams, W. 1970. Occas. Symp. Br. Grassl. SOC.6, 1-10. Williams, W. M. 1972. N. Z . J. Agric. Res. 15,363-370. Williams, W. M., and Barclay, P. C. 1972. N. Z. J. Agric. Res. 15, 356-362. Williams, W. M., and Hoglund, J. H. 1978. N. Z. J. Agric. Res. 21,491 -497. Williams, W. M., Lambert, M. G., and Caradus, J. R. 1982. Proc. N. Z. Grassl. Assoc. 43, 188- 195. Willis, C. B., and Thompson, L. S. 1969. Can. J. Plant Sci. 49,505-509. Wilman, D., and Asiegbu, J. E. 1982a. Grass Forage Sci. 37, 1 - 13. Wilman, D., and Asiegbu, J. E. 3982b. Grass Forage Sci. 37, 15-27. Wilman, D., and Hollington, P. A. 1985. J. Agric. Sci. 104,453-467. Wilson, E. R. L. 1978. N. Z. J. Agric. Res. 21,725-726. Witty, J. 1983. Soil Biol. Biochem. 15, 631 -639. Woledge, J. 1978. Ann. Bot. (London) [N.S.] 42, 1085- 1089. Woledge, J., and Dennis, W. D. 1982. Occas. Symp. Br. Grassl. Soc. 14, 3 1 1 - 3 12. Woledge, J., and Suarez, A. C. 1983. Ann. Bot. (London) [N.S.] 52,239-245. Woledge, J., Dennis, W. D., and Davidson, I. A. 1984. Adv. Photsynth. Res. 4, 149- 152. Wolton, K. M., and Brockman, J. S. 1970. J. Br. Grussl. SOC.25, 7- 19. Wolton, K. M., Brockman, J. S., and Shaw, P. G. 1970. J. Br. Grassl. SOC.25, 113- 118. Wong, E., and Latch, G. C. M. 1971. N. Z. J. Agric. Res. 14,633-638. Wood, M., and Cooper, J. E. 1985. Soil Use Manage. 1.66-69. Wood, M., Cooper, J. E., and Campbell, D. S . 1985. J. Soil Sci. 36,357-365. Wright, C. E. 1975. Rep. 1974-197s. Agric. Res. Inst. N. Ire., pp. 13-23. Wright, C. E., and Faulkner, J. S. 1977. Rec. Agric. Res. 25, 25 - 3 I . Yeates, G. W., Healy, W. B., and Widdowson, J. P. 1973. N. Z. J. Agric. Res. 15,81-86. Yeates, G. W., Crouchley, G. C., and Witchalls, J. T. 1975. N. Z. J. Agric. Rex 18, 149- 153. Young, D. J. B. 1958. J. Br. G r a d . Soc. 13, 106- 114. Young, N. R. 1984. Occas. Symp. Br. Grassl. Soc. 16,201-202. Young, N. R., and Mytton, L. R. 1983. GrassForageSci. 38, 13-19. Younie, D., and Black, J. S . 1979. North. Scotl. Coll. Agric., Bull. 16. Younie, D., Wilson, J. F., Carr, G., and Watt, C. W. 1984. Occas. Symp. Br. Grassl. SOC.16, 182- 183. Zaleski, A. I96 I. J. Agric. Sci. 57, 199 -2 12. Zohary, M. 1972. Bot. Not. 125,501-511.

ADVANCES IN AGRONOMY, VOL. 40

AGRONOMIC VALUE OF UNACIDULATED AND PARTIALLY ACIDULATED PHOSPHATE ROCKS INDIGENOUS TO THE TROPICS L. L. Hammond, S.H. Chien, and A. U. Mokwunye Agro-Economic Division, International Fertilizer Development Center Muscle Shoals, Alabama 35662

I. INTRODUCTION Considerable research has been conducted in recent years in search of alternative means of supplying phosphorus (P) for crop production in tropical soils. The motive for this effort has been not so much the agronomic ineffectivenessof the conventional high-solubility P fertilizers as the cost of these traditional commercial fertilizers. The primary component of the research has focused on the use oflow-cost indigenous material such as locally availablephosphate rock (PR) deposits, farmyard manure, or plant residues. The target groups for use of the alternative fertilizers are primarily the resource-poor farmers of subsistence and frontier-type agriculture. It is recognized that the greatest market for the conventional fertilizers is among the more modem commercial farmers who produce primarily cash crops such as sugar, cotton, soybeans, potatoes, coffee, fruits, or vegetables, and that the alternativesources may be most appropriate as complementsto the conventional sources to fill the needs of a specific user group with limited capital available for the purchase of production inputs. This review will focus on the use of indigenous PR deposits located in the tropics. Evaluation of these deposits was intensified around the 1974- 1975 period when the price ofphosphate fertilizers sharply increased on the world market and consumersin developing tropical countries began to realize that, although the PR essential for P fertilizer production was present in those countries, information was not available regarding means by which the resource could most appropriately be used to alleviate their dependence on foreign suppliers. The PR resources located in tropical regions had not been developed for a variety of reasons. Among the most important reasons is that many are considered “problem ores”: the chemical properties are not suit89 copyriebt 8 1986 by Academic h5.5, Inc.

AU rights of reproduction in any form m e n d .

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L. L. HAMMOND ET AL.

able for production of well-known fertilizer products using conventional process technology and many of the individual deposits are too small to warrant the investment required for conventional technology. The alternatives that have received the greatest attention include (1) use of the indigenous PR in a finely ground form for direct application to soil, (2) development of granulation techniques to improve the physical properties of unacidulated PR while minimizing a detrimental effect on agronomic performance, i.e., minigranulation, (3) mixing of finely ground PR with organic materials, sulfur, soluble P fertilizers, or other products capable of increasing the availability of P from the PR, (4) partial acidulation of the PR (PAPR) with available acids to increase the water and citrate solubility of the P in the PR while reducing the cost of the product relative to fully acidulated fertilizers, and (5) cogranulation of the PR with soluble P fertilizersto achieve a product similar to partially acidulated phosphate rock (PAPR) (i.e., extended superphosphate). Of the alternatives listed, the use of PR for direct application is one of the most appealing to developing countries because the low input of capital and energy required to prepare the product makes it one of the cheapest mineral P fertilizers possible. It is well known, however, that the agronomiceffectiveness of PR is highly variable and frequently lower than that of soluble P fertilizer. Several excellent reviews published earlier describethe factors that influence the agronomic effectiveness of PR (Lipman ef al., 1916; Rogers ef al., 1953; Russell, 1973; Khasawneh and Doll, 1978). These reviews have concentrated on the studies done in the temperate regions. It was generally concluded in each of these reviews that-paraphrasing both Russell (1973) and Khasawneh and Doll (1978)-PR is not suitable for use in intensive agricultural production because PR cannot maintain a sufficiently high concentration of P in the soil solution for high yields of crops with high P requirements but that finely ground PR may be a suitable source of P for many acid soils in tropical agricultural systemswhere very high yields are not sought and sulfur is not deficient, because it is the cheapest source of P that can be produced from PR. The purpose of this review is to evaluatethe degree to which PR and PAPR have, in fact, been observed to be suitable for use in tropical agriculture.

II. INDIGENOUS PHOSPHATE DEPOSITS IN THE TROPICS Phosphate rock is mined in many countries, but by far the greatest quantities available in the world market are produced in Morocco and the United States. The figures in Table I illustrate that 57.5% of the PR exports origi-

PHOSPHATE ROCKS IN THE TROPICS

91

Table I

Exports of Phosphate Rock by Country in 1983" Country of origin Morocco United States

U.S.S.R.

Jordan Togo Israel Nauru

s=d

Tunisia Christmas Island All others Total

Percentage of total exports 29.6 21.9 10.4 7.8 4.4 4.1 3.6 2.1

2.3 2.3 4.9 100.0

a From the Food and Agriculture Organization ( 1984).

nated from these two countries in 1983. Togo and Senegal, which lie within the region discussed in this review, accounted for 4.4 and 2.7%,respectively, of the PR exports. Other than those listed, all other countries combined accounted for only 4.9%of the exports. While the countries listed in Table I produce PR on a relatively large scale, the alternative P sources which are to be given attention in this review range from those that only show potential for commercial development to those that have only recently become available for internal use within the country of origin. Many of the deposits have not been exploited in the past because of their small size or inaccessibility; thus, their proximity to appropriate agricultural conditions an important factor. Some of these deposits located within the tropics are listed in Table 11. As will be shown in subsequent sections, these deposits exhibit a wide range in chemical composition, solubility, and, accordingly, potential for agronomic use. The economic potential of a deposit will depend on many factors, including market price or value, extent and grade of the deposit, location, cost of mining and beneficiation, quality and grade of the beneficiated product, and environmental factors [InternationalFertilizer Develop ment Center (IFDC), 19791. Also, when a deposit is to be mined for domestic use, the potential is related to the alternative of using imported ore or imported fertilizer. One country that has taken steps to reduce imports is Brazil.

92

L. L. HAMMOND ET AL. Table I1 Some Phosphate Rock Deposits Located in Countries with Agricultural Lands Lying within the Tropics"

~~

Country Africa Angola Burkina Faso Liberia Mali Mauritania Niger Senegal Tanzania Togo Uganda Zaire Zambia Asia Christmas Island India Paracel Island Philippines Sri Lanka Vietnam Latin America Aruba Bolivia Brazil

Colombia

Location Cabinda Kodjari Bomi Hills Tilemsi Valley Parc W Tahoua Taiba Thies Minjingu Hahotoe, Kpogame Sukulu Matadi Kaluwe. Nkomba Udaipur, Jaisalman Mussoorie Jhabua, Singhbhum, etc. Negros Island Eppawala Lao Cay

Total reserves and resources (mTX lo6)

I20 I500 1.5 20 5 100

-

1 100 2090 10 300 200 83 400

200 70 18 52 20 0.5

300 500

10

Capinota Anitapolis Araxa Catalao Ipanema Jacupiranga Olinda Patos de Minas Tapira Huila Pesca Sardinata Turmeque

-

320 500 370 I20 100

20 700 950 I38 600 80 10

Type deposit Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Igneous Sedimentary Igneous Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Igneous Igneous Sedimentary Sedimentary Sedimentary Igneous Igneous Igneous Igneous Igneous Sedimentary Sedimentary Igneous Sedimentary Sedimentary Sedimentary Sedimentary

PHOSPHATE ROCKS IN THE TROPICS

93

Table I1 (Continued)

Country Chile Curacao Ecuador Mexico Peru Venezuela

Location Mejillones Nape

Baja California Zacatecas Sechura (Bayovar) Lobatera Riecito

Total reserves and resources (mT X lo6) 4 10

-

lo00 140 6100

40

Type deposit Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary Sedimentary

a Adapted from the International Fertilizer Development Center (1979), Cathcart (1983), Harben (1983), Ricaldi and Escalera (1985), and PPCL (1 983).

Production of PR in Brazil has increased from 236,000 million tons in 1972 to 3,208,000million tons in 1983. As a result, imports of PR dropped from 1,156,000million tons in 1972 to zero in 1983 (Food and Agriculture Organization, 1984). Factors that must be considered in determination of the economic viability of developing a deposit for domesticconsumption must therefore include the value of savings on foreign exchange, the stability of price and supply of imported products, and the internal value of employment opportunities and infrastructure development. A prerequisite to all of these concerns is an understanding of the agronomic suitability of the alternative products that can be achieved with the ore.

Ill. AGRONOMIC POTENTIAL OF PHOSPHATE ROCK FOR DIRECT APPLICATION The term agronomicpotential as used in this review refers to the inherent capability of the P-containing rock to supply plant-available P under a specified set of conditions. It is determined primarily by the chemical solubility of the rock, and differsin meaning from the term agronomic eflectiveness in that the latter refers to the actual performance of a given PR source as influenced by both agronomic potential and external conditions under which it was used. The importance ofdefiningthe agronomicpotential ofthe PR source was not recognized for many years, thus leading to confusion in

94

L. L. HAMMOND ET AL.

interpreting the erratic results of agronomic trials with PRs. For decades, researchers failed to identify the source of PR used in the comparisonsunder the false assumption that all PRs would behave similarlywhen applied to the soil. It is now well recognized that this is not the case and that the observed effectiveness of PR relative to soluble P fertilizer will vary from source to source depending upon the mineralogy and chemistry of each rock as well as the influence of soil, crop, environment, and management factors. A. MINERALOGY AND CHEMISTRY OF PR SOURCES

Although a number of deposits of PR indigenous to countries within the tropics are characterized by high concentrations of iron and aluminum phosphate minerals [i.e., Phospal (Senegal),Christmas Island (C grade), and Maranhao (Brazil)],the P mineral of principal importance is apatite. Apatite varies widely in physical, chemical, and crystallographicproperties (Lehr et al., 1967). In general, the apatites are in the form of carbonate apatite (francolite)with varying degrees of isomorphicsubstitution of carbonate for phosphate. Precise identification of the apatite composition for a wide range of PRs located within the tropics has been accomplished during the past 20 years by the International Fertilizer Development Center (IFDC), the Tennessee Valley Authority (TVA), and other organizations using techniques including chemical analysis, X-ray powder diffraction, petrographic microscopy, infrared spectroscopy, and electron microscopy as described by McClellan ( 1979). Previous work has shown that the degree ofisomorphic substitution in the apatite structure is the key factor in determining the chemical reactivity of PR containing carbonate apatite (Lehr and McClellan, 1972). Thermodynamically, carbonate substitution for phosphate should increase the reactivity of the PR (Chien and Black, 1976; Chien, 1977). A plot of neutral ammonium citrate soluble P versus the mole ratio of C03 to PO4 of the apatites in 49 different PRs demonstrated that the solubility of those PRs increased as the carbonate substitution for phosphate in the apatite structure increased (Lehr and McClellan, 1972). These results agree with the earlier report by Car0 and Hill (1956) that the solubility increased as the “bound CO;’ in the apatitic minerals increased. Work done by Lehr and co-workers (Smith and Lehr, 1966; Lehr et al., 1967; McClellan and Lehr, 1969; Lehr and McClellan, 1972) has demonstrated that the unit cell a dimension of carbonate apatite can be measured by X-ray diffraction. The unit cell a dimension, in turn, is indicative of the degree of carbonate substitution for phosphate, decreasingas the mole ratio C03: PO4increases. This relationship, which is reviewed in greater detail by

95

PHOSPHATE ROCKS IN THE TROPICS

Khasawneh and Doll ( 1978), has been used to show that the composition of (sedimentary) apatites can be expressed by the following generalized formula: CaI0-a -dVaaM&(p04)6 -Ac03)22 +0.4x in which the u dimension = 9.374 - [0.204~/(6- x)]; u(Na) = 1.327(x)/ (6 - x); and b(Mg) = 0.5 15x/(6 - x). Table 111 shows the empirical formulae thus derived for the apatites of some representatives PRs from tropical regions in comparison with those of commercially developed phosphate rock deposits. B. CHEMICAL REACTIVITY OF THE PHOSPHATE ROCKS

Determination of the carbonate substitution for phosphate in the apatite structure as described above is theoretically the most appropriate means of characterizingthe chemical reactivity ofa PR sample, because it is a measure of the actual thermodynamicstability of the apatite structure. Its routine use, however, is limited by the requirement for sophisticated procedures and equipment not normally available in laboratories operating in developing countries. Instead, chemical reactivity is conventionally determined by measuring the solubility of the PR in extracting solutions. There are three commonly used chemical extractants for measuring the solubility of PR: neutral ammonium citrate (e.g., the United States), 2% Table I11 Unit Cell (I Dimension and the Empirical Formulas of Apatites in Some Representative Phosphate Rocks in Asia, Africa, Latin America, and the United States

Length of

Region Asia Africa Latin America United States

Rock sample

a axis (A)

El-Hassa, Jordan Mussoorie, India Kaiyang, China Gafsa, Tunisia Hahotoe, Togo Parc W, Niger Bayovar, Peru Pesca, Colombia Patos de Minas, Brazil North Carolina Central Florida Tennessee

9.339 9.3s2 9.372 9.328 9.35 9.365 9.337 9.346 9.370 9.322 9.345 9.357

Empirical formula C~.~N~.23M~.~(P04)5.12(C03)0.88F2.35

C~.~N~.14M~.06(P04)5.42(C03)0.~8F2.23 C~.9~N~.01M~.~l~P04~5.~~C03~0.06F2.02 C~.59N~.MM~.12(~4)4.90(C03)1.10F2.44

C ~ . ~ 9 N ~ . 1 5 M ~ . 0 6 ( P 0 4 ) 5 . 3 9 ( C 0 3 ) 0 , 6 1 F2.24 c~.93N~.05M~,02~p04~~.78~c03~0.22F2.~

C~.76Na.l,Mp0.0~P04)5,2a(C03)0.72F2.29 C~,~3N~.~M~.13(P04)4.77(C03)l.23F2.49

-35

N%.I I M h 4 ( P04)5.54(c03

)OM F2. I6

96

L. L. HAMMOND ET AL.

citric acid (e.g., Brazil until recently), and 2% formic acid (e.g., Europe). Other methods that have been found to be highly correlated with plant-available P include (1) absolute citrate solubility as proposed by Lehr and McClellan ( 1972),which is based on X-ray diffraction analysis of the apatite and (2) ammonium citrate, pH 3 (Chien and Hammond, 1978b). MacKay et af.( 1984)tried 5 and 15%citric acid and found that they were not suitable for measuring PR reactivity. Care must be taken in the interpretation of solubility measurements because of the presence of accessory minerals in the PR in association with the apatite. Free carbonates (calcite and dolomite), for example, are more soluble than apatite in neutral ammonium citrate solution (Silverman ez af., 1952)and tend to depress the extraction of citrate-soluble P during a single extraction. It has been shown, however, that if the first extract is discarded and the soluble P of the subsequent extract of the same sample is analyzed, a more accurate measure of agronomic potential can be obtained. This is most noticeable when measuring the relative potential of a number of PRs in which there is a wide spread in the content of free carbonates. In a study by Chien and Hammond (1 978b), for example, it was observed that in Huila PR from Columbia, which contains approximately 6% C02 in the form of free carbonates, the P extracted by neutral ammonium citrate increased from 0.4% by weight of the rock during the first extraction to 1.5% in the second extraction. Other PRs in the study contained negligible amounts of free carbonates, and the amounts of citrate-soluble P extracted during both the first and second extractions were the same. Thus, it is recommended that the second extraction by neutral ammonium citrate be employed for ranking of relative agronomic potential. Alternatively, sequential extractions as used by MacKay et af.( 1984)can be used to estimate the agronomic potential of PRs. Other accessory minerals can also influence the results of chemical solubility tests. Pesca PR from Colombia, for example, has siliceous minerals intermixed with the apatite in such a way that they render the apatite less accessible to attack by the chemical solvents. In this respect, the chemical solvents are in fact more indicative of the agronomic potential than is the absolute citrate solubility. Absolute citrate solubility, for example, shows that the C03:PO., ratios are the same for Pesca PR as for central Florida PR, whereas, in fact, the agronomic potential of Pesca is generally lower than that of central Florida PR because of the Si cementation (Chien and Hammond, 1978b). Differences in utilization of solubility tests for ranking the agronomic potential of PR are also encountered in the method of expressing the amount of P extracted by various methods. Lehr and McCellan ( 1972)pointed out that the conventional “apparent solubility,” as expressed by the amount of solubilized P as a percentage of total P in the material analyzed, can lead to

97

PHOSPHATE ROCKS IN THE TROPICS

an erroneous comparison of the PRs varying widely in grade. In Table IV it can be seen that the solubilities of the two low-grade PRs (Huila and Pesca) are apparently increased with respect to other rocks when the solubilitiesare expressed as percentage of total P rather than percentage of rock. Thus, expression of soluble Pas a percentage of rock, rather than as a percentage of total P, seems preferable where the PR sources vary widely in P content. Comparisons of chemical extractants have been made by various researchers. In early studies measuring the solubility of seven PRs, Car0 and Hill (1 956) reported that the method of 2% citric acid correlated better with the yield of alfalfa grown on three acid soils than did the method of neutral ammonium citrate. However, they did not provide the exact reason for this. Chien and Hammond (1 978b) compared various methods for predicting the agronomic potential of seven PRs (Table IV) for direct application. These measurements were evaluated by crop response data obtained from a greenhouse experiment with guinea grass and a field experiment with beans, both on acid Colombian soils. Among the five methods tested, neutral ammonium citrate (second extraction), 2% formic acid, and ammonium citrate, pH 3, were better correlated with the agronomic data than were the methods of neutral ammonium citrate (first extraction), 2% citric acid, and absolute citrate solubility. In a recent greenhouse study, MacKay et al. (1984) compared seven chemical extraction methods in terms of their ability to assess the agronomic effectiveness of five PRs using perennial ryegrass and white clover as testing crops. Of the conventional, single-extraction procedures Table IV Solubility of the Phosphate Rocks in Neutral Ammonium Citrate and Acid Ammonium Cltrate, pH 3" Soluble P (%) as expressed by Neutral ammonium citrate (second extraction)

Total P

Ammonium citrate, PH 3

Rock sample

(%)

Rock

Total P

Rock

Total P

Huila, Colombia Pesca, Colombia Bayovar, Peru Gafsa, Tunisia North Carolina Central Florida Tennessee

9.1 8.6 13.0 13.0 13.0 14.2 13.1

1.5 0.8 2.3 2.4 2.9 I .4 I .2

16.2 9.5 18.0

4.7 3.7 10.5 9.2 10.8 6. I 4.3

50.2 42.9 80.3 70.3 82.9 42.3 31.5

'

Data from Chien and Hammond (1978b).

18.6

22.4 9.7 8.9

98

L. L. HAMMOND ET AL.

evaluated, 2% formic acid appeared to be the most useful. Two 2% citric acid procedures (0.5- and 1-hr extraction time), neutral ammonium citrate, alkaline ammonium citrate, and two concentrations of citric acid ( 5 and 15%) at the first extraction were poor indicators of agronomic effectiveness. In the studies of Chien and Hammond ( 1978b)and MacKay et al. ( 1984), the poor correlations of the methods of neutral ammonium citrate (first extraction)and 2% citric acid with agronomic data were thought to be due to the depressed solubility caused by the presence of free carbonates in some PRs. Apparently, the associated free carbonatesdid not significantly depress the dissolution of apatites in the soils, and thus the two solubility measurements underestimated the agronomic effectivenessof these PRs. When the second extraction with neutral ammonium citrate was used, the solubilityof PRs then correlated well with crop data (Chien and Hammond, 1978b). It should be pointed out that the ability of chemical methods to predict the agronomic potential of PRs generally decreases as the residual effect of PRs progresses (Chien and Hammond, 1978b;Engelstad et al., 1974;MacKay et al., 1984). This is because of the influence of soil reactions with PR on the residual effect of PR, which is not measured by the initial chemical extractions. MacKay et al. ( 1984)proposed that a sequentialextraction procedure (sums of two to four extractions) be used to predict the long-term effect of PR. For example, the sum of two sequential extractionswith 2% formic acid method still correlated well with the yield of ryegrass at the fourth harvest, whereas the first extraction method failed to predict the residual effect of PRs. The motive for measurement of relative agronomic potential (RAP) has been threefold: First, it is recognized that the effectiveness of a P source measured under actual field conditionswill vary with changes in a number of climatic and agro-edaphic conditions. Solubility measurements, therefore, cannot be used to predict specific yield response, but they can serve as a means of predicting the relative performance of one source to another and thus assist in selection of the most appropriate source for a given set of conditions. Second, findings related to efficient use and management in studies with a given PR source can be expected to be relevant to other P sources with similar solubility.Third, for those countries with several undeveloped deposits of PR, measurements of RAP can serve as a guideline for selecting appropriate priorities for resource development. Studies reported by Hammond (1977), Le6n et al. ( 1986), and Nuiiez (1984) have attempted to rank a large number of PRs indigenous to countries within the tropics of Latin America. It was reported by Hammond (1977) that, on the basis of field trials in Colombia with beans (Phaseolus vulgaris) grown on a Typic Dystrandept and cassava (Manihot esculenta crantz) on a Tropeptic Haplustox as well as a greenhouse trial with Bra-

PHOSPHATE ROCKS IN THE TROPICS

99

chiaria decumbens on the same Oxisol, the seven distinct PRs studied could be ranked as having either high, medium, or low agronomic potential for direct application. It was found that RAP was correlated with the solubility of P in the PR when extracted with neutral ammonium citrate (second extraction). However, it was also concluded that due to the influence of the factors listed above which influence crop response, relative ranking of the source behavior represented the greatest degree of precision possible with solubility measurements. Of the Latin American sources included in that study, Bayovar PR from Peru was classified as having high RAP, Huila PR from Colombia as having medium RAP, and Pesca PR from Colombia as having low RAP relative to soluble P sources. This corresponded to category exhibited citrate solubility of the P in the range of 5.4 to 6.7% (high), 3.2 to 3.4% (medium), and 1.9 to 2.79/0(low). The three Latin American PR sources evaluated by Hammond (1977) were included with additional sources in a greenhouse evaluation reported by Lebn et al. (1986). In the later study, a total of 1 1 Latin American PRs (plus five additional sources) and triple superphosphate(TSP) were tested in a single experiment with guinea grass (Panicurn maximum) as the test crop grown in a Tropeptic Haplustox. The response curves obtained in that trial are illustrated in Fig. 1 and demonstrate the extreme differences among sources in their capability to supply plant-available P. 23

TSP North Carolina (USA)

21

Gaka (Tunisia) Bayovar (Peru)

19

Arad (Israel)

17

Huila (Colombia) Pesca (Colombia)

15

Lobaten (Venezuela) Sardinala (Colombia) Paloo de Mlnaa (Brazil)

13 11

Araxa (Brazil) Abaeti (Brazil)

9 7

Jacupiranga (Brazil)

5

Calalao (Brazil)

3

Taplra (Brazil)

1

50

ioo

2b0

400

Phosphorus Applled (mglpol)

FIG.I. Response by guineagrassto application ofphosphate rock.(FromL e h er al., 1986.)

100

L. L. HAMMOND ET AL.

With the inclusion of the additional sources, all of which were lower in solubility than those studied by Hammond (1 977), it was concluded by LeBn et al. (1986) that an additional RAP grouping (very low) should be recognized. The 16 PR sources (including the 11 from Latin America) were ranked into the four RAP groups by discriminant function analysis. The variables used in the discriminant function analysis included dry matter yield, P uptake, and P solubility in neutral ammonium citrate, 2% citric acid, 2% formic acid, and ammonium citrate, pH 3. Table V gives the average effectiveness and solubility measurements within each of the RAP groupings. In these evaluations, it was also concluded that solubility measurements with 2% citric acid, 2% formic acid, and ammonium citrate, pH 3, were effective, as was neutral ammonium citrate solution, in ranking the RAP of the PRs. This is in agreement with the findings of Chien and Hammond ( 1978b), who also found these methods highly correlated with initial crop response but not with residual P availability from the sources. In a review of studies conducted using Mexican PRs, Nuiiez (1984) reported the results obtained by Adan et al. (1980) with four PRs from Baja California and two from continental Mexico. It was observed in this review Table V Average Effectiveness and Solubility Measurements for Sources within Relative Potential Groupings' Relative agronomic potential (RAP)

Number of samples in the group Relative agronomic effectiveness (%) Mean Range Neutral ammonium citrate soluble P (%) Mean Range 2% Citric acid soluble P (%) Mean Range 2% Formic acid soluble P (%) Mean Range Ammonium citrate, pH 3, soluble P (%) Mean Range a

From LeBn et al. ( I 986).

High

Medium

Low

Verylow

4

4

5

3

94 85-99

16 74-19

55 42-67

21 12-28

2.6 2.4-2.9

I .2 0.8-1.5

0.5 0.2-0.8

0.2 0.1 -0.3

5.8 5.0-6.7

2.9 2.1-3.8

2.5 2.1-3.1

1.3 1.2- 1.3

9.1 7.6- 10.4

3.1 2.2-3.6

2.2 1.0-3.0

1.4 1.4-1.5

10.9 6.1 - 13.8

4.5 2.8-6.0

2.0 1.0-4.4

0.6 0.1-1.2

PHOSPHATE ROCKS IN THE TROPICS

101

that the PRs from Baja California were similar in reactivity to those from Morocco, Algeria, and Israel and were surpassed only by PRs such as those from North Carolina and Gafsa (Tunisia). The rocks from Zimapan and Saltillo, on the other hand, were found to be low enough in solubility that they were not recommended for direct application. It was also noted that plant response to direct application of these PRs on two soils in a greenhouse test with Italian ryegrass (Lolium multiflorum,cv. Westerwools)was closely correlated to the PR solubility in neutral ammonium citrate.

IV. PHYSICAL FACTORS INFLUENCING EXPRESSION OF AGRONOMIC POTENTIAL In the preceding section on agronomic potential of PR, it was pointed out that the degree to which the chemical reactivity of the source was expressed was measured as actual agronomic effectiveness. Although such external factors such as soil properties, crop type, and fertilizer management strongly influence agronomic effectiveness, the physical condition of the PR itself is also a critical factor. Since PR is water insoluble, the first step in utilization by the plant of the P applied to soil as PR is the partial dissolution of the rock. The physical form of the PR, therefore, influences both the rate of dissolution, because of the role of surface area contact in promoting the dissolution reaction, and the spatial availability of the P following dissolution. A. PARTICLE SIZEOF THE PHOSPHATE ROCK

In reviews of experiments with finely ground PR in the United States (Rogers el al., 1953; Khasawneh and Doll, 1978) and the United Kingdom (Cooke, 1956), it was concluded that increases in P availability were obtained by increasing fineness of grinding but that the degree of improvement rarely justified grinding to a size less than 100 mesh ( 150pm). It was additionally pointed out by Khasawneh and Doll (1978) that, because the concentration of P around the PR particle in the soil is low and the diffusion from the site is minimal, the availabilityof P from the PR is a function of the probability of plant roots intercepting zones in which PR is dissolving. This being the case, the influence ofparticle size is also interrelated with the rate of application, method of application, reactivity of the PR, and acidity of the soil. It can be predicted, for example, that the influence of fine grinding will diminish as the rate of application increases and as the distribution of particles in the soil is increased by more complete incorporation into the soil.

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Data reported by Adan et af. (1980) support these contentions. In his greenhouseresearch using six PRs from Mexico, one from Morocco, and one from Florida, the P sources were applied at two rates of application on two soils with the PRs ground to two size ranges: minus 8O/plus 100 mesh and minus 17O/plus 230 mesh. Analysis of variance based on P uptake by L. muftiflorumduring a 177-day cropping period with four cuts showed that there was no statistically significant difference between the two particle size ranges when applied at the high rate. In Africa the first trials involving the use of Senegalese PR from Thies showed a delayed response of groundnut to the fertilizer (Bouyer, 1951, 1954). Poulain and Mara (1965) suggested