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Trifolium repens L. |
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Leguminosae Author: Dr. John Frame Common name Description. Glabrous perennial with trifoliate leaves; leaflets ovate or circular with minutely serrate margins and usually whitish leaf markings on the upper mid surface. Stipules pale and translucent with a short point. Type and cultivar leaf sizes vary from very small in the prostrate, short-petioled types to large in the longer-petioled, more erect types. Stolons initiated from leaf axils form a branched network radiating from an initially tap-rooted seedling. When this dies, the individual stolons grow independently. The stolons are the main perennating organs which spread to colonize bare spaces in the sward, their apices being the main sites for leaf production. During the long daylengths and high temperatures of summer, the axillary buds on the stolons produce inflorescences rather than vegetative stolon branches. Shading, e.g. by a dense grass canopy, reduces stolon formation and increases stolon internode length. On grazed swards there is an annual cycle of stolon burial in winter, re-emergence of growing points in spring and surface development in summer. Severe defoliation or stress from other growth-limiting factors lead to dwarfing of the leaves and reduced stolon spread; amelioration of these factors leads to larger leaf size and better stolon proliferation. Initially the establishing seedling plant develops an extensively-branched, tap-root system which is short-lived and thereafter plantlets with adventitious roots develop from the stolon nodes. Clover roots can grow to a similar depth as temperate grasses but have less root mass in the upper soil layer, 0-20 cm, shorter root hairs and smaller root cyclinders (Caradus, l990). Clover cultivars with deep tap roots have been bred for dry conditions, e.g. in South Africa. Inflorescences are globular racemes, with 20-40 florets at the end of long peduncles originating from leaf axils on the stolons. Florets are white, often tinged pink, becoming deflexed with age. White clover is cross-fertilized by honey bees and bumble bees and the seeds, up to 3-4 per pod, ripen three to four weeks after pollination. Seeds are heart-shaped with a smooth surface, coloured bright yellow to yellowish brown, becoming darker with age. Distribution Widely distributed in moist temperate zones, Mediterranean areas and some cool subtropical parts of the world, viz. Europe, North America, southern Latin America, Australasia, Japan. Estimates indicate some 9 M ha of pasture with white clover in New Zealand, 5 M ha in Australia and 5 M ha in the USA (Marten et al., l989). Characteristics Grows well in a wide range of soil and environmental conditions but less vigorously on acid, poorly-drained or shallow, drought-prone soils. Stoloniferous growth habit makes it capable of colonising bare spaces in swards. Exhibits phenotypic plasticity in response to management factors imposed, e.g. reduced leaf size under severe sheep grazing. Perennation is dependent upon persistence of its stolon network. A range of types from prostrate small-leaved to more upright large-leaved is available. White clover contribution to the total yield of a grass/clover sward is variable and often unpredictable because of the dynamic interaction of the components. Season of growth Highest yielding in late spring and summer. Peak yield occurs later in the season in mixed swards than in clover monocultures. Temperature for growth Optimum temperature for growth, 20-25oC. In general slower growing than grasses below 10oC and faster above 20oC. Leaf appearance rate and petiole extension are increased by higher temperatures (Boller and Nösberger, l985). Low root temperature (5oC) causes cold-induced water stress and is a major reason for poor clover growth at low temperatures (Kessler and Nösberger, l994). Mediterranean types have a greater ability to grow at low spring and autumn temperatures than north European types (Eagles and Othman, l988). Growth ceases under the severe winter temperatures of cold temperate regions, e.g. Scandinavia but during milder winters, in the UK for example, growth continues slowly though maximum clover petiole length is shorter than grass leaf length so that the low- growing clover leaves are shaded and photosynthesis per unit leaf area is lower (Woledge et al., l989). Frost tolerance and regrowth Cultivars bred from north European ecotypes are generally more winter hardy than those of Mediterranean origin. Ability to grow at temperatures above zero is usually inversely related to ability to withstand sub-zero temperatures though some cultivars have been bred to do both (Rhodes et al., l989). Frost tolerance appears to be related to the duration of growth at low, positive temperatures since this allows cold hardening (Collins and Rhodes, l995) and the opportunity to photosynthesize and accumulate carbohydrates in the stolons (Harris et al., l983). Accumulation and utilization of non-structural carbohydrates (starch and water-soluble carbohydrates) are important in determining the extent to which white clover survives the winter and spring in a harsh climate, such as in Scandinavia (Frankow-Lindberg and von Fircks, 1998), also, late cold-dehardening seems to be valuable where frequent spring frosts occur. Light In response to low irradiance at the petiole tips, petiole extension is stimulated, thus enabling better light interception by the laminae at the surface of the sward canopy (Thompson, l995). Leaves developed under high irradiance levels have a higher photosynthetic potential than those developed under low irradiance (Dennis and Woledge, l983), particularly at high temperatures (Woledge and Dennis, l982). At the same height in the sward canopy, young leaves have a higher photosynthetic rate than old leaves (Boller and Nösberger, l985). Stolon branching and lengths of petioles and internodes are strongly influenced by the amount and quality of the light penetrating the base of the sward. This, in turn, is determined by the incident radiation, and canopy light interception and transmission. Long daylengths of over l4 hours stimulate inflorescence initiation and development, though high temperatures can compensate for shorter daylengths (Norris, l989). Drought tolerance Tolerates moderate but not severe drought. Summer drought is a limiting factor to clover performance in southern Australia (Hutchinson et al., l995). Water supply More susceptible to water loss than the more drought-resistant forage legumes e.g. birdsfoot trefoil, sainfoin. Water stress reduces the rates of stolon branching and leaf appearance, and also leaf size (Belayne et al., l996). Clover recovery can be rapid following cessation of drought (Aparicio-Tejo et al., l980). Stolons survive drought longer than leaves and so recovery or survival is related to stolon growing point density (Brock and Kim , l994). Above ground in mixed swards, grass foliage aids clover tolerance to drought by partially protecting clover from direct solar radiation and by keeping temperatures lower at ground level (Brock and Hay, l993). Tolerance of flooding Intolerant of long periods of waterlogging. Soil requirements Adapted to a wide range of soil conditions but not acid, ill-drained soils. Thrives on moisture-retentive but free-draining soils. An adequate soil pH, 5.8-6.0 on mineral soils and 5.5-5.8 on peaty soils, is necessary to ensure availability of plant nutrients including trace elements and to avoid toxic levels of exchangeable Al and Mn. Rhizobium relationships Colonisation of the nodules is by strains of Rhizobium leguminosarum bv. trifolii. Populations are highest in fertile soils with a history of growing white clover. Infertile acid soils frequently harbour ineffective strains and improvement of soil fertility and seed inoculation by effective Rhizobium is necessary when clover is introduced. A soil pH of at least 5.0 is necessary to avoid toxic levels of Al and Mn (Newbould et al., l982), (Cooper et al., l983). However, fertile soils can sometimes contain ineffective strains which compete with effective strains both natural or seed-inoculated. Rhizobial inoculation of seed is necessary when clover is newly introduced to land and strains of Rhizobium specific to different clover cultivars have been developed. Ability to spread naturally Clover produces seeds naturally in ungrazed patches or following rest periods of up to six weeks during clover flowering and seed setting periods. In addition to the directly-shed seed, seed from grazed seed heads retains its viability after passing through the gut of cattle or sheep and deposited in the dung. An open sward canopy is necessary for seed from soil seed bank to germinate and develop (Chapman, l987) though seedling emergence in established swards is very small in comparison with clover propagation from stolon proliferation (Chapman and Anderson, l987). Nevertheless, regeneration from the soil seed bank is significant in south-east USA. (Evers, l989) and in eastern Australia (Jones, l982). Land preparation for establishment Well-cultivated, uniform and firm seed bed required for good results. Sowing methods Broadcasting the seed mixture on a firm, ring-rolled seed bed and then harrowing and flat-rolling is the most suitable conventional method. Mixtures can also be drilled using a shallow depth due to the small size of clover seed. Drilling the grass seed component separately and then broadcasting the clover seed is an option. Direct sowing, i.e. without a cover crop, is best. If sown under a cereal, an arable silage crop is better than a cereal grain crop since the earlier removal of the shading cereal gives the clover seedlings more time to develop fully before winter. If a cereal grain crop is intended, the grass/clover mixture should be sown as soon as possible after the cereal using an early-maturing, stiff-strawed cereal variety at a seed rate 25% less than normal and a lower N rate. White clover can be oversown alone or in mixture with grasses into existing natural or sown swards by surface sowing, direct drilling (sod seeding) or partial cultivation. Aerial oversowing has been reported from New Zealand (Macfarlane et al., l987). Oversowing techniques are most successful on existing swards with low-density vegetation and provided soil moisture is adequate for germination and seedling development. Dense swards can be thinned out by severe grazing, cutting or partial chemical desiccation prior to oversowing. Partial spike rotavation to create tilth before sowing encourages good soil-seed contact. Heavy grazing by stock pre-sowing to create bare soil space and again post-sowing to trample in seeds is a cheap technique which can be effective provided the swards are not dense. Several types of direct drill have been developed, e.g. the Hunter Rotary Strip Seeder (Sheppard and Swift, l995) and the Aitchison Seedmaster Drill (Baker et al., l993). Drills can be fitted with an applicator to band-spray a chemical desiccant astride the slots made by the drill and so reduce competition to the sown species from the existing sward. Guidelines for successful oversowing are: control of perennial weeds before sowing; adequacy of bare space for clover seeds to germinate and develop; satisfactory soil pH and fertilizing with water-soluble phosphate; adequacy of soil moisture; grazing after sowing to limit competition from existing sward balanced with rest periods to avoid overgrazing the introduced species (Tiley and Frame, l99l). Sowing depth and cover The optimum sowing depth is l0-l2 mm with a light but firm soil cover. Sowing time and rate Spring is the best time though late-summer sowing is a practical alternative provided there is adequate soil moisture and sufficient time for the clover seedings to develop well before winter. White clover is most commonly sown at 2-5 kg/ha in mixture with grasses, the lower seed rates being used when seed cost is a consideration. While a high clover:grass seed rate ratio may be advantageous for initial clover establishment the effect does not persist beyond the first or second harvest year on account of its ability to spread by stolons (Laidlaw, l978; Frame and Boyd, l986). A seedling density of l50 clover plants/m2 at three months after sowing giving a 30% ground cover at one year after sowing is a target (Haggar et al., l985). In specialist seed-producing areas, 2-4 kg/ha are drilled evenly in rows 30-45 cm apart in late summer or early autumn. Number of seed per kg Circa 1 450 000 to 1 670 000. Percentage hard seed A high proportion of hard seed, 30-40%, can be present in seed lots. Seed treatment before sowing Seed is not normally treated to reduce the hard seed content since these may germinate over time following abrasion by fluctuating soil conditions after sowing. Nutrient requirements Liming may be necessary to ensure a satisfactory soil pH and hence availability of plant nutrients, including trace elements. White clover is the most sensitive component in mixed swards to deficiencies of nutrients such as P or K and will decline if these nutrients are in short supply (Dunlop and Hart, l987). On grazed swards, nutrients are recirculated through excretal return. However, as this is uneven, moderate P and K application is required annually. When a cut of hay or silage is taken, P and K replenishment is necessary in proportion to the nutrients removed, viz. circa 3 kg P and 30 kg K/t DM. Cutting trials show that fertilizer N applied repetitively during the growing season increases grass yield at the expense of clover contribution (Frame and Boyd, l987a). The effect is exacerbated on grazed swards since rhizobially-fixed N in the ingested forage is recycled through excretal return and transferred to the soil N pool underground which is also boosted from decaying clover roots and nodules. Eventually, clover can disappear from highly N-fertilized swards. Tactical N application at specific times during the growing season, such as spring or autumn, increases grass yield with minimum adverse effect on clover contribution provided there is a good stolon network to allow recovery (Laidlaw, l980; Frame and Boyd, l987). At 8-l0 kg DM/kg N applied, yield response from N-fertilized grass/clover swards is about half that from all-grass swards (Frame and Newbould, l986). The annual rate of fertilizer N needed on all-grass swards to match the yield from mixed swards – the fertilizer N equivalent – ranges from l24 to 278 kg/ha with an average of l72 kg/ha (Royal Society, l983). Cattle slurry is a rich source of available nutrients, especially K, e.g. 50 m3/ha at l:l dilution with water supplies circa 60 kg N/ha, l0 kg P/ha and l00 kg K/ha, but the clover-depressing effect of slurry N is less than for fertilizer N (Nesheim et al., l990). Critical levels of nutrients i.e. the minimum concentrations within the plant required to produce 90-95% optimum growth, are shown in Table l. The critical concentrations are generally higher in grass/white clover associations than white clover monocultures and differ from study to study because of different conditions and techniques. A technique of establishing ratio norms between different nutrient concentrations in white clover herbage has been devised – the diagnosis and recommendation integrated system (DRIS) – whereby actual ratios from compositional analyses are compared with the ratio norms derived from data sets in order to identify the nutrients likely to be limiting to growth (Jones and Sinclair, l99l). Table l Critical concentrations of nutrients in white clover (from Frame et al., l998)
In general non-aggressive grasses are the most and aggressive grasses the least compatible, but coincident growth rhythm of companion grass and clover also leads to incompatibility (Haynes, l980). Since grass is the highest yielding constituent of the mixed grass/clover sward, choice of seed mixture is usually made on the basis of the grass. Thus clover is grown with a range of grass species, compatible and incompatible. Perennial ryegrass (Lolium perenne) is an aggressive species but widely used in the UK and New Zealand. Tetraploid ryegrass is more compatible than diploid ryegrass due to lower tillering capacity (Fothergill and Davies, l993) and early-heading more compatible than late heading ryegrass (Gooding et al., l996). Meadow fescue (Festuca pratensis) is very compatible while cocksfoot (Dactylis glomerata) is poorly compatible. White clover co-exists with short-lived red clover in general-purpose mixtures and with sainfoin in mixtures for hay and aftermath grazing. Ability to compete with weeds Moderate during early establishment of mixed swards but improved ability with time. Monocultures have poor competitive ability. Tolerance of herbicides Tolerant to ‘clover-safe’ herbicides such as MCPB, 2,4-D, benazolin and bentazone types but not less-selective types. Manufacturers’ label recommendatiaons must be closely followed. If infestations of perennial weeds, e.g. docks (Rumex spp.), in sward are treated by effective herbicides such as those based on dicamba or fluroxypyr, the clover will be killed thus requiring re-introducion by oversowing or a full reseed after cultivations. Seedling vigour Slow growing during early establishment. Vigour of growth and growth rhythm Once established, capable of rapid vigorous growth under high soil fertility. Growth slow in early spring peaking in late spring- early summer. Some growth, albeit slow, in mild winters. Nitrogen-fixing ability Annual N-fixation can vary widely due to a number of factors including climatic and soil conditions, rhizobial effectiveness and stage of plant development. The presence of available mineral N in the soil reduces N-fixation, but the nutrients Ca, Mo, Co, Fe, Cu and B also influence nitrogen enzyme activity in the root nodules (Dunlop and Hart, l9987). Crush (l987) reported an N-fixation range of l7 to 680 kg/ha annually from a review of 30 studies in New Zealand but more than half the estimates were between l00 and 400 kg/ha. In the UK, the range cited for lowland swards was 74 to 240 kg/ha (Cowling, l982) and 100-l50 kg/ha for upland swards (Newbould, l982). Estimates can be influenced by the methodology used for measuring N-fixation. Response to defoliation Frequent, severe defoliation results in shorter petioles, shorter stolon internodes, smaller laminae, and, because total available carbohydrate (TAC) content is decreased, stolon branching is reduced (Jones and Davies, l988). Infrequent defoliation has a dual effect depending upon the density of companion grass. If grass is sufficiently dense to reduce irradiance at the sward base the number of stolon growing points is reduced. In contrast, if the grass density is thinned out in response to a rest interval, clover leaf apperance rate is increased and TAC builds up in the stolons. Advantage is taken of this plasticity by inserting a rest interval and a conservation cut in a severely sheep-grazed sward where clover content is declining (Gooding et al., l996). Grazing management White clover is suited to both continuous stocking and rotational grazing systems. In mixed swards, clover contribution is usually greater under rotational than continuous stocking, especially in early sward life (Curll and Wilkins, l983) though rotationally-grazed clover is less capable of withstanding stresses such as drought (Brock and Hay, l993). Under rotational grazing clover has time to produce larger stolons and leaves during the rest intervals, a reason why rotationally-grazed swards usually contain more clover than continuously-stocked swards. Large-leaved cultivars are best suited to rotational grazing and small-leaved to continuous stocking with sheep (Swift et al., l992; Brock and Hay, l996). In swards continuously stocked with sheep at high grazing pressure clover decline can be rapid (Orr et al., l990) since repeated defoliation at short intervals reduces stolon branching and lowers content of water soluble carbohydrates (WSC) in the stolons (Jones and Davies, l988). If stocking intensity is low, residual forage will shade the stolons, reduce branching, and thus diminish future clover contribution (Steen and Laidlaw, l995). Clover performance in continuously-stocked swards can be satisfactorily maintained by the use of small-leaved, highly-stoloniferous clover cultivars and control of sward surface height to avoid overgrazing (Davies et al., l991). Target swards heights for both continuous and rotational grazing systems, and for different classes of stock have been devised (Hodgson et al., l986). In swards continuously stocked with cattle clover content can remain relatively stable from year to year and correct stocking rates (Laidlaw et al., l995). Stolons from clover in rejected patches, where growth continues, can spread out and recolonise neighbouring areas depleted of clover (Teuber and Laidlaw, l995). In milder areas of western Europe stolon branching and leaf appearance continue, albeit slowly, during winter provided the clover is not overshaded by grass (Patterson et al., l995). Thus, winter defoliaton to reduce the grass canopy can improve clover contribution in spring by increasing stolon branching and photosynthetic rates (Laidlaw et al., l992). However, removal of the grass canopy exposes clover to more potential winter kill (Frankow-Lindberg et al., l998). The interactions between grass and clover in grazed swards makes a flexible approach to management essential, i.e. applying different facets of management at specific times, whether seasonally or as a result of previous management factors or effects, in response to the needs of clover (Frame and Laidlaw, l999). Breeding system White clover is a natural tetraploid with a chromosome number of 2n=4x=32. It is cross-fertilized through honey bee and bumble bee pollination. Breeding objectives Breeding programmes are ongoing in several countries but particularly in New Zealand and the United Kingdom. Objectives include : improved yield and competitive persistence in mixed swards under a range of grazing managements and livestock classes ; better early spring growth ; tolerance of fertilizer N application ; adaptation to temperature extremes, both cold and hot ; higher seed yields ; greater N-fixation ability ; removal of traits inducing bloat in ruminants. Dry matter yields Potential DM yield from a mixed sward in the UK is estimated at circa 20 t/ha annually though experimental yields achieved under various managements are in the range 7 to ll t/ha (Frame and Newbould, l984). Similar yields were recorded in trials in Atlantic Canada (Fraser and Kunelius, l995). The potential yield in New Zealand is about 25% higher than in the UK but in experiments over nine different sites, actual yields were between 6.7 and l4.9 t/ha with a mean of 8.4 t/ha and 37% clover content (Hoglund et al., l979). Also, DM yields from a 23-year-long cutting trial under dryland conditions there ranged from 4 to l0 t/ha with less than 5% clover, though under irrigation, yields increased to 8 to l3 t/ha with 20-30% clover (Rickard and McBride, l986). The highest DM yields obtained from grazing trials in New Zealand were l2 t/ha for hill sites, l6.2 t/ha for unirrigated dryland and 22.8 t/ha for favourable lowland sites, these values being a third higher than the amounts typically obtained in farming system trials (Brougham, l977). For clover monocultures, annual DM yields of l2 t/ha under irrigation and 9 t/ha without have been reported for the UK (Frame and Newbould, l984). In field-scale silage making white clover monocultures yielded 6-8 t DM/ha (Castle et al., l983). Actual yield data from grass/clover swards in commerical farming situations are not available but the levels of total yield and of clover content will be more variable than in experiments because of less precise management control. Suitability for hay and silage Mixed swards are highly suitable for conservation whether under continuous conservation or in an integrated grazing-conservation system. Grass/clover swards gave circa 80% of the yield from all-grass swards N-fertilized with 350 kg/ha annually (Bax and Browne, l995). In intensively-grazed swards where clover has declined, the introduction of a rest interval followed by a conservation cut aids restoration of the clover (Barthram and Grant, l995) due to improved stolon and leaf development ; also, the removal of circa 30 kg N t DM depletes the soil N pool and so reduces grass competition to the clover. Compared with grass, clover herbage has a higher buffering capacity, lower water-soluble carbohydrate (WSC) and DM contents, and a higher protein content. However, silage of high quality can be made provided the cut crop is wilted to concentrate the WSC and DM, short-chopped to aid release of sugars and consolidation in the silo, and an effective additive used. In hay making it is important to try to avoid loss of the nutritious clover leaf fraction during the curing process. Value as standover or deferred feed Canadian work indicated that white clover was a suitable component in mixed swards held over in late summer for use in late autumn (Kunelius and Narasimhalu, l993). Feeding value White clover is protein- and mineral-rich and retains a high digestibility since there is continual generation of new leaves from the stolons, which partially compensating for advance in maturity of existing foliage. Inflorescences and peduncles have lower digestibilities than leaves and petioles (Søegaard, l994). In contrast to other forage legumes and grasses, low-digestible stem tissue is not harvested in white clover crops and so digestibility declines less with age than in these other species. Compared with N-fertilized grass, grass/clover mixtures usually have higher contents of protein, minerals, including trace elements, pectin and lignin but lower contents of cellulose, hemicellulose and WSC (Thomson l984 ; Thomson et al., l985). The mineral composition is shown in Table 2. Table 2 Mineral composition of white clover (from Frame and Newbould, l986)
Acceptability Highly acceptable forage to livestock whether as silage, hay or when grazed at a leafy stage. Physical, chemical and anatomical features contribute to superior intake of white clover compared with grass. Anti-quality factors Bloat in ruminants is a hazard when grazing clover-rich swards but conventional prevention methods are available : gradual introduction to sward ; hay or straw supplementation ; provision of rumen anti-foaming agent such as poloxalene. The cyanogenic potential of white clover is of some concern, since after intake, hydrocycanic acid (HCN) is metabolized to inorganic thiocyanate which is goitrogenic. The concern is not widespread though in Switzerland, cultivars with more than 370 mg HCN/kg organic matter are not recommended (Lehmann et al., l99l). European cultivars usually have higher HCN potential than North American cultivars (Wheeler and Vickery, l989). Seed harvesting methods Seed crops may be directly combined or else cut, swathed and threshed. Crop desiccant applied before cutting speeds up drying. Correct timing of harvesting is necessary for maximum seed yields since the density of ripe inflorescences is at a peak for a brief period only (Hollington et al., l989). Spring defoliation by cutting or grazing removes the vegetation canopy and encourages the development of ripe inflorescences, but has to be carefully timed to avoid reducing the first and highest-yielding crop of flower heads (Marshall et al., l989). Legume seed crop management, including white clover, has been comprehensively reviewed (Marshall et al., l997). Seed yields In New Zealand, which produces 4500-5000 tonnes annually, i.e. about 50% of world production, yields range from l00 to l000 kg/ha, with a national average of 300 kg/ha (Mather et al., l996). In California, the main source of white clover seed in the USA, seed yields average 420 kg/ha from irrigated crops (Pederson, l995). Seed quality standards Using the Fodder Plant Seeds Regulations for the United Kingdom as an example, certified seed requires a minimum germination of 80% and a maximum permissible hard seed content of 40% by number of pure seeds in the sample. Required analytical purity is 97% by weight for the minimum standard but 98% for the higher voluntary standard. The maximum permissible content of seeds of other species is l.5% by weight for both the minimum and higher voluntary standards. Cultivars Clover cultivars have been classified by leaflet size as small, medium or intermediate and large. Small-leaved types are generally prostrate and highly stoloniferous with thin, multi-branched stolons whereas the larger-leaved types have longer petioles, less but thicker stolons and more robust root systems. However, leaflet classification is less relevant for modern cultivars which do not rigidly conform to these classes and which have more important distinguishing characteristics such as stolon and root development or differential response to environment or management. In practice, a simple blend of cultivars with different leaf sizes is often sown in mixtures so that the clover component tolerates both grazing and cutting management. A world check-list cites 3l9 cultivars (Caradus and Woodfield, l997) though probably two-thirds are not used or are only used locally since seed is not widely available. The New Zealand cultivar, Grasslands Huia, is the major cultivar, comprising 35-40% by volume of world sales (Mather et al., l996) though its dominance is steadily being eroded by the release of improved cultivars to the market. Some examples of cultivars are as follows :
Clover yield and percentage in swards have increased by 0.6% annually over the past 60 years, genetic gains being greatest for small-leaved types and large-leaved ladino types, and for cultivars originating in cool rather than cold or warm climates (Caradus, l994). Further improvement can be expected since white clover is a relatively wild species in comparison with the level of domestication attained with many crop species (Caradus, l993). Diseases Clinical, and sometimes sub–clinical, disease causes impaired clover performance and often there is no economic method of control. Several fungal diseases may strike simultaneously and complexes with other pathogens and pests can occur (Nelson and Campbell, l993). A number of reviews are available (Leath, l985 ; Clements, l994). Clover rot (Sclerotinia trifoliorum) seriously affects yield and persistence. Infection occurs mainly in autumn from fungal spores in the soil or on seed and it spreads by wind-borne spores from sclerotia in the plant tissues. Plant wilting, death and rotting of stolons and roots become evident the following spring. Selecting cultivars with some resistance and in the long run, breeding more resistant cultivars, is the main method of combating the disease. Necrotic breakdown of clover roots and stolons is caused by a root-rot complex of pathogenic fungi within which Fusarium spp. plays a prominent secondary role. Environment, management or pest attack are believed to be the main pre-disposing factors and so avoidance or amelioration of them reduces the incidence. Clover is damaged by several foliar and stem fungal diseases, such attacks often being severe in humid or wet weather (Skipp and Lambert, l984), the symptoms being weak plant vigour, stunting of growth, leaf fall and stem death. Specific examples include black blotch syn. sooty blotch (Cymodothea trifolii), leaf spot (Pseudopeziza trifolii), pepper spot syn. burn (Leptosphaerulina trifolii). Other less common leaf or stem diseases which occur from time to time include powdery mildew (Erysiphe trifolii), downy mildew (Peronospora trifoliorum) and rusts (Uromyces spp.) on leaves, and spring black stem (Asochyta imperfecta) and summer black stem (Cercospora zebrina) on stems. Viruses A number of viruses infect white clover, often simultaneously (Barnett and Diachun, l985 ; McLaughlin et al., l992). Aphids are the primary vectors and alternate host plants include other forage and crop legumes. Damage ranges from sub-clinical to severe and adversely influences yield, seed production and plant persistence. A review of the effects of five viruses – alfalfa mosaic, bean yellow mosaic, clover yellow vein, peanut stunt and white clover mosaic – reported range maxima of 35-59% for DM yield, 22-72% for root nodule number, 37-38% for nitrogenase activity in the nodules and a 90% maximum loss of seed production (Forster et al., l997). Pests Activities of pests are often covert and insidious and plant damage differs in frequency of occurrence and intensity. Pests which attack white clover were listed by Manglitz (l985). Costs and environmental considerations make pesticide use unsuitable for the control of many pests except possibly when populations are large. Developing resistant cultivars is perceived as the best way forward (Pottinger et al., l993 ; Pederson et al., l993). Slugs (Deroceras reticulatum, Limax spp., Arion spp.) can be serious pests of establishing clover, especially on moisture-retentive, heavy-textured soils. Populations usually peak in spring if the winter has been mild, and to some extent in wet autumns. Pesticide control is possible, e.g. by methiocarb, and is usually necessary when direct drilling clover into existing swards (Tiley and Frame, l99l). Cultivars vary in their resistance to slugs and acyanogenic types which produce hydrocyanic acid following tissue injury are less attractive to slugs, snails, weevils and other herbivorous-feeding insects (Kakes, l990). Leatherjackets, the larvae of the crane fly (Tipula spp.) feed on clover roots in spring but effective pesticides are available, e.g. chlorpyrifos, if the infestation warrants it. Stem eelworm (Ditylenchus dipsaci), which invades and lives within the plant, can cause significant damage. Clover-cyst nematode (Heterodera trifolii), root-knot nematode (Meloidogyne hapla) – which is a serious pest in southern USA – and free-living nematodes (Pratylenchus spp.) have been associated with root damage and weak plant growth. Breeding for resistance to nematodes is a plant breeding objective in several countries (Mercer et al., l990). Weevils (Sitona spp.) are a major pest of white clover (Clements, l994). Established plants may tolerate attack but seedlings are weakened or killed by weevil larvae feeding on their roots. The clover leaf weevil (Hypera punctata) and alfalfa weevil (Hypera postica) are also damaging pests. Weevils (and slugs) were significant pests in UK (Lewis and Thomas, l99l) and Swedish (Frankow-Lindberg, l997) surveys. Weevils (and leatherjackets) are less attracted to cyanogenic clover cultivars (Mowat and Shakeel, l989). In New Zealand, two root-feeding pests causing significant damage and loss of plants are grass grub (Costelytra zealandica) and porina caterpillars (Wiseana spp.). Main attributes Productive N-fixing, protein- and mineral-rich species adapted to a wide range of soil and environmental conditions. Stoloniferous growth habit makes it eminently suitable for grazing but morphologically is plastic enough for conservation too. High voluntary intake characteristics and nutritive value. Has a valuable role in organic farming systems. Major source of honey. Main shortcomings Persistence and yield variable from year to year. Slow spring growth. Less predictable agronomic performance than N-fertilized grass. Can be adversely affected by several diseases and pests. May cause bloat in ruminants. Performance The high nutritive value of white clover allied to its high intake characteristics combine to make clover forage, whether fresh, dried, hay or silage, superior to grass forage in animal performance, for example, with dairy cows (Thomson et al., l985) or lambs (Ulyatt, l98l) ; Ulyatt also reported superiority in animal performance of white clover over lucerne and greater lotus. The key role of white clover in southern Australasian pastures as a means of obtaining low-cost, market-competitive animal products is well recognized and documented. When grass/clover swards are compared with N-fertilized grass swards, the advantage to the mixed sward has been best for lambs (Vipond and Swift, l992), moderate for beef cattle (Clark, l988) and least for dairy cows (Bax and Schils, l993). Contributory factors to these differences may be the physiological state of animals used, clover content in the sward, different grazing patterns of sheep and cattle or even experimental technique. The challenge is thus to exploit clover’s advantages. Nevertheless, animal production systems based on grass/clover swards have proved their reliability and viability for dairy cows (Bax and Thomas, l992), (Bax and Browne, l995) for beef cattle (Stewart and Haycock, l984) and for sheep (Vipond and Swift, l993). Links:
Main references Frame et al. (l998) ; Fairey and Hampton (l998) ; Woodfield (l996) ; Pederson (l995) ; Hopkins et al. (l994) ; Frame and Newbould (l986) ; Gibson and Cope (l985). |