Trifolium pratense L.

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Leguminosae

Author:Dr. John Frame 

Common name.

Red clover 

Description.

Short-lived perennial. Develops a crown at or slightly above the soil surface, consisting of an accumulation of buds formed at the plant base. The basal buds produce upright, pubescent, hollow stems, 60-80 cm. These buds are more numerous in late flowering than early flowering types. Overwinters as a root-stock with a crown. Leaves are trifoliate, pubescent and alternate with a pale, inverted, crescent-shaped mark on the mid-upper surface. Tap root extends to one metre or more in depth but older plants have mainly adventitious and lateral roots emanating from the crown. Stems sometimes root at the nodes when in close contact with a moist soil surface. The cultivar, Astred, with a stoloniferous growth habit has been released in Australia. Inflorescences, terminal on main and axillary stems, consist of many ovoid, compound racemes with numerous pink or purple florets. The florets are cross-pollinated by bumble bees and honey bees. Seed pods contain one or two kidney-shaped seeds which are yellow, brown or purple in colour, with purple seeds being generally heavier than the others. Mature seed lots usually contain a proportion of hard seeds, depending on the incidence of high temperatures during seed set (Puri and Laidlaw, l984).

Distribution.

Widely distributed in the temperate zones of the world, viz. northern and southern Europe, North America, northern China, Japan, southern Latin America, southern Australasia.

Characteristics.

Will grow in a wide range of soil and environmental conditions but not excessively wet, acid or shallow soils. Upright growth habit but lodging can occur with increasing maturity of growth stage as the plant becomes stemmier. Usually productive for 2-3 years, but longer in disease- and pest-free environments (McBratney, l987). The most vigorous growth occurs in the spring to early summer period. Dormant in winter. Grown as an annual in south-east USA.

Season of growth.

Early-flowering types, previously referred to as ‘double-cut’ red clover, syn. medium red clover in the USA and syn. cowgrass in New Zealand, have two growth flushes during the season, resulting from two sequences of basal bud development and stem elongation. Late-flowering types, formerly referred to as ‘single-cut’ red clover have only a single main flush of growth, following a later start in the spring than early-flowering types but provide aftermath grazing following the main cut for hay or silage.

Temperature for growth.

Optimum temperature for growth, 200-250C, with growth limits, 70–400C. Increasing temperatures above the optimum affects roots more adversely than shoots. In response to elevated temperatures, respiration increases and total available carbohydrate (TAC) content decreases, resulting in weakened plants with poorer winter survival and greater susceptibility to soil-borne, root-rotting organisms.

Frost tolerance and regrowth.

Seedlings and young plants with less than six leaves are vulnerable to sub-zero temperatures, but plant cold tolerance increases with age and good winter hardiness is a recognised characteristic of red clover. A period of low temperatures – slightly above 00C – induces hardening to cold but plant damage from pests or diseases, excessive foliage cover before winter, or mineral deficiencies, reduce cold-hardening ability. Cold resistance is improved if plants have sufficient time after flowering to develop a vegetative, rosette stage prior to winter onset. High reserves of N in legume roots during winter are believed to aid cold-hardiness (Volonec and Nelson, l995). Red clover can survive better in severe winters when grown in association with grass than as a monoculture (Belzile, l987).

Light.

Critical leaf area index (LAI) ranges from 3.0-3.5 for stands at the vegetative stage to 5.5 at the reproductive stage. Individual leaves developed under high irradiance have a higher rate of photosynthesis and greater longevity than leaves developed under low irradiation. Shade leads to an increase in leaf area, petiole length, and leaf:stem ratio and a decrease in the root:shoot ratio (Bowley et al., l987). For example, the latter ratio decreases in plants undersown in a cereal crop when the transpirational load suddenly increases following grain harvesting. However, red clover tolerates low light intensity better than other forage legumes (Taylor and Smith, l995).

Long daylengths of at least l4 hours requqired at the stem elongation, flowering and reproduction stages with longer photoperiods being needed for late-heading than early-heading types. Shortening daylength in autumn encourages the induction of cold hardening in the plant.

Drought tolerance.

This is very high when grown on deep soils because of the deep-rooting characteristic.

Water supply.

Compared with many forage species, red clover is an efficient user of soil water supply as a result of its deep tap-root system. Water use in red clover crops ranges from 400 to 600 kg water/kg DM (Bowley et al., l984). Wilting can occur at a water potential of –0.8 to –l.2 MPa, though infection with vesicular arbuscular mycorrhizae (VAM) reduces this value, i.e. increases drought tolerance. Irrigation of crops for forage is not usually economic but seed crops grown in low-rainfall areas, e.g. the drier western states in the USA, require 90-l40 cm of irrigation water in summer. Supplementary water is particularly needed at peak flowering in order to encourage inflorescence development and seed set (Oliva et al., l994; Oliva et al., l994).

Tolerance of flooding.

Intolerant of long periods of flooding.

Soil requirements.

Adapted to a wide range of soil conditions though requires higher levels of fertility, including soil pH, and better soil drainage than alsike clover. Wheel tracking during crop harvesting or during the application of fertilizers and slurry can cause soil compaction which then adversely affects plant density and growth vigour (Frame, l987)

Rhizobium relationships

Rhizobial N–fixation in the nodules is by strains of Rhizobium leguminosarum bv. trifolii. Fixation can contribute up to 80% of total N assimilation but it can be reduced by factors such as accumulation of soil inorganic N, soil acidity, drought or defoliation. The proportion of plant N derived from fixation increases with the proportion of companion grass in the sward and associated competitive uptake of the soil inorganic N (Mallarino et al., l990).
 
 

Rhizobial inoculation of seed can increase herbage yield with only 20% occupancy of nodules by introduced strains (Martensson, l990), though introduced strains are not always highly competitive against existing indigenous strains of Rhizobium. More knowledge is required on the factors influencing Rhizobium competitiveness before inoculants with predictable effectiveness can be developed (Taylor and Quesenberry, l996). Effective strains selected from acid soil sites have increased herbage yields from red clover grown under acid soil conditions, even at pH 4.l, a level normally considered to inhibit nodulation (Lindstrom and Millyniemi, l987). Currently, rhizobial inoculation is recommended when introducing red clover into soils that do not have a history of growing red clover.

Ability to spread naturally.

Disposal of ripened seeds, whether from a late hay crop, a seed crop or rejected patches in a grazed stand can sometimes aid rejuvenation of an ageing stand.

Land preparation for establishment.

Well-cultivated, uniform and firm seed bed required for good results.

Sowing methods.

The seed is normally drilled or broadcast after conventional seed-bed cultivation. In areas with a mild winter climate, e.g. south-eastern USA, it can be sown in late season as winter annual forage. When sown under a cereal grain crop, the cereal curbs weed invasion and provides a valuable cash crop. There is a greater risk of poor clover establishment with undersowing compared with direct sowing due to competition from the grain crop, and possible late harvesting or crop lodging, both of which can retard development of undersown clover plants. Lowering the cereal seed rate, reducing N2-fertilization and cutting the cereal at a growth stage suitable for arable silage are options to reduce competitive stress on the clover.

Red clover can be oversown into existing grass swards in need of renovation by direct drilling (sod seeding) or following partial cultivation. These techniques are most successful on swards with low-density vegetation and when there is adequate soil moisture for seed germination and seedling development. Clover seed can also be introduced into healthy red clover stands in need of additional clover content. A range of direct drilling equipment and oversowing techniques has been developed in different countries.

Sowing depth and soil cover.

The optimum sowing depth is l0-l5 mm. An adequate initial seedling population is essential since plant numbers decrease with increasing age of stand. A population of circa 200 clover plants/m-2 by autumn of the establishment year is a reasonable target for a planned red clover-dominant stand.

Sowing time and rate.

Sowing monocultures or mixtures direct in early to mid season gives higher DM yields and clover contents during the establishment year and the following harvest year than later sowings (Frame, l990) Red clover can be sown conventionally, following an arable silage, or after an early-harvested cereal grain crop provided there is sufficient time for the plants to develop tolerance of winter cold. In general, mixed swards are preferred for grazing and pure-sown clover swards for hay or silage, using a clover seed rate of 3-6 kg/ha along with l8-24 kg/ha grass seed for mixtures and l0-l5 kg/ha for monocultures.

Number of seeds per kg.

Circa 550 000 for diploids and 295 000 for tetraploids.

Percentage hard seed.

A proportion of hard seed, l0-20%, may 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. Contamination of land by sowing seed infected with cysts of stem eelworm (Ditylenchus dipsaci) is preventable by pre-fumigation treatment with methyl bromide, normally carried out by the seed merchant.

Nutrient requirements.

The optimal soil pH for plant development and root nodule formation is in the range 6.0-6.5, but red clover will also grow at lower pH levels of 5.0-6.0, Mn and Al toxicity are problems in acid soils. Adequate P and K in the seedbed are necessary for seedling development, particularly a readily available, water-soluble source of P. A small ‘starter’ N application encourages early clover development in soils of low N status. Following removal of crops for conservation, plant nutrients P and especially, K require replenishment to maintain plant persistence and yield. Spring-applied 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 kgN/ha, l0 kgP/ha and l00 kgK/ha.

Compatibility with grasses and other legumes.

Compatible with non-aggressive grasses. Different companion grasses are favoured in different countries e.g. timothy (Phleum pratense) in Atlantic Canada; tall fescue (Festuca arundinacea) in parts of the USA; timothy and meadow fescue (Festuca pratensis) in Scandinavian countries, Italian ryegrass (Lolium multiforum) in France. Compatible with longer-lived white clover in general-purpose seed mixtures.

Ability to compete with weeds.

Relatively low at the early establishment phase when sown in monoculture but improves with the development of the canopy. Weed invasion is also a hazard in established stands in early spring before the canopy develops.

Tolerance of herbicides.

Tolerates so-called ‘clover-safe’ herbicides e.g. MCPB, 2,4-DB and, benazolin types, but not the less-selective herbicides.

Seedling vigour.

Initially poor on account of small-seed size. Hard seeds represent a reservoir from which seedlings may emerge after the initial establishment phase.

Vigour of growth and growth rhythm.

Vigorous in monoculture or in mixed stands with non-aggressive companion grasses. Relatively poor when it is a minor constituent of a multi-species, general-purpose seed mixture at medium to high fertility, except in the first harvest year. Growth rhythm differs according to whether the cultivar is a single-cut or double-cut type. For the latter in red clover-dominant stands, first and second cuts may contribute 50-60% and 30-40%, respectively, of annual production.

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. Published estimates are mostly in the range of l00 to 250 kgN/ha annually (Smith et al., l985) though a wider range of 76-389 kgN/ha has been cited by Taylor and Quesenberry (1996).

Response to defoliation.

Best suited to an infrequent defoliation regime in which cutting crops for hay or silage is the main objective and aftermath grazing secondary. Plant persistence and production are inversely related to defoliation frequency. For example, increasing defoliations from 3 to 6 reduced DM yields by over 30% (Sheldrick et al., l986). Over-frequent defoliation in autumn reduces carbohydrate and N reserves in the roots and this, together with its natural decline over winter, adversely affects plant persistence and subsequent yield.

Grazing management.

Some form of rotational grazing is the grazing system most favourable to plant persistence and production. Late-flowering cultivars are more tolerant of grazing than early-flowering cultivars since the growth buds on the plant crowns are more numerous. Continuous stocking at a high grazing pressure will reduce plant persistence due to foliage removal and crown damage by hoof trampling.

Breeding system.

Cross-pollinated, mainly by bumble bees and honey bees, though tetraploids have some self-fertility. The chromosome number is 2n=2x=14 for diploids and 2n=4x=28 for tetraploids. Both diploid and tetraploid cultivars have been produced by European plant breeders but North American breeders have favoured diploids. Breeding objectives. These include: improved yield, persistency, disease and pest resistance, low oestrogen content. Breeding programmes have been less intensive in recent years in comparison with lucerne or white clover.

Breeding objectives.

These include: improved yield, persistency, disease and pest resistance, and low oestrogen content. Breeding programmes have been less intensive in recent years in comparison with lucerne or white clover.

Dry matter yields.

Forage yields normally decline with ageing of the stand. Reported European yields over three successive harvest years were 9-l8, 9-l5 and 4-l4 t/ha (Laidlaw and Frame, l988). In France, grass/red clover associations receiving l50 kg N/ha annually gave similar yields to grass stands given 300 kgN/ha (Guy, l989). In Scotland, pure-sown red clover stands gave yields equivalent to those from pure-sown grass swards given fertilizer N at annual rates of 250, 2l0, and l40 kg/ha in successive harvest years (Hunt et al., l975). Red clover, sown alone or with white clover and/or lucerne gave DM yields of l4.5-l6.5 t/ha in the first harvest year and l0.0-l2.6 t/ha in the second, with red clover the dominant constituent in the mixed legume associations (Frame, l986).

Suitability for hay and silage.

Growth characteristics and plant response to infrequent defoliation make red clover a highly suitable species for conservation cropping. Two hay crops or a hay crop followed by a seed crop can be taken in north-west and north-east USA (Smith et al., l985). Shattering and loss of nutritious leaf is a hazard if haymaking is prolonged. Mechanical or chemical conditioning speeds up curing but conditioned hay will lose soluble carbohdyrates and minerals by leaching should there be rainfall (Collins, l982a).

Using techniques of wilting, short-chopping and the application of an effective additive, satisfactory silage can be made from red clover herbage in spite of its low DM and WSC contents, and high protein content and buffering capacity. These disadvantages are lessened in red clover/grass associations.

Value as standover or deferred feed.

Not a common method of utilization but autumn-saved forage can be utilized in early winter where the climate allows this practice.

Feeding value.

Largely determined by stage of growth at the time of utilization since feeding value falls with increasing maturity and associated increase in stem:leaf ratio. Tetraploids generally have higher digestibility, concentrations of protein and WSC than diploids (Mousset-Declas et al., l993). Increasing the number of harvests in a season by cutting at earlier stages of growth improves forage digestibility and protein content but at the expense of yield. Concentrations of N, Ca, Mg, Fe and Co, pectin and lignin are generally higher than in grasses but other constituents are similar or lower. Table 8 shows the mineral composition of red clover. Mineral contents, particularly P, K, Ca, Mg, Cu and Zn, are increased by early spring application of the growth regulators daminozide and mefluidide (Narasimhalu and Kunelius, l994).

Table 1 Mineral composition of red clover (from Spedding and Diekmahns, 1972)

Constituent Content range
  (g/kg DM)
N 23.4-47.0
P 1.4-4.5
K 8.8-41.0
Ca 11.9-24.2
Mg 1.7-3.8
S 1.5-2.4
Na 0.2-2.0
Cl 3.2-6.2
  (mg/kg DM)
Fe 74-362
Mn 36-75
Zn 21-35
Cu 5.8-11.6
Co 0.10-0.36
Mo 0.44

Acceptability.

Highly acceptable forage to livestock whether as hay, silage or when grazed at a young, leafy growth stage. Has high voluntary intake characteristics.

Anti-quality factors.

Bloat can occur in cattle when grazing lush, red clover-rich stands but conventional prevention methods can be implemented. Oestrogens present in the grazed forage can reduce reproductive performance in ewes, formononetin being the main phyto-isoflavone responsible. Formononetin content is lower in leaves than stems and lowest during summer (McMurray et al., l986). Reduced oestrus incidence and ovulation are the causal factors affecting lambing performance. Oestrogenic effects are most prevalent when fresh forage is consumed just prior to and during the mating period but this can be avoided by manipulating grazing management. Cattle are much less susceptible than sheep probably because they metabolize plant oestrogens more rapidly and also since they are usually fed a more mixed diet.

Seed harvesting methods.

In general, seed is harvested in the first harvest year from early flowering clovers after a cut or clipping in early season. The objective of the cut is to remove vegetative growth, thereby reducing the chances of later crop lodging, and to delay flowering until bee pollinator activity is enhanced by summer temperatures. Seed yields are higher after clipping than after a hay crop. Irrigation is valuable in dry summers to stimulate flower production and seed development.

Seed yield.

Seed yields in the USA average about 600-700 kg/ha (Rincker and Rampton, l985) though much higher yields are attainable under optimal growing conditions. In France, seed yield averages circa 700 kg/ha but can reach l000 kg/ha under the best conditions (Bouet and Sicard, l998). The USA is the major seed-producing country, with Canada and France also producing significant amounts.

Seed quality standards.

Taking The Fodder Plant Seeds Regulations for the United Kingdom as an example, certified seed requires a minimum germination of 80% and a maximum hard seed content of 20% by number of pure seeds in the sample. The 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.

Examples of North American medium or early-flowering red clovers are the diploids: Arlington, Florex, Kenstar, Marathon, Ottawa and Redland II, while late flowering types include Norlac and Altaswede. European early-flowering clovers include: diploids Merviot, Marcom, Kuhn and the tetraploid, Deben; examples of late flowering clovers are diploid Britta and tetraploid Sara. Cultivars with a low to medium content of the isoflavone, formononetin, responsible for reduced fertility in ewes, have been developed in both New Zealand and Australia: diploid early-flowering Grasslands Colenso, tetraploid late-flowering Enterprise and Redquin. An Australian cultivar, Astred, is noteworthy for its stoloniferous growth habit and persistence under grazing.

Diseases.

Clover rot (Sclerotinia trifoliorum) is considered the most serious disease affecting clover production and persistence. The infection generally appears in the autumn, encouraged by wet weather, as a necrosis of the clover leaves. Spread is most rapid in winter when large black patches of rotting foliage appear and plant population becomes severely decreased. Autumn defoliation will reduce the amount of foliage liable to infection and also the humidity in the canopy since a high humidity favours infection and spread of the disease. Breeding resistant clover cultivars is the main approach to combating the disease but application of a fungicide such as carbendazim is sometimes practised by seed producers. Following clover rot infection, fields should be given a five-year period of rest before sowing clover again because of the persistence of the fungal spores which can remain dormant for long periods. Necrotic breakdown and rotting of clover crowns and roots may occur from a range of fungi, notably Fusarium spp. However, this is mainly regarded as secondary since it generally follows other plant-weakening factors such as attacks by other pathogens or pests, mismanagement of the sward and physical and/or physiological stress (Rufelt, l983). Several leaf and stem diseases, varying in importance in different regions of the world, affect red clover adversely. Warm, dry summers favour the spread of powdery mildew (Erysiphe trifolii). Northern anthracnose syn. clover scorch (Kabatiella caulivora), which causes withering of the leaves and stem blackening is a major disease in northern USA, contamination of seed being the main source of infection. Southern anthracnose (Colletotrichum trifolii) is more prevalent in southern USA. Rust (Uromyces spp.), spring blackstem (Phoma trifolii), black patch (Rhizoctonia leguminicole) and Pseudopeziza leaf spot (Pseudopeziza trifolii) flare up from time to time. Breeding resistant cultivars is the main way forward against these fungal diseases.

Viruses.

A number of viruses infect red clover, often simultaneously (Barnett and Diachun, l985; Taylor and Quesenberry, l996), viz.,

  • BYMV bean yellow mosaic potyvirus
  • PSV peanut stunt cucumovirus
  • RCVMV red clover vein mosaic carlovirus
  • PStrV pea streak carlovirus
  • AMV alfalfa mosaic ilarvirus
  • WCMV white clover mosaic potyvirus
  • CYVV clover yellow vein potyvirus

Their net effect is to reduce plant vigour, production, persistence and N-fixing ability, but cultivars display differential resistance. Information on extent and specific effects of viruses is sparser than that for fungal diseases.

Pests.

Stem eelworm (Ditylenchus dipsaci), which invades and lives within the plant, is the major damaging pest. Damage occurs in patches which progressively increase in size with time and eventually merge until the whole field is affected. Dormant eelworm cysts on clover seed and infected plant debris on forage machinery aid the spread of the pest. Pre-fumigation with methyl bromide of seed to be sown and cleanliness of equipment when handling crops are advisable anti-infection procedures.

Root lesion nematodes (Pratylenchus spp.), root-knot nematodes (Meloidogyne spp.) and the clover-cyst nematode (Heterodera trifolii) are plant-damaging pests of importance in some regions, e.g. North America (Leath, l985). Breeding cultivars with resistance to nematode attack is the main means of alleviating the damaging effect of these pests. Slugs such as the netted slug (Derocera reticulatum) and weevils (Sitona spp.) feed on above-ground foliage. Slugs can pose a particular problem when establishing red clover on moist, heavy-textured soils and when direct drilling (sod seeding) clover into an existing sward. Leatherjackets (Tipula spp.) and weevil larvae attack roots and shoot bases. Control by pesticides is available but may only be economic if infestations are beyond specific population thresholds. In seed crops the larvae of the clover seed weevil (Apion trifolii) feed on the seeds following deposition of eggs in the flower buds but insecticide control is available.

Main attributes.

Highly productive, nitrogen-fixing, protein- and mineral-rich species adapted to a wide range of soil and environmental conditions. Has good winter hardiness. Its erect growth habit makes it suitable for hay and silage making. High nutritive value and voluntary intake characteristics lead to good animal performance. Valuable break crop in arable farming and organic farming on account of N- fixation ability and as a source of OM. Major source of honey.

Main shortcomings.

Relatively short-lived species. Unsuited to intensive grazing. Monocultures are prone to weed invasion in early spring because of bare space between dormant plant crowns. Yields decline markedly with ageing of stand. Can be adversely affected by several diseases and pests. May cause bloat in ruminants. Reduces fertility in breeding ewes if grazed during immediate pre-mating and mating periods.

Performance.

Its high nutritive value allied to its high intake characteristics combine to give improved individual animal performance from different classes of animals compared with grass forage (Thomas et al., l98l; Thomas et al., l985).

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Main references.

Frame et al. (l998); Taylor and Quesenberry, l996; Taylor and Smith (l995); Smith et al. (l985).