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Trifolium subterraneum L. |
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Leguminosae Author: Dr John Frame Common names. Subterranean clover, subclover. Description. Winter annual in areas with Mediterranean climates; trifoliate leaves, leaflets triangular to obcordate, hairy on both sides. Glabrous to pubescent foliage and stems, different incidence of leaf markings on upper surfaces of leaflets – presence or absence of white, red or brown markings, solid-coloured or flecked – according to ‘strain’. Stipules are small and pointed, varying in hairiness and colour of stripes which range from pale green to red. Leaf markings, particularly the shape and size of the central crescent, and stipule and calyx colouration are used as diagnostic aids to identify strains and cultivars. Anthocyanin markings are unreliable for identifying strains since they vary during the growing season. Prostrate growth habit with branching non-rooting stems up to l-2 m, close to soil surface. Tap root with many lateral branches. Inflorescences, arising from leaf axils, contain 3-6 florets, l-l.5 cm; corolla white, creamy white or pinkish depending on strain. Calyx pigmentation can be green or have a red band covering ¼ to ¾ of the tube depending on the strain, the intensity of colouration varying with the amount of light reaching the calyx. After self-fertilization, peduncles bend over and florets deflex, becoming brown. Several sterile, spiny florets emerge at the end of the peduncle, enclosing 2-4 ripening seeds in the dried-out corolla and calyx. These seed burrs become embedded or buried in the upper layer of the soil during summer for subsequent germination in autumn. Strains differ in their ability to bury burrs but burial is also affected by soil conditions, being greatest in moist, light-textured soils and least in dry, hard soil conditions. Seeds oval, with pronounced hilum. Two other closely related subterranean subspecies are T. subterraneum, subsp. brachycalycinum (Katzn. and Morley) and T. subterraneum, var. yanninicum (Katzn. and Morley) Zoh., (Zohary and Heller, 1984). T. subterraneum is the most widely grown of the 3 subspecies, and does best on well drained, slightly acidic soils. All current cultivars of this species have deep purple to black seeds but some non-commercial strains have cream or amber seeds. T. subterraneum, subsp. brachycalycinum foliage can be glabrous or pubescent and is most readily distinguished from the other two subspecies by its long peduncles which do not actively bury their burrs. This subspecies also has flattened seeds which are only partially enclosed in papery pods. While most brachycalycinums have black seed, the cultivar Rosedale and some non-commercial strains have amber-coloured seed. This subspecies is better adapted to heavier alkaline cracking or self-mulching soils. T. subterraneum, var. yanninicum has glabrous leaves and stems, and all have cream to light brown large seeds, which produce large seedlings. This subspecies is the most tolerant of temporary winter waterlogging and has greater resistance to root diseases. Hereafter, all text refers to T. subterraneum with added remarks on the other two subspecies, as appropriate. Distribution. Native to southern Europe, north Africa and southern England. Widely grown in southern Australia, and, to a lesser extent, southern Latin America, southern Europe, north-west and south-east USA, New Zealand, South Africa, Japan. Characteristics. Adapted to regions with a Mediterranean climate, viz. hot dry summers, moist, mild winters, and annual rainfall, 350-1200 mm; also to warm, temperate areas. In regions with a true Mediterranean rainfall pattern, early maturing strains will grow in areas receiving as little as 250 mm. Life cycle adapted to escape summer drought. Germinates in autumn with main growth during autumn to spring, produces seeds prolifically and remains dormant during summer with the seed burrs embedded or buried in the upper layers of the soil. The soil seed bank maintains large reserves of seed available for seasons of low seed production. Types range from prostrate to more upright, all with a high tolerance to grazing. Can flower and set seed under close grazing. Season of growth. Autumn to spring. Dormant in summer. Temperature for growth. Seed germination rate high at day/night temperatures of 25o/15oC but declines with decreasing temperatures (Evers, l99l) and at high (30oC) temperatures (Silsbury et al., 1984). Fluctuating daily temperatures reduce seed-coat impermeability (hardseededness) thus allowing germination (Taylor, l98l). Frost tolerance and regrowth. Subclover cultivars tolerate winter temperatures down to –4oC but frosts during flowering can reduce seed set. Light. Interception of light by the leaves is complete at a leaf area index (LAI) of about 5 (Morley, l96l). Converts up to 4.5% of incident light into dry matter. Tolerance to shade and N-fixation potential make subclover suitable as an understorey in silvopastoral systems (Bellon, l995). T. brachycalycinum in particular has a high tolerance to shading and its regeneration is aided by the accumulated litter of tree leaves providing a favourable seed bed (Pardini, l992). Drought tolerance. Subclover swards die back during summer drought but regenerate from the soil seed bank following autumn rainfall. Sometimes seeds germinate after summer or early autumn rainfall but then die off if drought re-occurs - this is referred to as a ‘false strike’. Provided the seed bank is adequate, and there is sufficient time and suitable conditions for further hard seed breakdown, the sward will re-establish from hard seed reserves. In these environments it is advantageous to use cultivars with higher levels of hard seed. Small seeds may also be more advantageous than large seeds since they need less water for germination because of a higher ratio of surface to volume (Pecetti and Piano, 1994) Tolerance of flooding. Only T. yanninicum is tolerant of waterlogged soils. Soil requirements. Subclover thrives best under high soil fertility, particularly when there is a high soil P and S status though some cultivars have yielded satisfactorily at low soil pH and soil P status (Chapman et al., l986). Unsuited to soils with a low water table since plants not deep-rooted. Some cultivars are relatively tolerant of high salt concentrations in the soil (Shannon and Noble, l995). T. brachycalycinum is better adapted to heavy-textured and moderately alkaline soils than the other two species. Rhizobial relationships. Colonisation of the nodules is by strains of Rhizobium leguminosarum bv. trifolii. Seed inoculation with effective strains is necessary when sowing on land without a previous recent history of growing subclover. Liming acid soils to raise soil pH and reduce Al ion activity improves nodulation and N-fixation (Unkovitch et al., l996). Ability to spread naturally. Regenerates each year after summer dormancy from the soil seed bank formed in late winter/early spring. Well-buried burrs are larger than unburied burrs and contain more and larger viable seeds. Establishment is also usually higher from well-buried burrs since the seedlings from shallowly-buried or surface-embedded burrs may suffer desiccation. Seed production of circa 600 kg/ha is required for successful pasture regeneration though this may not be achieved if there is drought during seed setting. For example, during a series of annual droughts in southern Australia subsequent seed banks were poor, particularly of soft seed-producing cultivars (Blumenthal and Ison, l993). Grazing of burrs by sheep during summer reduces seed banks. Most ingested seed does not survive passage through the rumen. Seed can be spread in hay or by attaching to sheep fleeces. Land preparation for establishment. Well-cultivated, uniform and firm seed bed required for good results when first establishing subclover. Sowing methods. Can be drilled into a conventionally-prepared seed bed in late summer/early autumn provided there is sufficient moisture for germination. Can also be direct drilled (sod seeded) into existing grassland (McGuire, l985). Sowing depth and cover. Best establishment when sown at 5-10 mm but with a larger seed size than most other forage legumes, it can be sown at a greater depth, l5-20 mm, with a firm soil cover. Sowing time and rate. Sown in early autumn when there is sufficient soil moisture to ensure good seed germination. Seed rate l0-l5 kg/ha in monoculture or 3-6 kg/ha in mixture with grasses. Number of seeds per kg. Ranges from 100 000 to 300 000. Percentage hard seed. Variable but can reach 40% or more. Western Australian classification uses a scale for cultivar seed characteristics of l (little or no hard seed) to l0 (very high, i.e. at least 40% hard seed). Seeds ripened under warm, moist soil conditions have lower proportions of hard seed (Smith, l988; Fairbrother and Lowe, l995). Seed treatment before sowing. Seed is scarified after harvesting to increase germination percentage to at least 70%. Most seed is inoculated and lime-pelleted prior to sowing, particularly when sown into new land. Nutrient requirements. P and S are critical nutrients and application of superphosphate which contains P and S has proved the key to good subclover performance. Liming infertile, acid soils enhanced establishment and subsequent persistence (Moreira et al., l994) though some cultivars have performed satisfactorily at a soil pH of 5.0 and low soil P status (Chapman et al., l986). When a cut of silage or hay is taken, nutrient replenishment, particularly P, S and K is necessary;Mo may be required in small amounts, especially on acidic soils, to ensure good plant growth and N-fixation. Compatibility with other grasses and legumes. Compatible with several grass species e.g. annual ryegrass (Lolium rigidum), perennial ryegrass (Lolium perenne), Bermuda grass (Cynodon dactylon), Harding grass (Phalaris aquatica), the choice of grass companion depending on the environment. Red and/or white clover are sometimes used in mixture with subclover and grass in warm, temperate areas which do not have a strict Mediterranean climate. White clover presence can substantially reduce seed yield and regeneration of subclover (Smith and Crespo, 1979). Ability to compete with weeds. Moderate during establishment but prostrate growth habit and spreading nature of stems in established swards enables subclover to compete aggressively with weeds. Tolerance of herbicides. Tolerates ‘clover-safe’ herbicides such as 2,4-DB, MCPB, benazolin and bentazone types but not less-selective herbicides (Dear et al., 1995; Sandral et al., 1995). Seedling vigour. Moderate to high. Vigour of growth and growth rhythm. Once established, capable of vigorous growth provided there is high soil fertility, adequate water supply from autumn to spring and good sward management. Growth peaks in spring. Sward dries off during summer. Nitrogen-fixing ability. Estimates of annual amounts of N fixed range from 50 to l88 kg/ha in Australia, the highest value being from grazed subclover (Bolger et al., l995; Sanford et al., l995). In south-east USA the range reported was l04 to 206 kg/ha (Brink, l990). Also in the USA, pure-sown Bermuda grass and Bahia grass (Paspalum notatum) required l60-254 kg/ha of fertilizer N to match the yields of swards of the same grasses oversown with subclover. Response to defoliation. Tolerates both continuous stocking and rotational grazing. Has the ability to flower and set seed under grazing. Grazing management. Grazing management must be such that a balance is struck between utilizing forage efficiently yet not so severely that seed production – the basis for annual pasture regeneration – is compromised. In general, prostrate types are suited to continuous stocking and the more upright types to rotational grazing or conservation. Maintaining a sward height of 3-6 cm under continuous stocking has been advocated as optimal for forage and seed production in the USA (Fairbrother et al., l992) though this is a lower sward height than usual in Australia. If grazing is too lax, leaf laminae which are of lower nutritive value than the stems are selectively grazed and so animal performance is depressed (Stockdale et al., l992a). Grazing should also be sufficiently intense to control the competitive growth of companion grasses. Breeding system. Strongly self-fertile; diploid with 2n=2x=l6 chromosomes. Breeding objectives. These include: improved seedling vigour, forage yield and quality, seed production, cold tolerance, and resistance to diseases and pests; lower formononetin content is also an aim so as to avoid reducing ewe reproductive performance. Dry matter yields. Over l6 t/ha has been achieved in experiments during a 5-month growing period (Cocks et al., l980) but actual on-farm yields are substantially less. Rainfall has a major influence and within a DM yield range of 3 to l2 t/ha, there was a linear relationship of yield with rainfall up to 440 mm over the growing season (Bolger et al., l993). Sustained drought in southern Australia restricted DM yields to circa 3 t/ha annually (Blumenthal and Ison, l993). In grass/subclover mixtures subclover yields are best with companion grasses which have a complementary growth rhythm together with a grazing management which favours the legume component (Brink and Fairbrother, l99l; Piano and Talamucci, l996). Because of its prostrate growth habit, cutting techniques to measure yield often underestimate subclover yields since some of the low-growing material is left below the cutting height. Suitability for hay and silage. The more upright-growing cultivars are best suited for conservation as hay or silage. Crops are usually taken in early spring after autumn/winter grazing. Hay making may reduce seed yield and subsequent seedling regeneration. Acceptability. Highly acceptable forage to livestock whether grazed or fed as hay or silage. Has high voluntary intake characteristics. Feeding value. Strongly influenced by growth stage since protein content and forage digestibility decline with advancing maturity while the total fibre content increases. Compared with stems and petioles, leaflets have higher protein and lower fibre contents but lower digestibility (Stockdale, l992; Mulholland et al., l996). Anti-quality factors. Bloat in ruminants is a hazard when lush subclover swards are grazed but this can be countered by conventional preventative measures, e.g. provision of rumen anti-foaming agent such as poloxalene. Oestrogens present in the forage can reduce reproductive performance in ewes, formononetin being the phyto-isoflavone responsible. The disorder is less likely when grazing cultivars with a low formononetin content or mixed grass/subclover swards. Seed harvesting methods. Combine harvesters with a special suction action are used to harvest the seed after soil disturbance to loosen the buried seed burrs. Seed yields. Yields are usually in the range 500 to l000 kg/ha. Australia is the world’s main seed producer. Seed yields benefit from early hard grazing to thin out the canopy. Seed yields peak 74 to 86 days after the onset of flowering (Collins and Quinlivan, 1980). Continued water supply after seed maturation can result in large losses of seed. Spraying herbicide six weeks before flowering to control weeds increased seed yields markedly due to increased seed number (Dear et al., l995). Seed quality standards. For certification in Australia the seed requires to have a minimum of 70% germinable seeds. The maximum % by weight allowed for inert matter plus weed seeds is 2.25, for weed seeds, 0.25 and for other crops seeds, 0.5. Cultivars. Breeding of cultivars has traditionally been concentrated in Australia but there are now also programmes in Italy, Spain, France (Piano and Talamucci, l996) and in New Zealand (Dodds et al., l995). Cultivars vary in time of maturity from early (77 days to flower) to late season (151 days to flower), in oestrogen content and in propensity for producing hard seed. Early-maturing cultivars are required for areas with short growing seasons. Long-established cultivars include Mount Barker, Tallarook, and Seaton Park; later-developed examples are York, Dalkeith, Riverina, Goulburn, Leura and Gosse. Currently subclover (T. subterraneum) is the most widely grown species with 24 cultivars registered in Australia while there are 5 of subspecies yanninicum and 3 of subspecies brachycalycinum. Diseases. In general, subclover is less prone to diseases than most other forage legumes. Phytophthora root rot (Phytophthora clandestina) affects plant survival and yield in dryland and irrigated pastures (Taylor and Greenhalgh, l987). Clover scorch syn. northern anthracnose (Kabatiella caulivora) can cause severe damage. Powdery mildew (Erysiphe polygonii) is relatively common in ungrazed stands of some cultivars in spring. A complex of organisms (Pythium, Fusarium and Rhizoctonia spp.) also cause root rot but are generally secondary invaders following damage by other pathogens and pests. The use of cultivars with resistance to diseases is the best strategy to combat disease since spraying fungicides is rarely economic. Viruses. Plant survival, yield and seed production can be affected by aphid transmission of a number of viruses, e.g. alfalfa mosaic virus, bean yellow mosaic virus, subclover mottle, stunt virus (Jones, l992; Helms et al., l993). Pests. Outbreaks of redlegged earth mites (Halotydeus destructor) can affect seedling survival and seed set. Blue-green aphid (Acyrthosiphon kondai) can damage some cultivars, particularly cv. Daliak and to a lesser extent cvs Dalkeith and Trikkala. Main attributes. Adapted to a range of dry environmental and edaphic conditions. Ability to self-regenerate annually in autumn from seed set and buried in the soil before summer drought. Highly tolerant to grazing. High voluntary intake and nutritive value characteristics. High degree of resistance to diseases and pests. Nitrogen-fixation ability makes subclover a valuable pivotal crop in pasture-arable crop rotations. Main shortcomings. Sward density and forage yield variable from year to year partly because of dependence on amount of seed produced. May cause bloat in ruminants. Oestrogen content may reduce fertility of breeding ewes if grazed during immediate pre-mating and mating periods. Performance. Good lamb performance achievable especially when stems and petioles form a high proportion of the diet (Mulholland et al., l996). Dairy cow performance is also satisfactory, e.g. daily DM intakes of 22 kg/cow and milk yields of 28 kg/cow reported from grazed subclover pastures (Stockdale et al., l992b). Links:
Main references. Frame et al., (l998); Hoveland and Evers (l995); McGuire (l985). |