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Medicago truncatula Gaertn. |
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Leguminoseae | Author: John Frame |
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COMMON NAMES Barrel medic, strong-spined medick. DESCRIPTION Procumbent to erect hairy annual. Trifoliate leaves, leaflets cuneate to obovate and serrated, tips mucronate; middle petiolule longer than lateral petiolules. Leaflet markings nil to purple-brown anthocyanin flecks and sometimes yellow or brown blotches on upper parts. Stipules toothed. Tap-rooted. Inflorescences are axillary racemes with 1–4 yellow florets, self-pollinated. Seed pods (burrs) barrel-shaped with 2–6 tight coils mostly wound anti-clockwise and with adpressed straight or slightly curved spines, 1.5–2.5 mm in length. Pods contain 4–12 creamy white sub-reniform seeds; seeds constitute 20–38 percent of pod weight. Number of seed kg-1: 190 000–320 000. Average 1000-seed weight: 3.9 g. DISTRIBUTION Indigenous to Mediterranean Basin countries, especially in West Asia
(Lesins and Lesins, 1979). Notably naturalized in southern Australia,
where accidentally introduced in the nineteenth century. Its spread
there, together with that of other annual medics, was encouraged initially
by the application of superphosphate and then by the breeding and
release of improved cultivars. Annual medics have also been introduced
to other parts of the world with Mediterranean-type climates, e.g.
parts of South Africa and Chile. Australian-bred cultivars have also
been introduced back to the countries of origin of the species (Piano
and Talamucci, 1996). Nevertheless there are breeding programmes in
some of these countries, e.g. for barrel medic in Tunisia. The annual
Medicago group is represented in nature by over 30 species (Lesins
and Lesins, 1979). In their review of breeding annual medics for Australian
conditions, Crawford, Lake and Boyce (1989) refer to 13 important
species: M. aculeata, M. arabica, M. intertexta,
M. littoralis, M. rugosa, M. scutellata,
M. tornata, M. truncatula and M. turbinate. Table 1 The most important Medicago spp. currently used in breeding.
The following description refers to barrel medic as representative of the annual medics. CHARACTERISTICS Barrel medic is adapted to different soil textured types, sandy to clayey, but particularly well drained neutral to alkaline soils, pH 6 to 8. It is intolerant of low soil pH (Evans, Dear and O’Connor, 1990), notably when soluble Al and Mn are present (Little, Chartres and Young, 1992), and intolerant of soils with a low water table since plants not deep rooted. Suited to warm temperate conditions, especially Mediterranean-type climates, 250–600 mm annual rainfall with dry hot summers and mild moist winters. Intolerant of winter frosts (Brandsaeter et al., 2002). Rapid growth rate after autumn seeding or regeneration of plants from soil seed bank after advent of autumn rainfall. There is a range of maturities, with early maturing cultivars being the most suitable for the driest conditions and shortest growing seasons. The life cycle is adapted to escape summer drought. After flowering and producing seeds, plants die off in summer and pods lie on the soil surface or become embedded in the soil. A high proportion of the seeds are hard. It is suitable for the ley part of pasture-cereal rotations. The ley farming system, which originated in Australia, is an integrated system of cereal and livestock production in which the N2-fixing annual legume, e.g. barrel medic or subterranean clover, rotates with the cereal crop and self-regenerates the pasture after the cropping phase (Puckridge and French, 1983). It has shown potential as an emergency forage in northern United States of America when sown in spring and harvested two months later, but has poor regrowth for a second harvest (Shrestha et al., 1998). Ability to spread naturally Good ability on account of its self-regeneration by seeds from the soil seed bank. Compatibility in mixtures Compatible with phalaris (Phalaris aquatica) or annual ryegrass (Lolium rigidum), but most benefit is likely from pure-sown stands. Nitrogen fixation Seed inoculation with an effective and competitive strain of Sinorhizobium meliloti is required at initial seeding to ensure a satisfactory level of N2 fixation. BREEDING Diploid with 2x=2n=16 chromosomes, and self fertile. Barrel medic
has been chosen as a model plant for the study of legume biology on
account of its short life cycle, rich natural diversity, diploid genetics,
small genome and ease of transformation (Colebatch et al.,
2002). Breeding objectives have included improved seedling vigour,
greater forage yield and seed production, greater resistance to pests
and diseases, and reduced oestrogenic potency. Cultivars Officially registered Australian cvs of barrel medic are Akbar, Ascot, Borung, Ghor, Hannaford, Jemalong, Jester, Parabinga, Paraggio and Sephi (all M. truncatula Gaertn. var. truncatula), and Caliph and Mogul (each M. truncatula Gaertn.). Seed production Yields of 336 kg ha-1 are obtained from low rainfall areas, but 2770 kg ha-1 has been obtained under irrigation in southern Australia (Anon., 1972a). Other yields reported include 500–590 kg ha-1 (Anon., 1972b) and 1030–1260500–590 kg ha-1 (Anon., 1982). AGRONOMY Establishment A well cultivated, uniform and firm
seed bed is required for good results when first establishing. Seed
can be broadcast, or drilled at 15–20 mm depth, into a conventionally
prepared seed bed in autumn when there is sufficient soil moisture
for germination. A seed rate of 8–12 kg ha-1 is usual. When
drilled or oversown into native grass swards overlying several soil
types on the northwest slopes of New South Wales, barrel medic established
satisfactorily, but on acidic soils with a pH of less than 6 it did
not persist (Fitzgerald, 1994). It can also be direct drilled into
cereal stubble. Nutrient requirements P and S are critical nutrients and so application of superphosphate proved to be the key to good annual medic performance in the past. For example, the DM yield of barrel medic oversown into native pastures in Queensland, Australia, was increased five-fold by the application of both P and S fertilizer (Clarkson, Swann and Chaplain, 1989). If deficient, mineral micronutrients, e.g. Co and Cu, may be required for the soils in some regions. Weeds Ability to compete with weeds is moderate during establishment. Potential problem perennial weeds should be controlled prior to initial sowing. It tolerates ‘legume-safe’ herbicides, e.g. 2,4–DB, but not less selective types. Pests A number of aphid species can adversely affect the performance of barrel medic (and other annual medics), the main ones being blue-green aphid (Acyrthosiphon kondoi), spotted alfalfa aphid (Therioaphis trifolii f. maculata), and, to a lesser extent, pea aphid (A. pisum) and cowpea aphid (A. craccivora). An indirect consequence of aphid attack is the transmission of viruses. Other pests include red-legged earth mite (Halytodeus destructor), lucerne flea (Sminthurus viridus) and sitona weevils (Sitona discoideus, S. humeralis). Pesticides are available if attacks warrant their application, e.g. perimicarb for blue-green aphid; chlorpyrifos for lucerne flea and red-legged earth mite; fenitrothion for sitona weevil. Some success has been achieved by breeding resistant cultivars, e.g. Mogul and Sephi are resistant to blue-green and spotted alfalfa aphids. All the current barrel medic cultivars are susceptible to lucerne flea and red-legged earth mite. There is resistance to these latter two pests in burr, gama, murca and snail medic cultivars. Diseases For barrel and other medics, diseases have been much less of a problem than pests. Seed-borne black stem (Phorna medicaginis) can cause damage (O’Neill, Bauchan and Samac, 2003), especially when there is above average spring rainfall (Crawford, Lake and Boyce, 1989). Leaf rusts (Uromyces anthyllidis and U. striatus) can reduce hay and seed yields (Crawford, Lake and Boyce, 1989). Diseases are most prevalent in dense, bulked-up stands. Developing resistant cultivars is recognized as the best solution to combat diseases. Viruses Alfalfa mosaic virus infection can cause significant reduction in growth and productivity (Dall, Randles and Francki, 1989). A number of other viruses can also attack the plants. Forage production In Western Australia, DM yields of 4.05–4.96 t ha-1 have been reported (Anon., 1972b). In irrigated mixtures with oats (Avena strigosa) or perennial ryegrass (Lolium perenne), the DM yields of the barrel medic constituent were 4.0 and 4.4 t ha-1, respectively (Lowe and Bowdler, 1988). Early winter DM yields of 1.14–2.91 t ha-1 were reported from South Australia (Anon., 1982). In northern United States of America, barrel medic has proved useful for summer forage when spring sown (Shrestha et al., 1998). In an evaluation of the forage potential of several species of annual medic, both spring- and summer-seeded in north-central United States of America, barrel medic was one of the highest yielding species (Zhu, Sheaffer and Barnes, 1996). In Alaska, DM yields of 3.39–3.84 t ha-1 were obtained when averaged over two N fertilizer rates, 0 and 90 kg ha-1, on a neutral soil; N application increased yields slightly on the neutral soil but on a moderately acid (pH 5.3) soil, N fertilizer decreased the DM yield from 0.72 to 0.63 t ha-1 (Panciera and Sparrow, 1995). NUTRITIVE VALUE It has high nutritive value, and the forage is highly acceptable
to stock and so has high voluntary intake characteristics. Anti-quality factors There is risk of bloat. Some older cultivars have phyto-oestrogenic potency, mainly from coumestans that inhibit ewe fertility. The coumestans are increased when the plants are damaged by fungal diseases or pest attack. UTILIZATION Grazing management A balance has to be struck between efficiency of utilization and allowing the plants to flower and produce seed for subsequent pasture regeneration. Grazing the pods with sheep in summer reduces the number of seeds available for regeneration (Cocks, 1988). Conservation Can be used for hay or silage. The more erect-growing cultivars are better suited for conservation than are decumbent types. ANIMAL PERFORMANCE The high nutritive value and intake characteristics lead to good animal performance. MAIN ATTRIBUTES It is a productive N2-fixing annual adapted to dry environmental (Mediterranean-type) conditions. It has a good ability to self-regenerate from hard seed, with a range of maturity types, and a high degree of resistance to diseases. MAIN SHORTCOMINGS It is susceptible to a range of pests, and unsuited to acidic soils. Sward density and forage yield is variable since it depends on the amount of seed produced for regeneration. MAIN REFERENCES ANONYMOUS (1972a) Medicago truncatula. Gaertn. var. truncatula
(barrel medic) cv. Jemalong. Register of Australian Herbage Plant
Cultivars, 2nd edn. 1972. B. Legumes 9. Annual Medics. |
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