Rhynchosia minima (L.) DC

Leguminosae

Synonyms

Dolichos minimus L.; Glycine rhombea Schum. et Thonn.

Rhynchosia (Australia); least rhynchosia (United States); burn mouth vine (Barbados).

A glabrous to somewhat pubescent, perennial, twining or sub-erect legume. Stems slender, numerous, 80 to 120 cm long. Stipules 2 to 3 mm long, lanceolate. Leaves trifoliate. Leaflets rhomboid, ovate or sub-orbicular to broad-acute, apex acute or rounded, 0.5 to 3 cm long x 0.5 to 3 cm wide, glabrescent to velvety. Inflorescence a lax 6- to 12-flowered raceme, 5 to 10 cm long. Calyx 3 to 4 mm long; five acuminate lobes. Corolla yellow, 1 cm long. Standard 5 to 7 mm long. Keel as long as the standard, wings shorter. Pods 1 to 1.5 cm long x 0.4 to 0.6 cm wide, black when ripe, two seeded, oblong, finely pubescent, slightly impressed between seeds, shortly beaked. Seeds monocoloured, black or brown, c. 3 mm long, with a short hilus.
There are numerous varieties. Those in south-eastern Queensland are var. minima pods with short fine hairs only, and var. australis (Benth.) C. Moore (syn. R. australis Benth.) with short fine hairs and long tubercular-based hairs on the pods (Stanley and Ross, 1983; Andrews, 1952; Pulle, 1976; Gooding, Loveless and Proctor, 1965).

Rhynchosia is an almost cosmopolitan plant on the heavier textured soils of the tropics and subtropics. It is widespread in the Sudan and East Africa, and is also prominent from the Syrian Arab Republic to India (on black cotton soils in Mysore State) and Nepal. It also occurs from the southern United States (South Carolina to Florida and Texas) to Mexico. Rhynchosia is common in Panama and Venezuela and extends to Argentina. It is also native to all mainland sates of Australia, on heavy clay soils, particularly those high in lime and phosphorus; it also grows in association with Astrebla spp. (Mitchell grasses) and Dichanthium sericium grasslands in the 450- to 750-mm rainfall belt.

Palatability of R. minima appears to vary widely from place to place. It is probable that it varies with the wide range of different ecotypes that exist. Hassell (1945) states it is eaten readily in Queensland when young but becomes rather fibrous and coarse when mature, while Beeston (1978) lists it as highly palatable in the Blackall District of central west Queensland and Boyland (1974) as moderately palatable in the far south-west of Queensland. Bogdan (1949) says its slight scent prevents it from being completely eaten in the flowering stage in Kenya. In Kenya, R. minima is more readily eaten by cattle than sheep (Edwards and Bogdan, 1951). However, Shukla, Ranjhan and Katiyar (1970) showed a daily consumption of 3.5 kg. per 100 kg. bodyweight by rams in India, and considered it palatable to sheep as well. Crude protein was only 15.1 percent and crude fibre 45.9 percent, with 60 percent digestibility of dry matter, 54 percent of crude fibre and 64 percent of crude protein. The animals showed positive balance for N, Ca and P, and gained 0.32 kg. per day. Studies with fistulated sheep at Toorak Research Station, on the northern end of the Queensland Mitchell grasslands, showed that R. minima was an important component of the "other species" in the pasture. These, while rarely exceeding 10 percent of the forage on offer, at times accounted for 60 percent of the diet consumed (Lorimer, 1978).
There are no known toxicities.
In Barbados, R. minima was among the most frequent plants on soils with a salt concentration of 1.6 meq/100 g, a concentration that prevented establishment of 30 other species (Eavis, Cumberbatch and Medford, 1974).
R. minima is a very hardy and widely adapted plant, to date much neglected. Many lines are lowly productive but others grow much more vigorously. The species warrants a thorough examination for pasture cultivars, especially for harsher, heavy-textured soils.