Crotalaria juncea L.
|Author: L.t Mannetje|
Sunn hemp, Indian hemp, Madras hemp (En), Chanvre indien (Fr.)
Origin and geographic distribution
Crotalaria juncea is generally considered to have originated in India, where it has been cultivated since prehistoric times, but is now widely grown throughout the tropics and subtropics.
Sunn hemp is a short-day, erect shrubby annual, generally 1 to 4 m in height. Stems up to 2 cm in diameter, cylindrical and ribbed. Leaves simple, spirally arranged along the stem, oblong-lanceolate, 4-13 cm x 0.5-3 cm, pilose; petiole up to 0.5 cm. Strong taproot, well developed lateral roots. Much branched and lobed nodules, up to 2.5 cm in length. Inflorescence a terminal open raceme to 25 cm in length with deep-yellow flowers, sepals 5, hairy; standard erect, suborbicular, ca. 2.5 cm in diameter. Flowering is indeterminate. Pod cylindrical, 3-6 cm x 1-2 cm, tomentose, light brown, containing ca. 6 seeds. Seeds heart-shaped, with narrow end strongly in-curved, up to 6 mm long, dark brown to black. Due to cultivar and environment, seed weight is highly variable, ranging from 18,000 to 35,000 per kg (Chee and Chen 1992).
Sun hemp is extensively cultivated for fibre or green manure and leaves are fed as a high protein supplement to other poorer feeds. In Sri Lanka dried leaves, bark and boiled seeds are fed to cattle. With restrictions, seed has been used as fodder in the former Soviet Union and southern Africa. It is showing promise as a forage legume for intercropping with upland rice. Leaves and stems are dried since animals do not eat sunn hemp when it is green. Sunn hemp should be cut for hay or ploughed in for green manure in the early flowering stage when it is 1.5-2.5 months old. Due to the shade of its dense canopy it is also used as a cover crop to suppress weed populations.
The presence of pyrrolizidine alkaloids is typical of the genus Crotalaria. Nitrogen concentrations of about 3% in hay and 5-10% in seeds have been reported from the former Soviet Union, but normally they are lower. Seeds may contain about 40% starch while stems contain about 40% fibre. There are about 33 seeds/g (Chee and Chen 1992).
Sunn hemp contains pyrrolizidine alkaloids, which are converted by oxidases or cytochrome P450 system of the liver into potent toxins (Mattocks 1978). Reddy et al. (1999) concluded that, despite its toxicity, sunn hemp hay can be safely incorporated at up to 45% level in rations of sheep under an intensive feeding system. Sheep will not suffer any adverse effects if forced to eat dried forage, but will suffer from toxicity if fed large quantities of seed. Sunn hemp should not be fed to horses, and the intake of hay by cattle should be restricted to about 10% of their diet.
Sunn hemp is drought resistant and is adapted to hot, semi-arid and arid areas, yet can tolerate light frosts. It is not tolerant of salt, nor of sustained waterlogging. It is photoperiod-sensitive and flowering occurs in response to short days; long daylengths favour vegetative growth and reduce seed-set, although daylength neutral selections exist.
Sunn hemp has a wide range of adaptation to soil types. It grows on poor soils, but growth on such soils is improved by fertilization. For fibre production, light, loamy, well-drained soils are preferred; on low-lying clay soils it makes vigorous growth, but then the fibre is coarser and yields are lower.
Propagation and planting
Sunn hemp is established by seed. Sowing rates of 40-45 kg/ha are used when it is sown as a forage crop or as green manure, but when it is sown for fibre, rates of 100-240 kg/ha are used.
It nodulates readily with native cowpea type rhizobia.
Growth and development
Sunn hemp is fast growing. Seedlings emerge 3 days after sowing, and rapidly produce a thick ground cover that smothers weeds. Extensive cross-pollination occurs in sunn hemp and self-pollination takes place after the stigmatic surface has been insect or mechanically stimulated.
Diseases and pests
Sunn hemp is attacked by many diseases and pests, including viruses, fungi, insects and nematodes, but they usually cause little economic damage. In India, anthracnose, caused by Colletotrichum curvatum, wilt caused by Fusarium udam and caterpillar larvae of the moth Utetheisa pulchella can be serious. Pod-boring insects can reduce seed production. However, Dempsey (1975) reported that significant shoot borer resistance has been identified. Beetles of the genus Exora can sometimes cause serious defoliation. Dey et al. (1990) reported that sources of resistance to anthracnose have been identified, indicating the potential to reduce disease losses through the development of resistant varieties. Germplasm possessing resistance to the fungus C. fimbriata has also been reported (Ribeiro et al. 1977). Damage from insects is more severe if crops are planted in the same area for more than 3 consecutive years. Fungicide seed treatments and crop rotations are the most recommended and practiced disease control measures.
Total green matter yields average 18-27 t/ha with forage yields ranging from 5-19 t/ha. When sown as a green manure crop after rice in Thailand, sunn hemp yielded 2 t/ha of high quality DM in 6-8 weeks. When grown for forage it can be harvested 4 times, starting 6-8 weeks after sowing and subsequently every 4 weeks. Seed yield of 1.13 t/ha has been reported.