1864

4: Forages

Crotalaria juncea L.

Sp. Pl. 2: 714 (1753).

LEGUMINOSAE

2n = 16

Sunn hemp, Indian hemp, Madras hemp (En). Chanvre indien (Fr). Indonesia: orok-orok lembut (Java). Philippines: putok-putukan (Tagalog), karay-kagay (Bikol). Cambodia: kâk'tung. Laos: po: th'üang, thwax chu:b. Thailand: po-thuang. Vietnam: suc sat, luc lac.

Sunn hemp originates from India but is now widely grown throughout the tropics and subtropics.

The major significance of sunn hemp lies in its valuable fibre which is extracted from the bark and used to make twine and cord, canvas, fishing nets, etc., and also to make paper and pulp. Moreover, sunn hemp is commonly used as forage, as green manure, and as a cover crop. It is one of the most widely grown green manures throughout the tropics. 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. In Indo-China it is used medicinally to treat urticaria.

Although sunn hemp contains poisonous glucosides, it is widely used as forage. The presence of compounds which cause unpalatability and/or are poisonous under some conditions is typical of the genus Crotalaria L. 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.
Sunn hemp fibre has greater tensile strength and is more durable under exposure than jute. It is not as strong as hemp (Cannabis sativa L.).

An erect, laxly branched annual, up to 3.5 m tall, with long strong taproot, many lateral roots and numerous irregularly branched and lobed nodules. Stem ribbed, pubescent, up to 2 cm in diameter. Leaves simple; blade oblong-lanceolate, 4—13 cm x 0.5—3 cm, pilose; petiole up to 0.5 cm long. Inflorescence a lax terminal raceme, up to 25 cm long; flowers yellow, showy; sepals 5, hairy; standard erect, suborbicular, ca. 2.5 cm in diameter. Pod cylindrical, 3—6 cm x 1—2 cm, tomentose, light brown, containing ca. 6 seeds. Seed heart-shaped, with narrow end strongly incurved, up to 6 mm long, dark brown to black.
It is very fast-growing. Normally multibranched, it may have a single stem up to 3.5 m tall in dense stands. It has vigorous lateral roots and a long taproot that can exploit deeply stored soil moisture. Sunn hemp is a short-day plant and long daylengths favour vegetative growth and reduce seed-set, although daylength neutral selections exist. Extensive cross-pollination occurs.

Sunn hemp is drought resistant and is adapted to hot, semi-arid and arid areas, yet can tolerate light frosts. It grows on poor soils, but growth on such soils is improved by fertilization. It is not tolerant of salt, nor of sustained waterlogging.
For fibre production, light, loamy, well-drained soils are preferred; on low-lying clay soils it makes vigorous growth, but the fibre is coarser and yields are lower.

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. Seedlings emerge 3 days after sowing, and rapidly produce a thick ground cover that smothers weeds.
Sunn hemp is attacked by many diseases and pests, including viruses, fungi, insects and nematodes, but they usually cause little economic damage. In India, wilt and caterpillar larvae of the moth Utetheisa pulchella can be serious. Pod-boring insects can reduce seed production. Beetles of the genus Exora can sometimes cause serious defoliation. Damage from insects is more severe if crops are planted in the same area for more than 3 consecutive years.
It can be harvested by hand or by machine. Total green matter yields average from 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. 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.
Leaves and stems are dried since animals do not eat sunn hemp when it is green. Sheep will not suffer any adverse effects if forced to eat dried forage, but will suffer from toxicity if fed large quantities of seed. It should not be fed to horses, and the intake of sunn hemp hay by cattle should be restricted to about 10% of their diet.
When grown for fibre the plants are usually harvested at the flowering stage or when the stems turn yellow. Stems are retted either in concrete tanks or in shallow open water for periods of 4—14 days, depending on the temperature, thickness of stems, and quantity of stems in relation to volume of water. After retting the material is dried, decorticated by machine and the fibre is then stripped from the stems by hand, washed, and hung up to dry in the sun for bleaching. For seed production the crop is harvested when the seeds are ripe; fibre is extracted from the stems afterwards.

There are no significant forage breeding programmes with sunn hemp, and few cultivars with predictable qualities are available. Cultivar 'Tropic Sun', of unknown origin, was selected in Hawaii as a green manure. Seeds and forage were found to be nontoxic in feeding trials. In India 'Kanpur 14' is the most common cultivar, and 'T6' flowers in 30 days after sowing. Existing lines of sunn hemp are usually variable, and it would appear there is scope for selecting in specific regions for specific objectives such as disease and pest resistance and yield of fibre or forage.

Sunn hemp is a fast growing species, tolerant of low fertility and of soil moisture stress. Since the crop is extensively cultivated for fibre or green manure, there is good potential for using sunn hemp foliage as a high protein source to supplement other feeds. It is also showing promise as a forage intercrop. Research is warranted on selection of improved lines and on clarifying and overcoming problems associated with its toxicity.

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Bogdan, A.V., 1977. Tropical pasture and fodder plants. Longman, London. p. 338.
Carangal, V.R., Tengco, P.L., Miah, N.I. & Topark Ngarm, A., 1990. Food and forage crop intercropping under rainfed lowland conditions. Proceedings Crop-Animal Systems Research Workshop, Serdang, Malaysia, August 15-19, 1988. MARDI/IDRC/ARFSN. pp. 435-451.
Duke, J.A., 1981. Handbook of legumes of world economic importance. Plenum Press, New York & London. pp. 63-66.
National Academy of Sciences, 1979. Tropical legumes: resources for the future. Washington, D.C. pp. 272-278.
White, G.A. & Haun, J.R., 1965. Growing Crotalaria juncea, a multi-purpose legume, for paper pulp. Economic Botany 19: 175-183.
Whyte, R.O., Nilsson-Leissner, G. & Trumble, M.C., 1953. Legumes of agriculture. FAO, Rome. p. 265.
Yost, R. & Evans, D., 1986. Green manure and legume covers in the tropics. Research Series 055. College of Tropical Agriculture and Human Resources, University of Hawaii. pp. 16-17.

Y.K. Chee & C.P. Chen

Chee, Y.K. & Chen, C.P., 1992. Crotalaria juncea L.. In: Mannetje, L.'t and Jones, R.M. (Editors): Plant Resources of South-East Asia No 4: Forages. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea