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Sunnhemp: fertilizer, fodder and fibre

Sunnhemp (Crotalaria juncea L.), one of the oldest known fibres of the Indo-Pakistan subcontinent, produces a strong bast fibre from its bark which has greater tensile strength and is more durable under exposure than jute. Sunn fibre is stronger when wet when wet, is also fairly resistant to mildew, moisture and micro-organisms in salt water, and is currently used in twine, rug yarn, cigarette and tissue papers, fishing nets, sacking, canvas and cordage. However, the potential of sunnhemp lies not just in its fibre but also in its use as a green manure and as forage for animals.

Crotalaria, meaning rattle, is indicative of the sound made by the seeds shaken in mature pods. Although the genus comprises over 350 species found throughout the tropics and subtropics, C.juncea is by far the most important and, being a legume, is often grown in rotation with tobacco, vegetables, cereals, cotton, sugarcane, pineapples, coffee and orchard crops. It grows fast and is therefore also effective as a cover crop for smothering weeds. Sunnhemp grown during the rainy season produces high biomass yields (18-27 tonnes/ha) and is used mainly as a green manure because the fibre is not considered of good quality. To produce fibre of the highest quality, sunnhemp must be grown on light, well-drained soils but the plant will also grow on marginal soils. It is also drought resistant and can be used to reduce the build-up of root-knot nematodes.

India, Pakistan and Brazil are the world's largest producers of sunnhemp fibre but the crop is also widely used as forage in parts of Sri Lanka and southern Africa. The presence of alkaloids in C.juncea make the leaves and stems unpalatable when fresh and the seeds poisonous. Sheep are unaffected by dried forage but it should not be fed to horses or pigs and the intake of hay by cattle should be restricted. However, the seeds are said to purify the blood and are also used medicinally in some regions to treat impetigo and psoriasis.

Research into sunnhemp in the US has been conducted since the 1930s but the difficulty in producing seed caused many farmers to abandon the crop. Kenaf has been the focus of more recent research because of its ability consistently to produce higher yields and to resist lodging. However research in Texas is currently investigating the use of the shorter core fibres of sunnhemp as a component of commercial nursery soil-less potting media and the US Development Agency is also re-examining its potential for the paper, plastics and composite board industries.

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