Sunn Hemp: Speediest Cover Crop?

By Don Comis
January 3, 1997

“Tropic Sun”, a variety of sunn hemp, grows to its full 6-foot height in 10 weeks, compared to 7 months for other cover-crop rivals such as hairy vetch and crimson clover.

Tests by scientists at the Agricultural Research Service show that sunn hemp, Crotalaria juncea--not to be confused with marijuana, Cannabis sativa--can be planted in the South immediately after a corn harvest. Then, it can quickly provide almost total ground protection against fall and winter rains. Other cover crops such as hairy vetch and crimson clover take 7 months to provide similar protection.

“Tropic Sun” hemp has proven useful as a forage for livestock, providing high levels of protein in its leaves when other pastures are normally in a lull.

The “Tropic Sun” variety makes as much nitrogen fertilizer--up to 120 pounds per acre--as slower growing legume cover crops. That leaves some nitrogen for next year’s corn crop but increases the possibility of nitrogen losses to groundwater over winter. Scientists are trying to work winter wheat into the corn-hemp rotation to use up the excess nitrogen, perhaps giving wheat a free ride with no additional fertilizer needed.

USDA’s Natural Resources Conservation Service released “Tropic Sun” seed for potential development as a green manure/cover crop. The sunn hemp it was bred from has been used for centuries as a green manure crop elsewhere, primarily in Southeast Asia.

People in India make cloth from sunn hemp fibers. ARS researchers in Weslaco, Texas, are investigating sunn hemp as a fiber crop similar to kenaf, which can be used to make a variety of products including paper. Weslaco researchers are testing the possibility of producing sunn hemp for paper and for a supplement for peat moss used to grow nursery plants.