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 years 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.
USDAs 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
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.
D. Wayne Reeves, ARS-USDA
Soil Dynamics Research Laboratory,
Auburn, Ala., phone (334) 844-4741 Ext. 138, e-mail