Sunn hemp a nitrogen source?

With cotton prices below 50 cents a pound for much of the growing season, a brief presentation by USDA-ARS researcher Phil Bauer drew plenty of attention from South Carolina cotton farmers.

Bauer showed the group two 10-foot tall stalks of sunn hemp. Growers looked with little interest until he mentioned the gangly plants produce 200 pounds of nitrogen per acre. The USDA researcher has been growing the green manure crop since 2004.

Cotton growers are not alone in their search for lower fertilizer input costs, especially nitrogen. Volatile nitrogen prices over the past two years, combined with uncertainties over using municipal waste and poultry litter, has led many growers to seek more natural sources.

Sunn hemp was planted in July of 2004 at the Pee Dee Agricultural Research and Education Center in Florence, S.C. It is a perennial, tropical legume that grows rapidly. It was harvested after the first frost of the season, producing over 200 pounds of nitrogen that first year.

“We are looking at sunn hemp primarily as a cover crop. It is a tropical legume that is an annual, which we think is a good thing because it fits in well with crop rotations in the Southeast. It has to be grown in the summer, perhaps after tobacco or maybe early corn. Or, you can plant it after wheat and have the whole summer to grow it,” Bauer says.

A problem is that it won’t go to seed here and seed costs are high, the USDA researcher says. At 50 pounds of seed per acre, the cost is well over $100 per acre, he adds.

He planted the variety Tropic Sun, which is non-toxic to cattle. Other varieties have shown toxicity to animals, so growers should be aware of potential problems there, Bauer points out.

“We are looking at several locations to grow seed. Puerto Rico looks like the closest site to the U.S. Sunn hemp doesn’t flower in South Carolina until early September, then the frost gets it before it sets seed,” Bauer says.

Sunn hemp has also been grown for biomass. It has more energy than switchgrass and being a perennial, growers in the Southeast can produce three times the biomass of other crops, he notes.

Bauer planted a new crop of sunn hemp at the research station in late May of 2009. The samples he showed cotton farmers in mid-August were already over eight feet tall.

Though Bauer let his crop of sunn hemp go full season, it can produce well over 100 pounds of nitrogen in 60 days. It can produce organic matter yields of as much as three tons per acre of air-dried weight within 60 days if growing conditions are favorable. In addition, it is resistant to root-knot nematodes (Meloidogyne spp.)

Bauer says the crop is highly adapted to the Southeastern U.S. It is adapted to well-drained soils that range from coarse to fine in texture, but not dense clay. It will grow in infertile as well as fertile soils with a pH from 5 to 7.5.

Sunn hemp seed should be broadcast and covered or drilled about one half to 1 inch deep in a well-prepared, weed-free seedbed. A soil test should be taken, and fertilizer and other soil amendments applied as recommended. If growers can’t or won’t soil test, a standard treatment is 100 to 150 pounds per acre of P2O5 applied as superphosphate, 0-20-20 fertilizer or the equivalent.

Sunn hemp is fairly drought tolerant, but needs at least an acre inch of moisture for optimum growth. Though Bauer let his crop grow until frost, optimum usage for nitrogen may come from plowing the crop under after 60 days or so of growth.

At this growth stage, nitrogen content is high and decomposition is rapid. If allowed to grow too tall or become over mature, sunn hemp becomes fibrous and will be difficult to turn under. If soil moisture is at the correct level and the plants are succulent, sunn hemp can be plowed (a single plow will usually do a better job than multiple plows), rotor-tilled with a rear-mounted tiller, or disked with a heavy disk. Mowing before turning under may be beneficial, especially if the crop is over four feet tall.

In tests in the Florida Panhandle sunn hemp has produced over two tons per acre of dry biomass and up to 125 pounds of nitrogen.

In Alabama, USDA Researcher Wayne Reeves grew sunn hemp for 9-12 weeks in the summer. His crops produced over 2.5 tons of biomass and over a hundred pounds of nitrogen.

A primary focus for researchers in recent years has been to use sunn hemp as a nitrogen source for cotton growers. If recent interest at the Pee Dee Research Station is an indication, sunn hemp could find a place among Carolina cotton growers.

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