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Sunn Hemp: Forage and Soil-Building Superhero

By Anne C. Randle

Sunn hemp, a tropical plant primarily grown as a cover crop or green manure, has increased dramatically in popularity over the last decade. Originally from India, it’s easy to understand what makes it so popular among vegetable and row crop farmers in the United States.

Sunn hemp possesses many soil-building traits, including high rates of biomass production — over 20 percent greater than crimson clover and hairy vetch in research trials. It is not only resistant to plant root nematodes but actively suppresses them. In as little as 60 to 90 days it can produce 120 pounds of nitrogen per acre and can suppress weeds up to 90 percent.

Sunn Hemp is adapted to a wide variety of soil and environmental conditions, thriving through hot, dry summers and continuing to grow until the first frost. But sunn hemp isn’t just a soil builder — it also offers benefits as a forage producer. Recent on-farm grazing trials have yielded an abundance of information on using this crop for grazing.

Sunn Hemp
Sunn hemp growing in a field.

What is Sunn Hemp?

According to the USDA NRCS Plant Guide, sunn hemp originated in India where it has been grown since the dawn of agriculture. It has been utilized as a green manure, livestock feed and as a non-wood fiber crop. It is a member of the legume family. It is a branched, erect, herbaceous shrubby annual growing 3 to 9 feet high with bright green simple, elliptical leaves.

Sunn Hemp has deep yellow terminal flowers (open raceme to 10 inches long), and the light brown pods are small (1 inch long and a half inch wide) and inflated. It has a well-developed root system with a strong taproot. The number of seeds per pound is 15,000.

Grazing Sunn Hemp

Sunn hemp is highly palatable and recovers quickly from grazing. In its leaves, the neutral detergent fiber (NDF) reaches 22-28 percent, acidic detergent fiber (ADF) 22-27 percent and crude protein 25-30 percent. These numbers rival the nutritive value of other forage legumes, including crimson clover. Its stems are of lower forage quality, so the key to sunn hemp management is grazing it early before the lower leaves begin to drop. Removing the top shoot also promotes branching, which increases leaf production.

Plants can be grazed when they reach 1.5 to 3 feet tall and can be eaten down to within about a foot of the ground without suffering mortality. After four to six weeks, forage quality declines rapidly. As long as animals can still reach its leaves, sunn hemp remains suitable for grazing until flowering.

This crop isn’t without drawbacks, however. It is a member of the Crotalaria genus, notorious for seeds that are high in toxic pyrrolizidine alkaloids. Ingesting the seeds at a high rate can cause damage to the liver, lungs, heart and nervous system. Susceptibility depends on the animal species: pigs are most vulnerable, followed by chickens, horses, cattle and sheep. Goats have the lowest risk.

Although the total content of toxic compounds in sunn hemp is much lower than other Crotalaria species, the alkaloids are still present in amounts that warrant special management.

Plants begin to flower five to six weeks after planting. At this point animals should be removed to avoid exposure to toxic compounds in the seeds, especially if the livestock are prone to grazing seed heads. This isn’t a major loss, as forage quality begins to drop at this point. It also isn’t an issue in northern U.S. climates: because plants are photoperiod-sensitive, flowering in response to shorter days, a killing freeze will usually occur before the plants are able to produce seeds in these areas.

However, one farmer in Alabama found that the plants began to flower while animals were still grazing in the late summer. It’s unclear if sunn hemp was directly responsible for any ill effects on his small ruminants, but it’s not a bad idea to proceed with caution once plants begin to flower. (It should be noted that leaves and stems do not contain any of the toxic alkaloids.)

Growing Considerations

Sunn hemp is easy to grow and amazingly productive. Plant when soils reach above 50°F and at least four to five weeks before frost. Plants will be killed when temperatures dip below 28°F. Optimal soil conditions include a pH between 5 and 7.5 and good drainage. Seed can be treated with cowpea inoculant to increase nitrogen fixation. For forage production, a seeding rate of 30-50 pounds per acre is recommended.

Seed should be drilled at ½-inch depth for best germination. Because plants can reach 4 to 6 feet in height, wide spacing between rows (6 inches is recommended) may make plants susceptible to lodging. With adequate moisture, temperature and fertility, researchers have recorded a growth rate of 1 foot per week. Plants can return to or exceed this growth rate if slowed by temporary drought.

Sunn hemp should be used sparingly, if at all, in mixes with other cover crop species. It has a tendency to hog nutrients and sunlight, suppressing the growth of other plants. The benefit of its growth rate is that it is highly competitive with weeds, even outpacing crabgrass in on-farm trials. Sunn hemp itself has a low potential to become a weed, unlike other Crotolaria species.

Even after grazing, sunn hemp leaves a substantial amount of organic matter in the field, unlike many other forage crops. It may be necessary to cut and chop up the fibrous stems before the pasture can be replanted.

While sunn hemp seed cost has in the past been a barrier to farmers, costs now compete with other legume cover crops, averaging around $70 per 50 pounds. This is slightly higher than current cowpea and crimson clover prices ($50 and $60 respectively) and lower than hairy vetch ($90).

Sunn hemp has the potential to fill an important gap in summer annual grazing. Its hardiness, productivity and palatability make it an option worth considering for farmers looking to build their soil and grow their stock.

This article appeared in the September 2018 issue of Acres U.S.A. magazine. Anne Randle is an Agriculture and Natural Resources Agent with the University of Georgia Extension.