Forage 2022: SUNN HEMP: From the tropics to Wyoming

Skyrocketing prices of fertilizer and the need to produce more feed in a limited growing season are leading producers to wonder if alternative plant species are right for their operation.  

Sunn hemp originated in India, where it has been grown since the dawn of agriculture. It has been used as livestock feed, green manure, and as a non-wood fiber crop. It has also been grown in Bangladesh and Brazil as a soil-improving crop. Sunn hemp (Crotalaria juncea l) and hemp (Cannabis sativa) have both been cultivated historically for fiber production, so both are called “hemp.” Sunn hemp is a legume, and hemp is a non-legume from China. Sunn hemp does not produce cannabinoids, according to the University of Florida Extension. 

According to a USDA agronomy technical note about Sunn hemp, research with Sunn hemp has been conducted in the United States since the 1930s. The crop was reported to be excellent at improving soil conditions; however, suitable areas for seed production were limited to mostly southern Texas. The difficulty in producing seed in other parts of the United States caused many farmers to abandon growing the crop.  

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More recently, Sunn hemp has been used as a cover crop that increases organic matter, provides nitrogen, grows in low fertility sandy soils, and does not harbor nematodes, giving it the designation of “green manure.” Sunn hemp, when grown as a summer annual, can produce over 5,000 pounds of biomass and over 100 pounds of nitrogen per acre. Farmers need only eight to 12 weeks of frost-free growth conditions to receive these results. 

Sunn hemp, because of its rapid growth and relatively short growing season requirement, can be an excellent choice as a winter cover crop or green manure to improve soil health and reduce erosion when planted after the cash crop harvest. Where conditions are favorable, it can provide the benefits of a winter legume prior to a killing frost in the fall, and also in the summer after the winter crop has been harvested. Sunn hemp is adapted to a wide range of soils and performs better on poor sandy soils than most crops. It grows best on well-drained soils with a pH from 5 to 7.5. 

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Sunn hemp has been used extensively as a soil improvement or green manure crop in the tropics because of its ability to produce large amounts of biomass in as little as 60 to 90 days. Because of this, it has the potential to build organic matter levels and sequester carbon. Also, as a legume it can fix large amounts of nitrogen.   

Forage and cover crop specialist Alex Guttormsson for Milborn Seeds in Brookings, South Dakota said, “We are always looking for alternative sources of nitrogen and legumes fix nitrogen (from the atmosphere). When the plant dies it breaks down and nitrogen leaches into the soil. We are always looking for ways to improve soil health, reduce tillage and soil diversity through different root systems. Sunn hemp has more of a tap root system while grasses have a fibrous root system. Different root systems help the soil microbes and make the soil healthier.” 

“It is a warm season plant and a legume, other than soybeans, that likes warm temperatures, but it does need moisture to grow. It is a plant that would thrive on pivot and as a rotational crop. It is also a little bit larger seed so can be a little trickier to set up the drill properly. Legume seeds are also more expensive,” Guttormsson said. 

Guttormsson doesn’t recommend planting straight Sunn hemp. “I like a cane, millet, turnip, radish and Sunn hemp mix. The mixes can be tweaked to the desired price point.” 

Sunn hemp is a broadleaf, so could be challenging to dry as a hay crop. Guttormsson said it would be better grazed or left in a cut swath for swath grazing. 

Carrie Eberle, agronomy and cropping systems specialist with the University of Wyoming Extension, has been growing Sunn hemp on a trial basis in Wyoming since 2016. In 2021, she, along with fellow researcher, Lauren Shortnacy, published a study about what they’d discovered in the five years they’d been growing the tropical crop in the semi-arid West.  

“Overall, Wyoming-grown Sunn hemp (Crotalaria Juncea L) produced a high-nutritive value feed with biomass accumulations in the range of those previously reported. Sunn hemp has potential to be a valuable crop for the northern high plains and its ability to maintain biomass accumulation and nutritive value under rain fed conditions is promising for the region. Future studies on irrigation management, field establishment, harvest time, and livestock feeding will be essential to determine the best management practices for sunn hemp for the northern high plains.”