Sunn hemp is a warm-season legume that producers are adding to their grazing and pasture management plans. Traditionally, producers do not use sunn hemp because of limited seed availability. Now, newer varieties, such as AU Golden and AU Darbin, are available for producers. These new varieties allow the plant to produce seed in moderate climates.
Sunn hemp is a drought tolerant annual, and therefore producers would plant it each year. It can grow in soil with moderately low pH levels, ranging from 5.0 to 8.4. While sunn hemp tolerates low soil fertility, fertile soils greatly enhance its productivity. Sunn hemp grows best in sandy, well-drained soils. It is not tolerant of standing water or heavy, clay soils.
Planting Sunn Hemp
Dr. Leanne Dillard, an Alabama Extension forage specialist, said that soil temperatures must be fairly warm to plant sunn hemp.
“Once soil temperatures reach 65°F, producers can plant sunn hemp into a prepared seedbed,” Dillard said. “Producers should plant at a rate of 25 to 30 pounds of pure, live seed per acre, and inoculate with a cowpea type inoculant. Seeding depth is between 1/4 of an inch to 1 inch.”
According to Dillard, because it is a legume and because of its wide tolerance of soil pH levels, adding nitrogen fertilizer and lime is not necessary.
“Because sunn hemp is a legume, it does not require nitrogen fertilizer. While adding lime is not mandatory, it is, however, recommended,” Dillard said. “Producers should add phosphorus and potash based on soil test results.”
After planting, producers will see little above ground biomass production. However, by Day 60, plants can be 6 feet tall. To maximize the length of the grazing season, producers can stagger sunn hemp plantings (May and July). This allows for a higher quality forage throughout the grazing season and into early fall.
Cattle, goats and sheep are the only livestock that can graze sunn hemp. Livestock can start grazing approximately 45 days after planting, when the plants reach 1 ½ to 3 feet tall.
Dillard said that the leaves are high in nutrition, and while the stems are of much lower quality, they provide the fiber needed for proper rumen function.
“To maintain forage quality, maximizing the leaf-to-stem ratio is important,” Dillard said. “A field allowed to grow until flowering may lose lower leaves and have reduced forage quality. To ensure forage quality, early grazing is important.”
At first, livestock may not find sunn hemp palatable, but within a few days they will develop a taste for it. Because of the high nutritional value, it works well in limit grazing plans in combination with warm-season perennial pastures. In this instance, producers should allow their livestock to graze one to three hours per day. Once the forage reaches 12 to 18 inches, livestock should rotate out of the field.
Dillard said that there are several things that affect the growth of sunn hemp.
“If sunn hemp is grazed too early, livestock will overgraze, possibly killing it. If plants are grazed too high, the livestock will break the plants and it will not regrow,” Dillard said. “Mowing or grazing sunn hemp to less than 12 inches can also prevent plant regrowth. Cutting it for hay or silage is not recomended.”
Possible Toxicity Concerns
While there is no evidence of the leaves nor the stems being toxic, the seeds could pose a chance for livestock poisoning. Sunn hemp is in the genus Crotalaria, which is characterized by presence of pyrrolizidine alkaloids in the seeds. It contains only low levels of two to three different alkaloids.
Non ruminants are more susceptible than ruminants to acute toxicity from ingesting seeds. The consumption of a small amount of seeds, however, will not cause toxicity in livestock. Because of weight loss concerns, producers should not incorporate sunn hemp seeds into an animal’s diet.
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