According to USDA-NRCS, sunn hemp – which is not related to that other kind of hemp – can improve soil properties, reduce soil erosion, conserve soil water and recycle plant nutrients.

When it’s grown as a summer annual, sunn hemp can produce over 5,000 lbs. of biomass and 100 lbs. of nitrogen/acre. And it can produce these amounts within 60 to 90 days, so it has the potential to quickly build organic matter levels and sequester carbon in the soil. It’s also known to suppress harmful nematodes.

Because of its superior biomass accumulation and nutritive value, there is interest from producers to use sunn hemp as forage for livestock – but is it possible?

Looking at the use of sunn hemp as forage for early-weaned beef calves recently were J.V.S. Lazarin, J.M.B. Vendramini, H.M. de Oliveira and P. Moriel of the University of Florida’s Institute of Food & Agricultural Sciences (IFAS); J. Garzon of the University of Maine; and D. Cook and D. Gardner with Poisonous Plant Research, USDA-ARS, Logan, Utah. Their findings were presented at the American Forage & Grassland Council meeting earlier this year.

A legume, sunn hemp is originally from India. It’s been utilized as a green manure, livestock feed and as a non-wood fiber crop, according to USDA.

Sunn hemp is adapted to a wide range of soils and performs better on poor sandy soils than many other crops. It tends to grow best on well-drained soils with a pH from 5.0 to 7.5.

Is there a bright side to using sunn hemp as a cover crop?

Sunn hemp and sorghum/sudangrass grown in mixture at five weeks after planting (left) and mature sunn hemp and sorghum sudangrass roots (right). Photos courtesy of Stacy Swartz and Daniel Calzadilla

However, sunn hemp contains pyrrolizidine alkaloids (PAs), which can affect animal performance. The objective of this study was to evaluate the effects of feeding sunn hemp hay to early-weaned beef calves in a dry lot.

The experiment took place at the IFAS Range Cattle Research & Education Center in Ona, FL, from April to May 2021 over the course of 42 days. Beef calves were fed either stargrass or sunn hemp hay, and body weight, blood and feces samples were collected at the beginning and the end of the trial.

The end results showed that stargrass performed a bit better, with calves showing an average daily gain of 1.1 lbs. vs. 0.9 lbs. with sunn hemp. The forage dry matter intake as a percentage of bodyweight was 1.2% vs. 1% (stargrass being the larger amount). There were no PAs in stargrass; calves ingested about 0.25 grams of PAs each day with sunn hemp. (Notably, though, there was no detectable amount of PAs in their feces.)

When looking at other data, though, the parasite egg counts in stargrass-fed calves were nearly double those fed sunn hemp. Coccidia egg counts were higher with stargrass as well. Plasma cortisol – a stress hormone – was higher in calves ingesting sunn hemp, however.

In general, those early-weaned calves receiving sunn hemp had decreased forage and total dry matter intake and tended to have less ADG – not great for those raising beef cattle. The presence of PAs in sunn hemp may be the cause of the calves’ decreased forage intake and animal performance.

That’s not to say sunn hemp is harmful – it’s just not as useful as other legume cover crops.

The University of Florida reported, “Although sunn hemp is of tropical origin, it has been cultivated in temperate zones as far north as Washington State. It is recommended for planting in the warm season in USDA plant hardiness zones 8 – 13. Southern varieties such as ‘Tropic Sun’ may be less tolerant to northern climates than northern varieties such as ‘AU Golden’ and ‘Ubon.’”

If you’re willing to experiment with your cover crops and want more information on using sunn hemp, visit edis.ifas.ufl.edu/publication/AG443.

by Courtney Llewellyn