Research by USDA-ARS scientists in Florence, SC, suggests that Southeastern farmers could use the tropical legume sunn hemp in their crop rotations by harvesting the fast-growing annual for biofuel.

They compared the energy content of sunn hemp with cowpeas – another common regional summer cover crop. The crops were grown in experimental plots and both were harvested on the same day, three times in 2004 and 2006. The last harvest in both years was right after the first killing freeze of the season.

The scientists measured potential thermal energy production of both feedstocks via direct combustion. That provided their “higher heating value,” which indicates how much energy is released via combustion.

In 2004, when there was ample rainfall, the resulting sunn hemp biomass yield exceeded 4.5 tons per acre. That’s equivalent to 82.4 gigajoules of energy per acre – close to the energy contained in 620 gallons of gasoline and well in the ballpark of other bioenergy crops, which have yields of 30-150 gigajoules per acre.

The higher heating value of sunn hemp biomass exceeded that of switchgrass, bermudagrass, reed canarygrass and alfalfa. And although reduced rainfall resulted in lower hemp biomass yields in 2006, sunn hemp’s higher heating value for both study years was 4-5% greater than that of cowpeas.

The researchers believe growing sunn hemp as a cover crop could one day help U.S. farmers meet growing demands for environmentally sustainable biofuel feedstocks. But they say more research is needed, particularly in regard to managing sunn hemp’s content of minerals known to affect biofuel production.

The work was done at the Coastal Plains Soil, Water and Plant Research Center in Florence.