Continue to watch for prussic-acid poisoning in alternative forages grown as a result of this summer’s drought and forage shortfalls, suggests Mark Sulc, Ohio State University (OSU) Extension forage specialist.

Sudangrass and its hybrids, forage sorghums and sorghum-sudangrass crosses can produce good forage yields rapidly. But they also contain compounds called cyanogenic glucosides, which convert quickly to prussic acid in plant tissue damaged by frost.

“Animals can die within minutes,” says Sulc, if they feed on frost-damaged forages containing high levels of prussic acid. “Ruminants are more susceptible to prussic-acid poisoning than horses or swine because cud chewing and rumen bacteria help release the cyanide from plant tissue.”

Prussic-acid poisoning interferes with oxygen transfer in the bloodstream of the animal, causing it to die of asphyxiation. Symptoms include staggering, labored breathing, spasms, foaming at the mouth and convulsions.

Parts of Ohio and Indiana already have experienced frost, says Jim Noel, with the National Weather Service. Noel's weather updates are featured in the OSU Agronomic Crops Team's weekly C.O.R.N. Newsletter.

To reduce the risk of poisoning, Sulc suggests making hay and silage from these forages soon after a frost. Prussic-acid levels decrease during wilting and hay drying. But hay or silage not properly cured and dried before baling or ensiling should be tested for prussic acid before being fed to livestock.

“Because prussic acid is a gas, the longer the gas has to dissipate out of the plant, the less it is dangerous for livestock.”

To avoid prussic-acid poisoning, try the following:

• Don't graze on nights when frost is likely. High levels of the toxin are produced within hours after a frost.

• After a killing frost, don’t graze animals until plants are dry, usually within five to seven days.

• After a non-killing frost, wait two weeks before grazing the crop because of the potentially high toxic levels.

• New growth may appear at the base of plants after a non-killing frost. Don’t graze that growth until 10-14 days after a hard, killing freeze.

• Don't let hungry or stressed animals graze young growth of species that may contain high levels of prussic acid.

• Graze or greenchop sudangrass only after it is 18” tall. Sorghum-sudangrass should be 30” tall before grazing. Never graze immature growth.

• Don't graze wilted plants or plants with young tillers.

• Greenchop frost-damaged plants. It lowers the risk compared with grazing directly because animals will be less likely to selectively graze damaged tissue. But the forage can still be toxic, so feed greenchop with great caution.

• Feed greenchop within a few hours; don't leave it in wagons or feedbunks overnight.