The first few frosts of fall bring the potential for prussic-acid poisoning when certain forages are grazed or fed to livestock, warns Steve Barnhart, Iowa State University Extension forage agronomist. He says sorghums and closely related species contain cyanogenic glucosides, which are converted quickly to prussic acid in freeze-damaged plant tissue.

“Historically in Iowa, there are very few documented cases of prussic-acid poisoning,” says Barnhart. “However, the risk is present and good management practices are necessary to minimize the risk.”

Prussic acid is a cyanide compound that can kill animals within minutes of ingestion under the right circumstances, he says. Cyanide interferes with the oxygen-carrying function in the blood, causing animals to die of asphyxiation. Besides breathing difficulties, symptoms include excess salivation, staggering, convulsions and collapse. Affected animals have bright red mucous membranes from the cyanide. Ruminants are more susceptible than horses or swine because they consume large amounts of forage quickly and the rumen bacteria contribute to the release of cyanide from consumed plant tissue.

Sudangrass is low to intermediate in cyanide-poisoning potential, sorghum-sudangrass hybrids and forage sorghums are intermediate to high, and grain sorghum has high-to-very-high poisoning potential. Pearl and foxtail millets have very low levels of the compound.

Prussic acid doesn’t form in plants until the leaf tissue is ruptured, as with grazing or chopping. Young, rapidly growing plants pose the greatest risk because the cyanide-producing compounds are more concentrated in young leaves. Plants growing under high nitrogen levels are more likely to have even higher cyanide potential.

Freezing can cause a rapid change in prussic-acid risk in plants of any age or size. Frozen forage tissues rupture and cyanide gas forms. The gas can be present in dangerously high concentrations within a short time, and remain in the frosted leaves for several days. Because cyanide is a gas, it gradually dissipates as frosted tissues dry. That means frosted sorghums and sudangrasses that are still green pose the greatest risk when being grazed. New growth of sorghum species following frost can be dangerously high in cyanide.

Prussic-acid content decreases dramatically as hay dries and during ensiling. Frosted foliage contains very little prussic acid after it’s completely dry. Likewise, sorghum and sudangrass forage that has undergone silage fermentation is generally safe to feed.

When grazing or greenchopping species with prussic-acid potential this fall, Barnhart suggests following these guidelines:

  • Don’t graze on nights when frost is likely.
  • Immediately after frost, remove the animals until the grass has dried thoroughly. Generally, the forage will be safe to feed after drying five to six days.
  • Don’t graze wilted plants or plants with young tillers or new regrowth. If new shoots develop after a frost, they will have high poisoning potential. Sudangrass should not be grazed until the new growth is at least 18-20” tall (24-30” for sorghum-sudangrass).
  • Don’t allow hungry or stressed animals to graze young growth of species with prussic-acid potential. Greenchopping the frost-damaged plants will lower the risk compared with grazing directly, because animals have less ability to selectively graze damaged tissue; however, the forage can still be toxic, so feed it with great caution. Feed greenchopped forage within a few hours, and don’t leave it in wagons or feedbunks overnight.

When making hay or silage from sorghum species this fall, consider the following:

  • The forage can be mowed anytime after a frost. It’s very rare for dry hay to contain toxic levels of prussic acid. If the hay was not properly cured, it should be tested for prussic-acid content before feeding.
  • Waiting five to seven days after a frost to chop frosted forage for silage will limit prussic-acid risks greatly. Delay feeding silage for eight weeks after ensiling. If the forage likely contained high prussic-acid levels at chopping, hazardous levels might remain, so analyze the silage before feeding.