Protect Livestock From the Threat of Prussic Acid Lurking in Pastures

— Written By and last updated by Kim Davis

With the approach of cool weather comes the threat of frost. After the first frost of the year, we get questions concerning grazing and the potential for prussic acid poisoning in livestock. Since it seems to be a common topic every year, I’ll go ahead and address it now!

The main grasses in question are the sorghums, sorghum-sudangrass crosses and sudangrass. The greatest danger seems to be after a drought or a series of frost; the grasses with the highest potential for problems are the forage sorghum varieties and less so with the sudangrasses. The type of grass is not the only critical factor; the fertility of your soil may also affect the chances of prussic acid poisoning. Those soils that are high in available nitrogen and low in phosphorous tend to be the most problematic.

A few weeks rest between freezing and grazing reduces the risk of poisoning because the cyanide release levels have time to decrease. When plants grow and become mature, the risk is also reduced since the higher levels are seen in the leaves of the plant, rather than the stalk and stem; times of stress (drought or frost) increase the potential of toxicity in plants, even mature ones. On high risk forage, toxicity will take its toll before you even have a chance to figure out your animals are sick.

Keep an eye on your animals if you suspect the possibility of prussic acid poisoning. Obtain a forage sample for testing if possible. The active compound in these grasses, and the cause for toxicity, is HCN (hydrocyanic acid, i.e. cyanide poisoning). Symptoms of prussic acid poisoning include gasping, staggering, trembling muscles, and possible death from respiratory failure.

Once a frost occurs, and you’re concerned about your animals, take them off the pasture and prevent grazing of the sorghum, sorghum-sudangrass, or sudangrass. Sufficient drying and recovery should occur within 5-8 days following the frost, after that it is probably safe to return your animals to that field. Here are some other tips for handling and/or preventing possible prussic acid poisoning in livestock:

  • Do not graze sheep on sudangrass or hybrids until it is 12-15 inches tall
  • Do not graze cattle on sudangrass or hybrids until it is 18-24 inches tall
  • Sorghum may not be safe to graze until fully headed
  • Have the plants tested for toxicity levels before grazing
  • Do not graze hungry livestock on sorghum or sorghum-sudangrass hybrids. Potential for poisoning increased with the amount of this high risk forage that is consumed.
  • Follow proper fertilizer recommendations to ensure proper nitrogen and phosphorous levels
  • Select grass varieties that are low in prussic acid

There are other plants that may contribute to the possibility of prussic acid (cyanide) poisoning, these include, but are not limited to: arrowgrass, black cherry trees (leaves and twigs), johnsongrass, peaches, plums, etc.

Call your local vet if you suspect your animals have prussic acid poisoning and remove them from the pasture. Don’t delay—time is of the essence with any poisoning!

Stefani Garbacik is the Livestock and Forage agriculture extension agent for North Carolina Cooperative Extension in Wayne County. If you need further assistance with livestock, animal waste, horse, or forage concerns please contact her at or (919) 731-1525. Livestock information can be found at