We have had our first frost of the year. Many producers are concerned about grazing forages that have the potential to cause prussic acid poisoning and what they can do to minimize the risk.

Prussic acid or cyanide can build up to dangerous levels anytime the plant is stressed, like after a drought or following a frost. Prussic acid hinders the oxygen-transferring ability of the red blood cells, which causes animals to suffocate. Ruminant animals such as cattle, sheep, and goats are more susceptible than non-ruminant animals like pigs. The main grasses that pose a problem are sorghums, sorghum-sudangrass crosses, and sudangrass. Other plants that may contribute are arrowgrass and cherry trees and can include but are not limited to Johnson grass. Plant parts that are especially high in prussic acid include the leaves and young or new growth.

Allowing the plants to rest after frost reduces the risk of poisoning, because it allows the cyanide levels to decrease. Once a frost occurs, take your animals off the pasture and prevent grazing of the sorghum, sorghum-sudangrass, or sudangrass. Sufficient drying and recovery should occur within 7-10 days following the frost; after that it is probably safe to return your animals to that field. Always err on the side of caution and wait a full 10 days before turning your animals back onto pasture.

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 increases with the amount of this high-risk forage that is consumed.
  • Select grass varieties that are low in prussic acid.

What do you do if you think your animals may be suffering from prussic acid poisoning? Call your local vet and remove them from the pasture. Don’t delay – time is of the essence with any poisoning! Check your pastures for a cause of the problem to avoid other animals from getting sick. Avoid plants that can cause prussic acid poisoning and monitor animals.