Prussic acid poisoning is caused by hydrocyanic acid. As a cyanide compound the substance can quickly kill animals. Death can occur within minutes of ingestion in some cases. Cyanide interferes with the oxygen-carrying function in the blood resulting in asphyxiation. Symptoms include staggering, difficulty breathing, convulsions, and immobility.
Sorghums and sorghum sudangrass crosses are the most affected. Millets are generally safe and have low risk of prussic acid. Young regrowth can easily be affected by a mild frost. As the sorghum plant grows taller, and thus older, the risk of prussic acid poisoning decreases. However, during a frost or continued frost events the mature plant can be just as dangerous.
It is recommended that cattlemen remove animals from sorghum or sorghum sudangrass pastures when frost and freeze are likely. Animals can return 7 days after a killing frost. If the forage must be harvested, cutting and baling is the best option. Make sure to allow the hay to cure. This will reduce the risk of prussic acid in the hay. It can be chopped and ensiled, but the levels of cyanide may not be fully reduced by ensiling. As a result the silage should be tested prior to feeding. I do not recommend green chopping.
Producers should have availability to cornstalks or other stockpiled grasses right now. These feeds and grazing situations will not pose risk of prussic acid. Watch your weather forecast if you are grazing sorghum or sorghum sudangrass and prevent problems with prussic acid poisoning.