As frost appears so does the potential for hydrocyanic (prussic) acid in forages. The toxicity usually occurs after the first frost and lasts for about 10 days. If the frost doesn’t kill the plant and another frost follows, the toxicity will reappear. Grain sorghum, sudan grasses, sorghum-sudan hybrids and johnsongrass all contain compounds that produce cyanide when plant cells are ruptured by a frost. To reduce toxicity problems follow these guidelines:


  • Don’t graze for two weeks after a non-killing frost.
  • Don’t graze for 10 days to two weeks after a killing frost or freeze, or until plants have thoroughly dried.
  • Don’t graze prussic acid-producing plants at night if possible.