Even a light frost can turn certain common forage plants into deadly poisons for cattle and other grazing animals. Grain sorghum, sudan grasses, sorghum-sudan hybrids and johnson-grass all contain compounds that produce cyanide when plant cells are ruptured by a frost.

Any stress, such as drought or frost, has the potential for producing prussic acid. The plants reach toxic levels quickly after a frost. The danger decreases over the next few days and is usually gone within 10 days to two weeks if there are no more frosts. The danger can be extended if there's a light frost that induces prussic acid toxicity but doesn't kill the plant, followed by another frost a week or two later. Remove cattle from areas with stands of plants that produce prussic acid from first frost until at least 10 days after the plants are killed. Here are some tips to reduce toxicity problems:

  • Graze sorghum or sorghum-cross plants only when they're at least 15 inches tall.
  • Don't graze plants during and shortly after droughts that severely reduce growth.
  • Don't graze wilted plants or plants with young tillers.
  • 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 the plants have had time to dry.
  • Don't graze prussic acid producing plants at night if frost is possible.
  • Don't allow access to wild cherry leaves, whether they're wilted or not. Always check pastures for fallen wild cherry limbs following storms.