The following guidelines will help you avoid danger to your livestock this fall when feeding species with prussic acid poisoning potential:

  • Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost.
  • Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days.
  • After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of toxic compounds.
  • New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a hard, killing freeze, then wait another 10 to 14 days before grazing the new growth.
  • Don't allow hungry or stressed animals to graze young growth of species with prussic acid potential.
  • Graze or greenchop sudangrass only after it is 18 inches tall. Sorghum-sudangrass should be 30 inches tall before grazing. Never graze immature growth.
  • Do not graze wilted plants or plants with young tillers.
  • Green-chopping the frost-damaged plants will lower the risk compared with grazing directly, because animals be less likely to selectively graze damaged tissue. However, the forage can still be toxic, so feed greenchop with great caution after a frost.
  • Feed greenchopped forage within a few hours, and don't leave greenchopped forage in wagons or feed bunks overnight.

Hay and silage are safer: Prussic acid content in the plant decreases dramatically during the hay drying process and the forage should be safe once baled as dry hay. The forage can be mowed anytime after a frost if you are making hay. It is very rare for dry hay to contain toxic levels of prussic acid. However, if the hay was not properly cured and dried before baling, it should be tested for prussic acid content before feeding to livestock.

Forage with prussic acid potential that has undergone silage fermentation is generally safe to feed. To be extra cautious, wait 5 to 7 days after a frost before chopping for silage. If the plants appear to be drying down quickly after a killing frost, it is safe to ensile within a shorter time period after the frost.

Delay feeding silage for 8 weeks after ensiling. If the forage likely contained high levels of cyanide at the time of chopping, hazardous levels of cyanide might remain and the silage should be analyzed before feeding.