Prussic Acid Potentials

When grazing or greenchopping species with prussic acid potential this fall, follow these guidelines:

  • Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost.
  • Immediately after frost, remove the animals until the grass has dried thoroughly. Generally, the forage will be safe to feed after drying five to six days.
  • Do not graze wilted plants or plants with young tillers or new regrowth. If new shoots develop after a frost they will have high poisoning potential, sudangrass should not be grazed until the new growth is at least 18 to 20 inches (24 to 30 inches for sorghum-sudangrass).

Best management is to allow the final, killing freeze to kill the crop, and then wait five to six days before grazing. Other practical managements may be to harvest as hay or silage since. In most cases, adequate growth for safe grazing cannot be obtained after a later, killing freeze occurs.

Don’t allow hungry or stressed animals to graze young growth of species with prussic acid potential.

Green-chopping the frost-damaged plants will lower the risk compared with grazing directly, because ani-mals have less ability to selectively graze damaged tissue; however, the forage can still be toxic, so feed with great caution. Feed greenchopped forage within a few hours, and don’t leave greenchopped forage in wagons or feed bunks overnight.

When making hay or silage from sorghum species this fall, consider the following:

  • Frosted/frozen forage should be safe once baled as dry hay. The forage can be mowed any time after a frost. It is very rare for dry hay to contain toxic levels of prussic acid. If the hay was not properly cured, it should be tested for prussic acid content before feeding.
  • Waiting five to seven days after a frost to chop frosted forage for silage will limit prussic acid risks greatly.

Delay feeding silage for eight weeks after ensiling. If the forage likely contained high HCN levels at time of chopping, hazardous levels of prussic acid might remain and the silage should be analyzed before feeding. Other common forages such as alfalfa, clovers and cool-season perennial grasses do NOT produce toxic compounds after a frost and can be fed safely. The only concern is a slightly higher potential for bloat when grazing legumes within a day or two after a killing frost.