Toxic Forage, Livestock a Potentially Lethal Combo

Be mindful of prussic acid poisoning when conditions are right for it.

Published on: Oct 20, 2005

Producers who graze their livestock on sorghum, sudangrass or sorghum-sudangrass crosses should keep in mind toxicity issues associated with the forages.

Steve Boyles, an Ohio State University Extension beef specialist, says now's the time of year where prussic acid -- also referred to as cyanide or hydrocyanic acid -- poisoning could become an issue under certain production situations.

"Sorghum and sudangrass varieties are excellent forage crops. They produce excellent tonnage, especially in drought situations," Boyles says. "Producers just have to be mindful of the dangers associated with prussic acid poisoning when conditions are right for it."

Prussic acid is a gas produced by sorghum and sudangrass varieties during growth, which can increase to dangerously high levels under dry conditions. Prussic gas can be toxic to livestock, inhibiting oxygen utilization in the animal. Symptoms of prussic acid poisoning include labored breathing, staggering, trembling muscles, convulsion and death.

Prussic acid poisoning may be a concern, but the good news is that it is easily manageable if producers follow a few simple guidelines:

  • Choose the right variety. The level of toxicity is relative to the forage variety. Sorghums contain the highest levels of prussic acid, followed by sorghum-sudangrass crosses and then sudangrass.
  • Do not immediately graze livestock on these forages following a series of light frosts, especially if a warm period follows. The potential for prussic acid poisoning increases under these conditions.
  • Livestock should not graze on extremely short forage. Plants should be 18 inches to 24 inches tall.
  • Plants should be completely killed by frost or dried before grazing. Young, re-growth forage can be highly toxic. "After a killing frost, producers should let the stems break up. By doing so, you are allowing the gas to escape during field curing," Boyles says.
  • Sorghum used for silage should remain in the field several days after a killing frost before harvesting. "Field curing or drying releases over 50% of the prussic acid," Boyles says. "And remember, silage is not silage until it has been in the silo for three weeks. Hay stored for two or more months gradually losses all of its poisoning potential."
  • Don't graze hungry livestock specifically on sorghum or sudangrass varieties. Make sure they are fed something else first.

Boyles says that sorghum, sudangrass and sorghum-sudangrass crosses aren't the only forages that are a source for prussic acid poisoning. Others include Johnsongrass and wild black cherry, apple, apricot, elderberry and peach trees.