Cyanide Potential in Frost Affected Forage

September 25, 2012 07:43 AM

Winter is on it's way and as a hard freeze approaches, experts are quick to advise caution when feeding forages affected by frost. Mark Sulc of the Ohio State University Extension warns that annual sorghum related grasses can become toxic to livestock within hours of a hard freeze.

Sulc reports, "Animals can die within minutes if they consume forages such as the sorghum species that contain high concentrations of prussic acid in the plant tissue. The prussic acid is released from the forage and interferes with oxygen transfer in the blood stream of the animal, causing it to die of asphyxiation. Before death, symptoms include excess salivation, difficulty breathing, staggering, convulsions, and collapse."

Prussic acid is the concern - you may know prussic acid by its more common name...cyanide. While grain sorghum has the highest potential for cyanide poisoning following a frost, sundgrass hybrids, forage sorghum and piper sudangrass can all contain the toxin. In addition, johnsongrass, chokecherry, black cherry, indiangrass, elderberry and birdsfoot trefoil can also be affected. Soils with high nitrogen content or low P&K levels are most prone to producing toxic plants.

Mr. Sulc offers the following grazing precautions:

  • 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 tend to be much safer. Toxic gasses are released during the drying or ensiling process so make certain crops cut for hay have fully dried before baling, and wait on feeding silage for 8 weeks after chopping. Even then, Sulc advises having silage and hay tested for cyanide.



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