Frost can create prussic acid problems with forage sorghum

Contact: Dan Undersander, 608-263-5070,

There has been an increase in the number of acres of forage sorghum and sorghum-sudangrass planted in Wisconsin this past year due to the long winter, late wet spring, drought last year and some areas of the state having a lot of alfalfa winter kill. Recent frosts can create prussic acid problems with these crops if they were not harvested prior to the frost and are not utilized in a safe manner according to Dan Undersander, University of Wisconsin-Extension forage specialist at UW-Madison.

Undersander noted sorghums, and sorghum-sudangrass crosses may be poisonous if grazed or fed improperly. The danger of prussic acid poisoning is greatest for forage sorghum varieties, less for sorghum-sudangrass crosses, and least for surdangrasses. Fall is a difficult time since repeated light frosts may continually rupture new cells and release prussic acid

The young, dark green growth or regrowth (less than 18 to 24 inches) is the most dangerous stage. Shortly after frost, prussic acid release potential increases slightly. However, they can be safely grazed a few weeks after freezing if there is no substantial regrowth.

Undersander said, “Fall is a difficult time since repeated light frosts may continually rupture new cells and release prussic acid.”

He provided the following recommendations and noted, in all cases, mixing with other feedstuffs reduces the problem compared to when the sorghum or sorghum-sudangrass is the sole source of feed:

  • Silage. Sorghum and sorghum-sudangrass silage is generally safe for feeding. Although it may have contained toxic levels of prussic acid when harvested, while in storage much of the poison escapes as a gas during fermentation and when being fed. However, as a precaution, do not feed new silage for at least 3 weeks after harvesting and storing.
  • Hay. The prussic acid content of sorghum and sorghum-sudangrass hay decreases as much as 75 percent while curing and is rarely hazardous when fed to livestock. Hay stored for two or more months gradually losses all its cyanide potential.
  • Greenchop or grazing. Do not graze sorghum or sorghum-sudangrass hybrids following a series of light frosts, as the potential for poisoning increases for a period of time after frosts. Allow 7 to 10 days after a light frost before feeding greenchop or grazing. Do not green-chop graze sorghum or sorghum-sudangrass hybrids following a killing frost until the plant has dried, approximately 7 days. Do not graze hungry livestock on sorghum or sorghum-sudangrass hybrids. Poisoning potential increases with the amount of high- risk forage consumed.

Since prussic acid poisoning is very fast-acting, death will occur quickly. Watch animals closely for any signs of toxicity. Symptoms of prussic acid poisoning are gasping, staggering, trembling muscles, convulsions, and death resulting from respiratory failure. The mucous membranes of the mouth and eyes may have a blue coloration as evidence of cyanosis. In cases of recovery, there appears to be no permanent effects.

For more information about prussic acid poisoning please contact your local county UW-Extension agriculture agent.


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