Prussic acid poisoning potential in frosted forages

The first few frosts of the fall bring the potential for prussic acid poisoning when feeding forages. Some forage species, primarily sorghums and closely related species, contain cyanogenic glucosides, which are converted quickly to prussic acid in freeze-damaged plant tissue. Historically in Iowa there are very few documented cases of prussic acid poisoning. However, the risk is present, and good management practices are necessary to minimize the risks.

Prussic acid, or more precisely, hydrocyanic acid, is a cyanide compound that can kill animals within minutes of ingestion under the right circumstances. Cyanide interferes with the oxygen-carrying function in the blood, causing animals to die of asphyxiation. Symptoms include difficult breathing, excess salivation, staggering, convulsions and collapse. Affected animals will have bright cherry red mucous membranes from the cyanide. Ruminants are more susceptible than horses or swine because they consume large amounts of forage quickly and the rumen bacteria contribute to the release of the cyanide from consumed plant tissue.

Sudangrass varieties are low to intermediate in cyanide poisoning potential, sorghum-sudangrass hybrids and forage sorghums are intermediate to high, and grain sorghum has high to very high poisoning potential. Pearl millet and foxtail millet have very low levels of cyanogenic glucosides. A few other plants also can produce prussic acid, including cherry trees.

Prussic acid does not form in sorghum and sudangrass plants until the leaf tissue is ruptured, as with grazing or chopping. Young, rapidly growing plants will have the highest levels of prussic acid. The cyanide-producing compounds are more concentrated in young leaves. Minimum plant heights of 18 inches are recommended, to avoid using risky, young leaf tissue. Plants growing under high nitrogen levels are more likely to have even higher cyanide potential.

Frost and freezing can cause a rapid change in prussic acid risk in plants of any age or size. With frost, forage tissues rupture, and cyanide gasses form. The cyanide gas can be present in dangerously high concentrations within a short time, and remain in the frosted leaves for several days. Because cyanide is a gas, it gradually dissipates as the frosted/frozen tissues dry. Thus, risks are highest when grazing frosted sorghums and sudangrasses that are still green. New growth of sorghum species following frost can be dangerously high in cyanide due to its young stage of growth. Prussic acid content decreases dramatically during the hay drying process and during ensiling. Frosted foliage contains very little prussic acid after it is completely dry. Sorghum and sudangrass forage that has undergone silage fermentation is generally safe to feed.

Precautions to take to limit risk

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