Forages and Frost: What forages should I be concerned about? What should I do to manage against prussic acid poisoning?

There are many warm season forages that we recommend utilizing as forage crops. Johnsongrass, Sorghum, Sudangrass (members of the sorghum family) are used regularly in Alabama with great success as a quality forage crop, but these forages have the potential of having high amounts of prussic acid (cyanide) in leaves. Prussic acid poisoning can occur when these forages are grazed while under stress. Stress can occur during severe drought, nutrient deficiency, after herbicide application, and after freeze injury due to a killing frost. Most often in Alabama the concern is with the latter (potential poisoning immediately after a killing frost).

Ruminants are more susceptible to prussic acid poisoning due to the neutral pH levels in the rumen and the large number of microbes and enzymes that can hydrolyze the sugar, thus liberating the cyanide molecule. All livestock have the potential of prussic acid poisoning to some degree. Cyanide poisoning symptoms can be characterized as excessive salivation, rapid breathing, and muscle spasms, and can occur quickly after grazing. Unfortunately the ingestion of high concentrations of cyanide usually results in quick death (less than 30 minutes) once signs have developed.

Good forage and grazing management and pasture awareness can dramatically decrease the poisoning potential. Remember: Prussic acid levels elevate due to stress, and deteriorate with time. Avoid grazing areas where known cyanide-containing forages are present during periods of potential stress. Forage with high prussic acid levels can be ensiled or harvested for hay with little fear of poisoning when fed. Standing forage killed by frost should be avoided for a minimum of two weeks after killing frost.

*Prepared by Jennifer M. Johnson, Ph.D, Extension Agronomist, Alabama Cooperative Extension System