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Prussic Acid Poisoning May Be A Problem In Drought Stressed Forages

Livestock Update, September 1998

John B. Hall, Ph.D., Extension Beef Cattle Specialist, Animal and Poultry Sciences, Virginia Tech

Hydrocyanic acid or prussic acid poisoning is a condition usually associated with frosted sorghum, sudangrass, sorghum x sudangrass, or Johnsongrass. However, when these grasses are drought stressed they can also produce prussic acid. Prussic acid poisoning is rapid and usually fatal in cattle and sheep, and causes severe problems in horses. Dr. Dennis Blodgett, toxicologist at the Virginia-Maryland Regional College of Veterinary Medicine, said that nitrates may also be a problem in these plants, and in fact may be more of a problem than prussic acid.

Drought-stressed plants are most toxic during and shortly after the drought. Stunted plants can contain high levels of prussic acid. In addition, the new regrowth will also be high in prussic acid. Because animals are selective grazers, it is hard to limit the exposure to prussic acid by grazing management. Sorghums, sudangrasses, and their cousins should not be grazed during the drought or when young regrowth is occurring. Producers should wait until these grasses are actively growing and have at least 2 weeks or 15 to 18 inches of new growth. If producers are unsure about the prussic acid content of a field, they should hold off on grazing, test the field or use a few closely observed tester animals to gauge its safety.

Prussic acid problems can also be avoided by harvesting plants for silage or hay. This results in about a 50-75% reduction in prussic acid levels. However, it is best to wait for regrowth before haying or ensiling also. Prussic acid levels are reduced by HCN being lost as a gas during hay curing or silage wilting. Some prussic acid is also lost during handling of silage for feeding. Silage should not be fed until it has ensiled for 3 weeks. DO NOT enter unventilated silos once ensiling has started. Little information is available on the safety of round bale silage.

Testing forage for prussic acid may be beneficial. Hay and silage should be tested after proper curing or ensiling. Levels of 25 mg/100 g of dry matter are considered safe. The Toxicology Laboratory at the Virginia-Maryland Regional College of Veterinary Medicine can test for prussic acid. Samples must be kept frozen or well refrigerated in plastic bags from the field to the laboratory. The University of Nebraska and Texas A&M also have testing laboratories. Other commercial laboratories may offer this service.

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