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.