Every years, as cold weather reaches Northeast Texas, concerns about toxicity of feed and feedstuff are common in our ranchers. Prussic acid poisoning also called hydrocyanic acid or cyanide poisoning. Cyanogenic compounds can develop in plants that are stressed; in the rumen the compounds are converted to cyanide, which can kill livestock. Livestock can show symptoms of intoxication within 5 minutes of eating plants with the poison, and may die within 15 minutes. Salivation and labored breathing occur first, followed by muscular tremors, uncoordinated movements, bloating, convulsions and death from respiratory failure. Prussic acid can accumulate in plants in the sorghum family, such as johnsongrass, sudangrass, forage sorghums and grain sorghum. It appears to occur when plants are injured by frost. Severe drought stress can also cause prussic acid to form. High concentrations may be associated with rapid growth, such as shortly after a rain irrigation on previously drought-stressed fields, or warm weather after a cool period. Under good conditions, toxic concentrations can also form in young, rapidly growing plants. Prussic acid dissipates from plant properly cured for hay.
To prevent prussic acid poisoning:
- If plants have been damaged by frost, defer grazing until they either are well recovered from injury or cut for hay, or after a killing freeze and the plants have been allowed to dry.
- Do not graze plants in the sorghum family until they are 2 to 3 feet tall.
- Remove all livestock from the feed source when an animal is found to have died suddenly after grazing forages under poor growing conditions.
- After plants have grown rapidly, such as shortly after a rain irrigation on previously drought-stressed fields or warm weather after a cool period, wait at least 2 weeks after the plants begin to grow before grazing.