Freezing weather can bring grazing problems with certain plants. Eddie Baggs, Denton County Extension agent, says prussic acid poisoning is one of the most toxic and rapidly acting of any common poison. It also is called hydrocyanic acid or cyanide poisoning. Cryogenic compounds can develop in plants stressed in the rumen, with compounds 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, Baggs notes. Salivation and labored breathing occurs first, followed by muscular tremors, uncoordinated movements, bloating, convulsions and death from respiratory failure.
Although there is usually little danger of prussic acid poisoning, it can accumulate in plants in the sorghum family such as Johnsongrass, Sudan grass, forage sorghum and grain sorghum. It also is found in Bahia, corn, cocklebur, white clover and other minor plants but seldom at toxic levels.
"One problem with prussic acid is that it tends to 'come and go' in the plant," Baggs notes. "It may be present for a short time and then dissipate. It appears to occur when plants are injured by herbicides or frost."
Severe drought stress can also cause prussic acid to form. High concentrations of prussic acid may be associated with rapid cell division or rapid growth such as shortly after a rain or irrigation on previously drought-stressed fields—or warm weather after a cool period. Under the right conditions, toxic concentrations also can form in young, rapidly growing plants. Prussic acid dissipates from plants properly cured for hay. However, in hay baled early at high moisture or plants chopped for immediate feeding, prussic acid may not have had time to dissipate.
Baggs says to prevent prussic acid poisoning: Dot not graze any of the cyanogenic-accumulating plants (sorghums) that have been subject to drought or injury unless they are tested for hydrocyanic acid. If plants have been damaged through herbicides or frost, defer grazing until they either are well recovered from injury or cut for hay, or after a 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," Baggs advises. "Graze second-growth sorghums with caution if growing conditions are poor. Remove all livestock from the feed source when an animal is found to have died suddenly after grazing forages under poor growing conditions. Prevent animals from grazing wilted plants or those with young tillers. "
When plants have grown rapidly, wait 2 weeks after the plants begin to grow before grazing.