Mark Arnold

Ellis County Ag Agent

Prussic acid poisoning is one of the most toxic and rapidly acting of any common poison.

It is also called hydrocyanic acid or cyanide poisoning. Cyogenic 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 five 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.

Although there is usually little danger of prussic acid poisoning, it can accumulate in plants in the sorghum family, such as Johnson grass, Sudan grass, forage sorghums and grain sorghum. It is also 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: 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 good conditions, toxic concentrations can also form in young, rapidly growing plants.

On the positive side, prussic acid dissipates from plants properly cured for hay. However, in hay baled early at high moisture or plants chopped for immediate feeding, the prussic acid may not have had a chance to dissipate.

To prevent prussic acid poisoning:

• Do 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 by herbicides or 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.

• Graze second-growth sorghums with caution of 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.

• After plants have grown rapidly, such as shortly after a rain or irrigation on previously drought-stressed fields or warm weather after a cook period, wait at least two weeks after the plants begin to grow before grazing.

• When moving livestock in to new pastures containing cyanogenic-accumulating plants, don’t turn in on cloudy days or early in the morning.

For further information, contact Mark Arnold, county extension agent for agriculture/natural resources, at 701 S. Interstate 35E, Suite 3, Waxahachie; 972-825-5175; or email