Prussic acid poisoning

Use caution when grazing sorghum/sudan grass during periods when these forages may be experiencing stress, whether drought or frost stress, as 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 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 accumulations

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.

Preventing losses

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 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.

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 2 weeks after the plants begin to grow before grazing.

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

Prussic acid testing:

Cyanide begins to leave the sample as soon as the plant begins to die. Therefore, it is critical that producers hand-carry or ship overnight all samples to be tested for prussic acid.

The plant sampling method is similar to that for nitrate. A good sample for prussic acid testing consists of leaves from 10 to 12 plants. Refrigerate but do not freeze the samples in transit to the lab.

Sampling and handling baled hay presents problems, because prussic acid is lost rapidly after the bale is opened. Use a hay probe, empty the sample immediately into a pint canning jar (only one probe per sample jar), seal the jar and send it to the lab overnight.

Plant samples may be sent to the Texas Veterinary Medical Diagnostic Laboratory (TVMDL), 1 Sippel Road, College Station, Texas 77843. Phone (409) 845-3414.

For further information, contact Mark Arnold, County Extension Agent-Agriculture/Natural Resources, 701 South I-35 E, Suite 3, Waxahachie, or call 972/825-5175 or email:

Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, religion, sex, national origin, age, disability, genetic information or veteran status. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating