Producers should be aware that a killing frost can be deadly for livestock

Fall has arrived with its cool days and colorful landscapes. While producers enjoy the break from the summer heat, it’s a good time to keep in mind changes that can occur to forages after the first killing frost. Some grasses produce potential toxins in high stress conditions. The first killing frost may be weeks away, but it’s not too early for producers to brush up on the basics of prussic acid poisoning. 

Prussic Acid

Several species found in pastures in the region contain prussic acid or hydrocyanic acid (HCN). Prussic acid can be found in grasses such as sorghum, sorghum-sudan grass hybrids and Johnsongrass as well as wild cherry trees. “These plants contain cyanogenetic glucosides (dhurrin); during periods of plant stress prussic acid is formed by enzymatic action on the glucosides,” Sarah Kenyon, Ph.D., field specialist in agronomy with the University of Missouri Extension, explained. 

Different conditions can cause plants stress and trigger the production of toxic levels of prussic acid. “Plant stress can be from drought, inadequate rest following grazing or frost.  With the possibility of frost nearing, the risk of prussic acid poisoning increases,” Dr. Kenyon said. 


Producers with pastures containing forages conducive to creating prussic acid when stressed, should keep a close on livestock for symptoms of prussic acid poisoning. The symptoms include anxiety, progressive weakness, labored breathing and death. Additionally, animals may exhibit symptoms such as an increased rate of respiration, increased pulse rate, gasping, muscular twitching and convulsions. “Death from prussic acid poisoning often occurs more rapidly than nitrate poisoning in affected animals,” Dr. Kenyon stated.


If producers suspect an animal is suffering from prussic acid poisoning, they need to take quick action.

First, the animals need to be immediately removed from the pasture where they are grazing. Next, producers should contact their veterinarian. “Animals affected by prussic acid poisoning may be treated with a sodium nitrite-sodium thiosulfate combination,” Dr. Kenyon said. The combination of sodium nitrite and sodium thiosulfate must be injected intravenously and very slowly. The dosage and method of administration are critical.

Management Strategies

Though the threat of prussic acid poisoning may be concerning, there are management strategies producers can put in place to mitigate the risk. One practice to avoid prussic acid poisoning in a herd is to bale the potentially affected pastures into hay. The prussic acid contained in the forages will dissipate during drying, thus making the hay safe for animals to consume.

  If livestock is going to graze the pastures where prussic acid could develop, then wait until the plants are completely recovered from drought stress or grazing.  In the case of a frost, wait for two weeks before allowing livestock to graze. 


Nitrates pose a different set of challenges for producers. Nitrates can build up in forages over time and can linger in hay after it is cut and dried. “Nitrate poisoning is still a concern in many types of forage, including corn and Johnsongrass. Continue testing these forages for the presence of nitrates,” Dr. Kenyon recommended. 

Rapid nitrate testing can be done at many county extension offices.  Prussic acid testing requires other considerations since it volatilizes quickly. The University of Missouri Veterinary Diagnostic Lab in Columbia, Mo., has the capabilities to test forages for prussic acid. 


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