Livestock and prussic acid

Prussic acid poisoning can be a lethal problem for cattle grazing sorghum, sorghum-sundangrass crosses and sundangrasses. Other plants that potentially contain prussic acid include white clover, vetch seed and chokecherry.

Prussic acid, also called hydrocyanic (HCN), is typically not present in plants. However, some common plants can accumulate large quantities of cyanogenic glycoside. When plants are actively growing, prussic acid levels are low because the compound breaks down over time. However, stress causes cyanogenic glycoside to accumulate in plant tissues. Drought and frost can damage plant cells and cause glycoside to degrade to form free HCN.

The environment in the rumen also favors the degradation of glycoside to free HCN. Therefore plants containing the glycoside have the potential to cause HCN toxicity when consumed by ruminants. Horses, hogs, and other non-ruminant animals have stomachs that have the ability to convert prussic acid to less toxic formic acid and ammonium chloride.

Hydrocyanic levels are affected by soil fertility and plant's growth stage. Soils high in nitrogen and low in phosphorus will increase prussic acid. Furthermore, toxicity is greater when there are periods of rapid growth followed by cool, cloudy weather. Lush regrowth is particularly dangerous.

Hydrocyanic is one of the most potent poisons. It interferes with oxygen usage at the cell level. Once animals consume a lethal dose, they will die from suffocation within minutes. Clinical signs occur rapidly. Animals will exhibit excitement and muscles tremors initially followed by rapid difficult breathing. Pupils will be pink.

A characteristic sign of HCN toxicity is a bright cherry red blood several hours after death, unlike with nitrate poisoning, blood will be dark brown in color. Sodium nitrate and sodium thiosulfate or methylene blue can be used to treat animals if administrated soon after symptoms are seen.

Problems associated with prussic acid can be avoided with proper management. Sorghums fertilized heavily with nitrogen, stunted by drought or cool, cloudy weather should be suspected. Younger plants have a higher potential to accumulate HCN. Therefore, do not graze sorghum plants until they are 18 to 24 inches tall. This also applies to regrowth. If regrowth occurs after a frost, delay grazing until the entire plant is killed by frost. Do not graze sorghum after a frost until plants thaw and wilt for a few days. The forage is usually considered safe to feed after drying for 5 to 8 days.

Leaves are higher in HCN potential than stems with the highest concentration in new growth. Glycoside levels are higher in the morning, then level off in the afternoon and evening. Contact the Extension Office for sampling instructions and testing options.

The variability in sample and shipping and changes in field conditions limits the value of a quantitative test. If forage does contain prussic acid, livestock should not be allowed to consume it until the level declines to the point it is no longer detected. Toxicity varies among animals depending on rate of intake and individual animal tolerance.

Nitrates have also been found in sorghum and sundangrasses. The Extension Office provides free qualitative and quantitative nitrate testing to producers. For more information contact the Fallon-Carter County Extension Office (406) 778-7110 or Elin at (406) 852-3693.