As the year progresses, many producers are looking to move livestock to alternative pastures. Unfortunately, certain weather conditions, including drought or frost, can make certain plants in the sorghum family, including Johnsongrass, poisonous. Even after limited grazing, deaths can be seen due to ingestion of prussic acid, also known as hydrocyanic acid or cyanide. A classic call to the vet is, “My cattle are dropping like flies.
Prussic acid toxin is created when harmless hydrocyanic glycosides in plants are stressed and degraded. Once hydrocyanic glycosides in plants are damaged by actions such as chewing cattle or a windrower and seamer, they quickly convert to prussic acid. After ingestion, prussic acid is released in the rumen and rapidly absorbed into the bloodstream.
Once in the circulatory system, the toxin prevents cells from absorbing oxygen. The blood therefore becomes saturated with oxygen, resulting in bright cherry-red blood. The most frequently observed clinical signs are excitation, muscle tremors, increased respiratory rate, excessive salivation, staggering, convulsions and collapse. Asphyxiation at the cellular level is the cause of death due to oxygen deprivation.
When producers encounter animals showing clinical signs of prussic acid toxicity, they should immediately remove any animals that appear normal to a new pasture and contact their veterinarian. The veterinarian will treat sick animals with two drugs (sodium nitrite and sodium thiosulfate) which can reverse the toxicity. Treatment should be initiated quickly but may be difficult due to the rapid progression of the toxin.
Medications used to treat prussic acid toxicity can be difficult to obtain. It is advisable to contact your veterinarian before grazing potentially toxic plants to ensure that your veterinarian will have the availability to intervene and the necessary medications on hand to treat the livestock in the event of a problem.
Breeders may want to take the following steps to prevent prussic acid toxicity:
- Never turn starving cattle into new pasture
- Take soil samples and fertilize accordingly
- Graze mature plants 18 to 24 inches tall
- Wait until plants are dry before grazing after frost (usually at least 7 days)
- Rotation of pastures to prevent livestock from consuming lush regrowth
- Place 1 or 2 cows in a pasture and observe the problems before turning all the cattle
- Test the plants for the presence of prussic acid. Care should be taken however as false negatives can be seen if the test is not performed correctly.
Two types of tests exist to determine prussic acid levels. The first is quantitative and involves submissions to a diagnostic laboratory, such as the Oklahoma Animal Disease Diagnostic Laboratory. The second is a qualitative test that simply detects the presence of hydrocyanic acids and cyanide in fresh plant material. Most agricultural extension educators in the county have access to testing supplies.
Producers must be careful because several toxins can cause the death of cattle. It is recommended that appropriate veterinary diagnostics and testing be performed to determine the ultimate cause of death. A fact sheet with information on prussic acid is available at Prussic Acid Poisoning | Oklahoma State University (okstate.edu).
Source: Oklahoma State University, which is solely responsible for the information provided and is fully owned by the source. Informa Business Media and all of its affiliates are not responsible for any content contained in this information asset.