As the year progresses, many producers look to move cattle to alternative pastures. Unfortunately, certain weather conditions, including drought or frost, can set up some plants in the sorghum family, including Johnson grass, to become toxic. Even after limited grazing, deaths may be seen due to the ingestion of prussic acid, also known as hydrocyanic acid or cyanide. A classic call to the veterinarian is, “My cattle are dropping like flies.”
Prussic acid toxin is created when the harmless hydrocyanic glycosides in plants are stressed and break down. Once the hydrocyanic glycosides in the plants are damaged through actions like cattle chewing or a swather and crimper, they quickly convert to prussic acid. Following ingestion, the prussic acid is released in the rumen and rapidly absorbed into the bloodstream. Once in the circulatory system, the toxin prevents cells from taking up oxygen. The blood, therefore, becomes saturated with oxygen, leading to blood that appears bright cherry red. The clinical signs most often seen include excitement, muscle tremors, increased respiration rate, excess salivation, staggering, convulsions and collapse. Asphyxiation at the cellular level is the cause of death due to the deprivation of oxygen.
When producers encounter animals displaying clinical signs of prussic acid toxicity, they should immediately remove all of the animals that appear normal to a new pasture and contact their veterinarian. The veterinarian will treat the sick animals with two drugs (sodium nitrite and sodium thiosulfate) that can reverse the toxicity. Treatment must be initiated quickly, so it can prove difficult due to the rapid progression of the toxin.
The drugs used to treat prussic acid toxicity can be difficult to obtain. It is advisable to contact your veterinarian before grazing potential toxic plants to make sure that your veterinarian will have availability to respond and the necessary drugs on hand to treat the cattle if a problem arises.
Cattlemen may want to take the following steps to prevent prussic acid toxicity:
• Never turn hungry cattle into a new pasture.
• Take soil samples and fertilize accordingly.
• Graze mature plants with 18 to 24 inches of height.
• Wait until plants are cured before grazing after frost (usually at least seven days).
• Rotate pastures to keep cattle from consuming lush regrowth.
• Place one or two cows in a pasture and observe them for problems before turning in all the cattle.
• Test plants for the presence of prussic acid. Care should be taken though, as false negatives can be seen if the test is not performed correctly.
Two types of tests exist for determining prussic acid levels. The first is quantitative and involves submissions to a diagnostic lab, such as the Oklahoma Animal Disease Diagnostic Laboratory. The second is a qualitative test that detects hydrocyanic acids and cyanide in fresh plant material. Most county agriculture Extension educators have access to test supplies.
Producers should be cautious because there are multiple toxins that can cause cattle deaths. It is recommended that appropriate veterinary diagnostics and testing be done to determine the ultimate cause of death. A fact sheet that contains information about prussic acid is available at okstate.edu by searching for “prussic acid poisoning.” — Rosslyn Biggs, DVM, beef cattle Extension veterinarian; and Barry Whitworth, DVM, Extension food animal health specialist
