Prussic Acid Toxicity

Barry Whitworth, DVM, Senior Extension Specialist, Oklahoma State University, Department of Animal & Food Sciences

 

Cattle producers throughout the state have been reporting cattle losses from cyanide/prussic acid toxicity. Some producers have expressed surprise that cyanide/prussic acid is a problem since there has been an overall abundance of moisture in the state this spring. According to the Mesonet website (www.mesonet.org), precipitation has been near or above normal for many parts of the state. However, as of July 1st, the same website revealed several counties that have recently been dry. These counties have received less than one quarter of an inch of rain, and during this same period, temperatures have skyrocketed. High temperatures and no rain for short periods of time stress plants. This stress leads to certain plants becoming toxic, including plants in the sorghum family.

 

Hydrocyanic acid (HCN), also referred to as cyanide or prussic acid, is the toxin in these plants that causes problems. The toxin is created when the harmless hydrocyanic glycosides in plants are stressed and break down. Once the hydrocyanic glycosides in the plants are damaged, they quickly convert to prussic acid which can kill an animal within minutes when consumed. When cattle ingest the plants high in hydrocyanic glycoside and break them down by chewing, the prussic acid is released in the rumen and absorbed into the blood stream. Once in the circulatory system, the toxin prevents cells in the body from taking up oxygen. The blood becomes saturated with oxygen which cannot be absorbed by the cells which is why normally dark venous blood appears bright red. The clinical signs are excitement, muscle tremors, increased respiration rate, excess salivation, staggering, convulsions, and collapse. The cattle actually die of asphyxiation. 

In plants, especially in the sorghum family, prussic acid is highest in the leaves of young plants with the upper leaves containing the very highest amounts. The amount of prussic acid increases when the plant is stressed such as in drought situations or following a frost. Fertilizing with large amounts of nitrogen can also increase potential for prussic acid toxicity as does nitrogen and phosphorus soil imbalances. Certain sorghum families are more prone to prussic acid toxicity than others. For example, Johnson grass has a high potential for toxicity while Pearl or Foxtail millet are low. When planting sorghums for grazing, producers may want to check the toxic potential of the particular variety.    

     

When producers encounter animals displaying clinical signs of prussic acid toxicity, they should immediately remove all the animals that appear normal to a new pasture and contact their veterinarian.  The veterinarian will treat the sick animals that reverse the toxicity. Treatment can result in a full recovery if initiated quickly.

 

Producers 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
  • Wait until plants are cured before grazing after frost (usually at least 7 days)
  • Rotate pastures to keep cattle from consuming lush regrowth
  • Place 1 or 2 cows in a pasture and observe for problems before turning in all the cattle

 

Plants can be tested for prussic acid, but it can be challenging. If not done properly, producers may get a false since of security. The best practice is to visit with your local veterinarian or Local Oklahoma State University County Extension Agriculture Educator before grazing forages that may contain prussic acid. A fact sheet that contains information about prussic acid is available from Oklahoma State University. The fact sheet title is Prussic Acid Poisoning PSS-2904 which can be found at https://extension.okstate.edu/fact-sheets/prussic-acid-poisoning.html.


Oklahoma State University, in compliance with Title VI and VII of the Civil Rights Act of 1964, Executive Order 11246 as amended, Title IX of the Education Amendments of 1972, Americans with Disabilities Act of 1990, and other federal laws and regulations, does not discriminate on the basis of race, color, national origin, sex, age, religion, disability, or status as a veteran in any of its policies, practices or procedures. This includes but is not limited to admissions, employment, financial aid, and educational services.  References within this publication to any specific commercial product, process, or service by trade name, trademark, service mark, manufacturer, or otherwise does not constitute or imply endorsement by Oklahoma Cooperative Extension Service.