Prussic Acid Poisoning

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Knowledge Nuggets

  • Prussic acid toxicity (hydrocyanic poisoning) is caused by hydrogen cyanide (HCN) production in several types of plants such as some varieties of birdsfoot trefoil, white clover, flax green feed, green flax straw, sorghum-sudan grass, hybrid pearl millet, foxtail millet, Indian grass, chokecherry, elderberry, arrow grass.
  • Young rapidly growing plants generally have a higher concentration of prussic acid than mature plants, especially if the plants were under stress while growing. Anything that retards normal plant growth may increase prussic acid content. Drought-stunted plants accumulate HCN and can possess toxic levels at maturity. Leaves generally produce 2 to 25 times more prussic acid than the stems of forage grasses; seeds contain none. Test before feeding.
  • The concentration of prussic acid is higher in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries. If hay is not properly cured, toxic levels of HCN may be present. Analyze all suspect feeds to avoid prussic acid poisoning. Do not allow hungry cattle to graze if prussic acid may be a problem. Don't graze potentially troublesome plants after a frost or after rain has been received during a drought.
  • Freezing ruptures the cells and releases the HCN. After a killing frost wait at least 7 days before grazing, since HCN is volatile and will dissipate. Test suspect fields before grazing.
  • Plants may have high levels of prussic acid accumulators when grown in soils high in nitrogen and low in phosphorous and potassium. Avoid applying excessive amounts of fertilizer in one application.
  • Application of foliar herbicides may also increase prussic acid levels for several weeks.
  • If the leaves are damaged or the plants are under stress an enzyme is released and HCN is produced. Once the plants are eaten, the toxin is absorbed directly into the blood stream, and binds to the cell enzymes. This cyanide complex prevents hemoglobin from transferring oxygen to individual cells and suffocation results.
  • Hydrogen cyanide levels above 200 ppm on an as fed basis (0.02%) or 500 ppm on a dry matter basis (0.05%) is potentially toxic. Since the toxic level is very low and adequate forage mixing equipment is not found on most farms, attempting to dilute forages high in prussic acid is not recommended.
  • Hydrogen cyanide is released when the leaves are physically damaged by trampling, cutting, crushing, chewing or wilting. Cud chewing will release the cyanide from the plant leaves. Horses and pigs are rarely affected by prussic acid toxicity. The acidity of the monogastric stomach helps to destroy the enzyme that releases the toxin.
  • Signs of prussic acid poisoning can occur within 15 minutes to a few hours after the animals eat the potentially toxic feed. Initially, you may notice excitability, rapid pulse rate and muscle tremors; followed by rapid and laboured breathing, staggering and collapse. Excessive tearing, salivation, and voiding of urine and feces may occur; the smell of bitter almonds is often detected. If the animal is lying down and going through severe convulsions, death can occur in a matter of minutes.
  • Prussic acid toxicity is similar to nitrate poisoning, but the blood from a HCN poisoning is bright red in colour and clots slowly. Nitrate poisoning turns the animals blood a dark chocolate colour.
  • When submitting forage samples for prussic acid analysis, ideally they should be quick frozen as soon as possible and maintained in a frozen state. Dry hay or silage may also be submitted.

Minimize Livestock Prussic Acid Poisoning Risk

Nitrates and Prussic Acid in Forages - Sampling Testing and Management Strategies

Nitrate and Prussic Acid Toxicity in Forage: Causes, Prevention, and Feeding Management - available in PDF format only

Prussic Acid Poisoning

The Merck Veterinary Manual
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This document is maintained by Linda Hunt.
This information published to the web on November 19, 2003.
Last Reviewed/Revised on February 16, 2018.

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