Prevention of Prussic Acid Poisoning for Cattle

By Dr. John Comerford

Sorghums, sorghum-Sudan grass hybrids, Sudan grass, and wild cherry leaves and twigs are sources of poisons that are collectively referred to as prussic acid, cyanide or hydrocyanic acid. Most beef producers are well aware of the danger of wild cherry trees and limbs falling in the pasture because the wilted leaves are toxic sources of cyanide. Less well-known are the effects of the sorghum family of forages that can also produce toxic levels of prussic acid.

Some of the plants that may contain prussic acid:

Prussic acid occurs normally in these forages in a bound, nonpoisonous state. Certain environmental conditions result in the combination of the chemicals emulsin and dhurrin to produce prussic acid in an unbound state. There are several factors that contribute to prussic acid formation:

1. Plant species - The vegetative portion of all sorghum-variety plants contains prussic acid, but Sudan grass may contain as much as 40 percent less than other sorghums. There are some varieties of sorghum plants that have been developed that are lower in prussic acid content. Observation for the presence of any of these plants, including in fencerows and sacrifice areas, will help prevent poisoning of cattle. Pearl millet does not contain high levels of prussic acid.

Photo courtesy of Helmut Gevert/

2. Plant parts - Leaf blades contain the highest levels of prussic acid and are generally the grazed portion of the plant. Stems contain less, and grains and seeds contain none. The leaf tillers produced after a drought, frost or mowing are the most dangerous parts of the plant.

3. Plant maturity - Younger, growing plants will have higher concentrations of prussic acid, and plant maturity will dilute the concentration with more stem proportion. However, cattle will normally selectively graze the leaf portion regardless of the plant maturity, so the plant can remain dangerous in a pasture regardless of maturity.

4. Drought - Drought is the primary cause of most prussic acid poisoning. Drought-stricken plants have a higher proportion of leaf material, and in the case of regrowth right after a drought, the new leaf shoots will be high in prussic acid.

5. Frost - Similar to the effects of drought is the effect of freezing. The tops of the plant containing the highest levels of prussic acid that are frosted may appear dead, but the prussic acid content is not diminished until significant wilting occurs. Frosted plants should not be baled or chopped until five to six days after a killing frost. Plants that are frosted and then produce new leaf shoots in warmer weather should be avoided completely.

6. Fertilizer - High levels of nitrogen fertilizer applied to sorghums when phosphorus and potassium levels remain low will increase the prussic acid content of the plants.

Safely feeding sorghum plants:

Symptoms and treatment of animals poisoned with prussic acid are similar to other toxins. Symptoms include staggering, salivation and a high respiratory rate. In many cases, because prussic acid can be highly concentrated in grazed material, the producer will just find dead cattle. Death can occur in less than 20 minutes in some cases. The cumulative effect of consuming lower concentrations of prussic acid - staggering gait, salivation, etc. - is similar to many other health problems. A veterinarian should always be called in under these conditions, and they should be informed about intake of sorghum-based plants if they are available. Treatment with nitrogenous compounds can be effective, but only if administered quickly after the onset of symptoms.

A checklist of actions for the use of sorghum-based plants for cattle:

Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania State University.