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Prussic Acid: A Forage-Related Animal Disorder

by Wyoming Livestock Roundup

In one of my previous columns, I mentioned that animal disorders may result from toxic substances or mineral imbalances in forges and weeds consumed by livestock. As a result, reduced animal productivity such as visible symptoms of ill health or even death of grazing animals may occur.

Prussic acid poisoning is one the important animal disorders. 

Prussic acid or hydrocyanic acid (HCN) can build up to toxic levels in leaves of several plants, such as sorghum, sudangrass, sorghum-sudan hybrids, johnsongrass and wild cherry. However, pearl millet does not produce prussic acid. 

Prussic acid reaches to its dangerous concentrations especially immediately after a frost. Also, immediately after long drought, tender and young growing leaves may have potential toxic levels of prussic acid. In general, young, tender and fast-growing plants are more likely to be toxic than older plants. 

Additionally, herbicides including 2,4-D (2,4-dichlorophenoxyacetic acid) may temporarily increase the prussic acid concentrations.

In severe cases, prussic acid can cause death of an animal. This is because of interference with the oxygen-transferring ability of red blood cells, which causes suffocation. 

Symptoms of animal disorders may occur within 10 to 15 minutes after animals eat forages with prussic acid. Typical symptoms include rapid breathing, excessive salivation and muscular contraction. As a result, animals stagger, collapse and finally die.

Prussic acid deteriorates with the advancement of time. 

Ensiled forages with high prussic acid are usually safe to feed within three weeks after silo fill. Standing forage killed by frost is normally safe after about one week. Dried hay, at a level of 18 to 20 percent moisture commonly does not contain toxic levels of prussic acid.

It is recommended that grazing should be avoided after severe drought for at least one week. 

On the other hand, once frost occurs, grazing or feeding greenchop should be stopped for at least one week after the last green forages have been frosted. Also, it is advisable not to turn hungry cattle in to potentially toxic pastures. Caution should be taken in feeding animals newly regrowth plants especially after frost kill.

Anowar Islam is an assistant professor and the University of Wyoming Extension Forage Agroecologist in the Department of Plant Sciences in the College of Agriculture and Natural Resources. He can be reached at 307-766-4151 or

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