Livestock call ‘drops like flies’ | Community


(Rosslyn Biggs, DVM, state beef cattle extension veterinarian)

(Barry Whitworth, DVM, SE Area Extension Food Animal Health Specialist)

As the year progresses, many producers look to move cattle to other pastures. Unfortunately, certain weather conditions, including drought or frosts, can make some plants in the sorghum family, including Johnson grass, poisonous.

Even after limited grazing, deaths can be seen due to ingestion of prussic acid, also known as hydrocyanic acid or cyanide.

A classic call to the vet is, “My cattle are dropping like flies.

Prussic acid toxin is created when harmless hydrocyanic glycosides in plants are stressed and degraded. Once hydrocyanic glycosides in plants are damaged by actions such as chewing cattle or a windrower and seamer, they quickly convert to prussic acid. After ingestion, prussic acid is released in the rumen and rapidly absorbed into the bloodstream.

Once in the circulatory system, the toxin prevents cells from absorbing oxygen. The blood therefore becomes saturated with oxygen, resulting in blood that appears bright cherry red.

The most commonly observed clinical signs include excitement, muscle tremors, increased breathing rate, excessive salivation, staggering, convulsions and collapse.

Asphyxiation at the cellular level is the cause of death due to oxygen deprivation.

When producers encounter animals showing clinical signs of prussic acid toxicity, they should immediately remove any animals that appear normal to a new pasture and contact their veterinarian.

The veterinarian will treat sick animals with two drugs (sodium nitrite and sodium thiosulfate) which can reverse the toxicity.

Treatment should be initiated quickly but may be difficult due to the rapid progression of the toxin.

Medications used to treat prussic acid toxicity can be difficult to obtain. It is advisable to contact your veterinarian before grazing potentially toxic plants to ensure that your veterinarian will have the availability to intervene and the necessary medications on hand to treat the livestock in the event of a problem.

Breeders may want to take the following steps to prevent prussic acid toxicity:

• Never turn starving cattle into new pasture.

• Take soil samples and fertilize accordingly.

• Graze mature plants 18 to 24 inches tall.

• Wait until plants are dry before grazing after frost (usually at least 7 days).

• Rotate pasture to prevent livestock from consuming lush regrowth.

• Place 1 or 2 cows in a pasture and observe the problems before turning all the cattle.

• Test plants for the presence of prussic acid. Care should be taken however as false negatives can be seen if the test is not performed correctly.

Two types of tests exist to determine prussic acid levels. The first is quantitative and involves submissions to a diagnostic laboratory, such as the Oklahoma Animal Disease Diagnostic Laboratory.

The second is a qualitative test that simply detects the presence of hydrocyanic acids and cyanide in fresh plant material.

Most county agricultural extension educators have access to testing supplies.

Producers must be careful because several toxins can cause the death of cattle. It is recommended that appropriate veterinary diagnostics and testing be performed to determine the ultimate cause of death.

A fact sheet with information on prussic acid is available at Prussic Acid Poisoning – Oklahoma State University (


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