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Posted 31 October 2017. PMN Crop News.

Prussic Acid and Nitrate Poisoning Are Concerns After a Light Frost

Source: Oklahoma State University Press Release.

Stillwater, Oklahoma (October 30, 2017)--Cooler temperatures are coming into Oklahoma about three weeks ahead of what is typical for late October and early November, meaning temperatures could stress certain forage plants and make them potentially toxic to livestock.


It was discovered in the early 1900s that under certain conditions sorghums are capable of releasing hydrocyanic acid, commonly called prussic acid. Prussic acid, when ingested by cattle, is quickly absorbed into the blood stream and blocks the animal's cells from utilizing oxygen.

The animal dies from asphyxiation at the cellular level, reminds Glenn Selk, Oklahoma State University Cooperative Extension emeritus animal scientist and editor of the university’s popular Cow-Calf Corner newsletter. Animals affected by prussic acid poisoning exhibit a characteristic bright red blood just prior to and during death.

“Lush young regrowth of sorghum-family plants are prone to accumulate prussic acid especially when the plants are stressed by conditions such as drought or freeze damage,” Selk said. “Light frosts that stress the plant but do not kill it are often associated with prussic acid poisonings.”

OSU’s Division of Agricultural Sciences and Natural Resources recommends producers avoid grazing fields with sorghum type plants following a light frost.

“The risk of prussic acid poisoning is reduced if grazing is delayed until at least one week after a killing freeze,” said Wes Lee, McClain County Extension director and agricultural educator. “As the plants die and cell walls rupture, the hydrocyanic acid is released as a gas and the toxicity is greatly reduced in the plants. Still, one can never be absolutely certain a field of forage sorghum is 100 percent safe to graze.”

DASNR recommendations for cattle that must be grazed on forage sorghum pastures during this time of year should be fed another type of hay before turning in on the field and should be watched closely for the first few hours after turn in.

“If signs of labored breathing such as would be found in asphyxiation are noted, cattle should be removed from the pasture immediately,” Selk said. “Also, call your local veterinarian for immediate help for those animals that are affected.”

Selk and Lee contend producers also should read OSU Extension Fact Sheet PSS-2904, “Prussic Acid Poisoning,” prior to turning cattle out onto potentially dangerous pastures. The publication is available online at

“Think of it as preventive management,” Selk said. “Awareness and knowledge are keys and can save the producer a lot of headache down the line in terms of risk management and animal well-being.”

Plant stress from frosts also will impair the normal metabolism of the plant. Therefore, the plant continues to take up nitrates from the soil but is inefficient at converting the nitrates to protein.

“This creates the opportunity for nitrate accumulations in the plant to reach dangerous levels,” Lee said. “Testing the forage before grazing or cutting for hay will provide important knowledge about the safety or danger in the forage.”

Producers who are uncertain about proper testing procedures should speak with the agricultural educator at their OSU Cooperative Extension county office, typically listed under “County Government” in local directories.

Additional information is available online at by reading OSU Fact Sheet PSS-2903 “Nitrate Toxicity in Livestock.”

The Oklahoma Cooperative Extension Service is one of two state agencies administered by DASNR and one of three equal parts of the university’s state and federally mandated teaching, research and Extension land-grant mission.