Fall Alert: Prussic Acid and Nitrate Poisoning After Frosts for Livestock

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In Franklin County, we have had our first “killing” frost. This is the time of year to be careful with several summer forage grasses such as the sorghums. Pearl millet does not present a problem with prussic acid however, it will accumulate nitrates in the same manner as the sorghums under stress conditions.

It was discovered in the early 1900s that under certain conditions sorghums are capable of releasing hydrocyanic acid or commonly called prussic acid. Prussic acid when ingested by livestock, is quickly absorbed into the blood stream, and blocks the animal’s cells from utilizing oxygen. Thus, the animal dies from asphyxiation at the cellular level. 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 such as drought or freeze damage. Light frosts, that stress the plant but do not kill it, are often associated with prussic acid poisonings.

Producers should avoid grazing fields with sorghum type plants following a light frost. The risk of prussic acid poisoning will be reduced, if grazing is delayed until at least one week after a “killing” freeze. As the plants die and the cell walls rupture, the hydrocyanic acid is released as a gas, and the amount is greatly reduced in the plants. One can never be absolutely certain that a field of forage sorghum is 100% safe to graze.

Livestock 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, animals should be removed immediately. Call your local veterinarian for immediate help for those animals that are affected.

Frosts also stress the plant before a hard freeze kills it. Plant stress from frosts 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. Nitrate accumulations may reach dangerous levels. Testing the forage before grazing or cutting for hay will provide important knowledge about the safety or danger in the forage. You can have your hay tested for nitrate levels before feeding at no costs with the NCDA & CS forage lab in Raleigh. Visit NCSU’ s Forage site for more information, http://www.forages.ncsu.edu/extension.html, or give us a call at the Franklin County Cooperative Extension Center at 919-496-3344.