Prussic Acid Poisoning

The average first frost date for the Wise County area is about November 12-15.  However, on last Monday morning we did see a light frost in some of the lower lying areas.   You can already see the effects of the frost on some of our bigger lush weeds, like the cocklebur. With the weather changing, it is quite possible we could see an earlier than normal frost.  Light frosts, that stress Johnson grass and other sorghum type plants but do not kill the plants, are often associated with prussic acid poisonings. 

            In the early 1900’s it was discovered that under certain conditions sorghum 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 using oxygen.  Therefore, the animal dies from asphyxiation.  Animals affected by prussic acid poisoning exhibit a characteristic of bright red blood just prior to and during death.  Lush young regrowth of sorghum-type plants are prone to accumulate prussic acid especially when the plants are stressed from drought or freeze damage.

            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 sorghum is 100% safe to graze.  Although pearl millets have been shown to be potential sources of nitrate toxicity (like the other forage sorghum types), they are generally considered to be unlikely to accumulate prussic acid.

            Cattle that must be grazed on 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.  Call your local veterinarian for immediate help for those animals that are affected.