Prussic acid poisoning a concern after a light frost
The average first frost date for central Oklahoma is about Nov. 1. However, light frosts often occur in the northwest part of this state much earlier. Of course, producers that live in states north and west of Oklahoma can expect these cool nights to occur almost at any time. Light frosts, that stress the plant but do not kill it, are often associated with prussic acid poisonings.
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 bloodstream 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 plants are prone to accumulate prussic acid, especially when the plants are stressed, such as from drought or freeze damage.
Producers should avoid grazing fields with sorghumtype 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 percent 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. — Glenn Selk, Oklahoma State University Extension Cattle Reproduction Specialist