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McClain County

Prussic Acid Poisoning is a Concern after a Light Frost

It was discovered in the early 1900’s that under certain conditions sorghums are capable of releasing hydrocyanic acid or 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.  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.  Light frosts that stress the plant but do not kill it, are often associated with prussic acid poisonings.  Lush young re-growth of sorghum plants are prone to accumulate prussic acid as well as slightly frosted mature leaves.  

Producers should avoid grazing fields with sorghum type plants (including johnsongrass) 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.  Haying, if properly cured, also will eliminate prussic acid problems.  One can never be absolutely certain that a field of sorghum is 100% safe to graze.  Millets are much less likely to accumulate prussic acid than other forage sorghum types.  Millets, however, like other forage sorghums, can be high in nitrate.  Prussic acid toxicities can occur at any time when these types of plants are stressed.  Light frosts are just one of several stressors that can interrupt normal plant metabolism.  Other potential stressors are drought, excessive heat, and consecutive cloudy days.

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 immediately.  Call your local veterinarian for immediate help for those animals that are affected. 


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