Some of northwestern Oklahoma and the panhandle have already experienced a frost or freeze from the last cold front However the rest of the state has yet to experience the first real frost of autumn 2018. Soon a cold front will bring near-freezing to sub-freezing nighttime temperatures to the rest of Oklahoma.
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 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. 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". A hard freeze is a period of at least four consecutive hours of air temperatures that are below 25 degrees Fahrenheit. Many plants can survive a brief frost, but very few can survive a hard 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.
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 immediately. Call your local veterinarian for immediate help for those animals that are affected. Be certain to read OSU Fact Sheet PSS-2904 “Prussic Acid Poisoning” before turning cattle to potentially dangerous fields.
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. Therefore 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. Visit with an OSU County Extension office about testing procedures and read OSU Fact Sheet PSS-2903 “Nitrate Toxicity in Livestock”.