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A University of Tennessee animal science professor says cool weather moving into Tennessee could cause certain grasses to become deadly to cattle.

Dr. Jim Neel, beef cattle specialist for UT’s Agricultural Extension Service, says each year’s first frost triggers a condition known as prussic acid poisoning in sorghum grasses, which are commonly used as cattle feed.

Neel says prussic acid poisoning can occur when feeding or grazing frost-damaged sudangrass, sorghum-sudangrass hybrids, forage sorghum, or grain sorghum.

“Cold weather is expected for the next few days here in Tennessee, and could drop down cold enough to frost,” Neel says. “Frost on sorghum type forages, which are used by many cattle producers use in Tennessee to graze cattle, will create a condition called prussic acid poisoning.”

Neel says prussic acid can kill grazing cattle in minutes. Symptoms include excess salivation, difficult breathing, staggering and convulsions. The animals die of asphyxiation.

“When the cattle consume this frosted forage, it goes into their digestive system and produces hydrogen cyanide, which is poisonous,” Neel says. “It is absorbed into the bloodstream and prevents the carrying of oxygen to the cells of the body. So, the animal actually dies due to lack of oxygen.”

Neel says sorghum grasses and grains should not be grazed for 2 weeks after killing frost. The prussic acid disappears once the grass is dead.

“Usually after the grass has frosted, turned brown and freezes and moves into winter or the month of November, you no longer have any problem.”

Neel says the safest approach is to remove cattle from fields containing plants that are prone to produce prussic acid.

Cyanide potential is less if the sorghum grasses are put up as hay or silage, he says. Fresh forage is generally higher in cyanide than in silage or hay because cyanide is volatile and dissipates as the forage dries.

However, hay or silage likely to contain high cyanide levels at harvest should be analyzed, and should not be fed to cattle for 6 to 8 weeks after ensiling, he says.