Use Caution When Grazing Johnsongrass After Frost

With the arrival of low fall temperatures hinting at the winter surely on its way, ranchers and pasture managers should be keeping an eye on what their cattle are grazing, experts with the University of Arkansas cautioned this week.

John Jennings, extension forage specialist for the Division of Agriculture, said that scattered frosts will likely be occurring across the state over the next few weeks and will increase toxicity risk when grazing pastures containing johnsongrass.

“When johnsongrass becomes stressed, it can produce prussic acid, also known as hydrocyanic acid, which is very toxic to livestock,” Jennings said. “Prussic acid toxicity can kill cattle quickly, often before a producer has a chance to observe that the animal is under stress.”

Forages prone to prussic acid are johnsongrass, sorghum, sudangrass, Green Graze (a family of hybrid varieties), grain sorghum and forage sorghum, Jennings said. Freeze damage from fall frosts can cause these forages to become toxic.

When frosts do occur, all is not lost, Jennings said — the forages just need time to dry out, allowing the prussic acid time to dissipate.

“These forages should not be grazed following a hard frost until the plants become completely dried out and brown paper colored,” Jennings said. “Do not graze at night when frost is likely. To reduce risk even further, don’t turn hungry cattle directly out on johnsongrass pasture. Make sure they have grazed other forages first or fill them up on
hay.

“Properly dried johnsongrass hay does not contain prussic acid and is safe to feed,” he said.

While silage may contain toxic quantities of prussic acid, it usually escapes in gaseous form while being moved and fed. If frosted forage is ensiled or wrapped in plastic as baleage, Jennings said producers should allow the fermentation to take place for at least six weeks before feeding it to livestock


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