Frost can turn good pasture deadly

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LITTLE ROCK -- Scattered frost across the state can turn good forage deadly, said John Jennings, professor-forage, for the University of Arkansas System Division of Agriculture.

“Late summer rains brought on a flush of johnsongrass in many pastures and it became dominant in some fields,” he said. “When johnsongrass becomes stressed from drought or frost, it can produce prussic acid, also known as hydrocyanic acid, which is very toxic to livestock.”

Immature plants and regrowth following haying or grazing contain the highest levels. Jennings said the light frosts that occur in fall could wilt the tops of the plants, causing them to become toxic.

“Prussic acid toxicity can kill cattle quickly, often before a producer has a chance to observe that the animal is under stress,” he said. “Sorhgum/sudan, green graze, grain sorghum, and forage sorghum can also develop prussic acid after frost.”

Jennings recommends that frost-damaged johnsongrass should not be grazed for at least seven days after the first killing frost.

“It is best to delay grazing until the frosted plants become completely dried out and brown paper colored,” he said. “Do not graze it 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.”

Silage may contain toxic quantities of prussic acid, but it usually escapes in gaseous form while being moved and fed. If frosted forage is ensiled, allow fermentation to take place for at least six to eight weeks before feeding. Prussic acid dissipates as the plants dry. Properly dried johnsongrass hay does not contain prussic acid and is safe to feed.

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