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Scattered Frost Causes Concern In Pastures

October 25, 2013

John Jennings – Extension Forage Specialist

Scattered frost across the state can turn a good forage deadly. Late summer rains brought on a flush of johnsongrass in many pastures and it became dominant in some fields. When johnsongrass becomes stressed from drought or frost, it can produce prussic acid (hydrocyanic acid) which is very toxic to livestock. Immature plants and regrowth following haying or grazing contain the highest levels. Light frosts that occur in fall can wilt 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. Sorhgum/sudan, greengraze, grain sorghum, and forage sorghum can also develop prussic acid after frost. 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 paper brown colored. Do not graze it at night when frost is likely. To reduce risk even farther, 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 out. Properly dried johnsongrass hay does not conatin prussic acid and is safe to feed.

For more information ask for FSA 3069 Prussic Acid at your county extension office.

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