Use caution when grazing johnsongrass after frost

John Jennings-Extension Forage Specialist

Scattered frosts will 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 (hydrocyanic acid) which is very toxic to livestock. Prussic acid toxicity can kill cattle quickly, often before a producer has a chance to observe that the animal is under stress. The forages that are prone to prussic acid are johnsongrass, sorhgum/sudan, greengraze, grain sorghum, and forage sorghum. Freeze damage from fall frosts can cause these forages to become toxic. These forages should not be grazed following a hard frost until the plants become completely dried out and paper brown colored. Do not graze 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. 

Prussic acid dissipates as the plants dry out.  Properly dried johnsongrass hay does not contain prussic acid and is safe to feed. 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 weeks before feeding. 

For more information see our factsheet at  https://www.uaex.edu/publications/PDF/FSA-3069.pdf   at the Bradley County Cooperative Extension Service office at 870-226-8410.