Temperatures are expected to fall below freezing across much of Arkansas by this weekend, which should prompt cattle producers to use caution before allowing livestock to graze fields of johnsongrass. Prussic acid (hydrocyanic acid) can result from the frost and can be toxic to the livestock. 

“Frosts in the fall can wilt tops of the plants, causing them to become toxic,” said John Jennings, extension forage specialist at the University of Arkansas System Division of Agriculture. “Prussic acid toxicity can kill cattle quickly, often before a producer has a chance to observe that the animal is under stress.” 

Johnsongrass has spread widely across the state in the past two years because of summer rains and has become dominant in some fields. Johnsongrass isn’t  the only forage to produce prussic acid. It can also develop in sorghum/sudan, greengraze, grain sorghum and forage sorghum after a frost. 

Jennings advised producers that livestock shouldn’t graze johnsongrass for at least seven days after the first killing frost. “The key visual clue for safe grazing after frost is when the frosted plants become completely dried out and paper-brown colored,” he said. “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.” 

Prussic acid dissipates as the plants dry out, Jennings said. 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, ask for a copy of FSA 3069 Prussic Acid at your county extension office.