When the first frost hits, beef producers should be concerned for grazing cattle if the field contains johnsongrass. Cattle may suffer from prussic acid poisoning caused by this grass.

In normal years producers don't have to worry about this problem, but Georgia experienced a lot of rain this summer. Repeated grazing by cattle prevents johnsongrass from getting established. But the abundant rainfall may have caused some situations where the grass grew faster than the cattle could eat it. It is also prevalent in many hay fields, and some farmers may allow cattle to graze those fields this fall.

Cyanide-producing compounds in living plant cells are converted to prussic acid when cells are crushed or otherwise ruptured. The prussic acid potential of plants is affected by species and variety, weather, soil fertility and stage of plant growth. Plants of the sorghum group like johnsongrass and leaves of wild cherry trees can produce toxic levels of prussic acid, especially when stressed during cold temperatures.

Prussic acid is one of the most potent toxins in nature. As ruminants like cows and goats consume plant materials containing cyanide-producing compounds, prussic acid is liberated in the rumen, absorbed into the bloodstream and carried to body tissues where it interferes with oxygen usage.