Key points:
- Freezing damages plant cells resulting in the rapid release of large quantities of prussic acid
- Withhold grazing of sorghum species for 10 to 14 days following frost
- Look for regrowth of sorghum species after a frost and avoid grazing new leaves and shoots
As we’ve gotten further into fall and cooler weather moves in, I’ve had a few folks ask about grazing johnsongrass after a frost due to prussic acid concerns. Sorghum species (johnsongrass, sudangrass, sorghum-sudangrass, etc) are some of the many plants containing a cyanogenetic glucoside (dhurrin) that is stored in the plant cell’s vacuole. When cells are damaged, say through chewing or freezing, dhurrin is released from the cell and free to interact with enzymes in the rumen or within the plant that break the dhurrin molecule apart resulting in the formation of toxic hydrocyanic acid (prussic acid). When consumed in high enough quantities, prussic acid can result in cattle death within minutes.
When sorghum plants experience frosts the cells in their leaves are damaged resulting in the rapid formation and accumulation of prussic acid in the plant. This accumulation happens in more mature foliage as well as newer growth, so grazing sorghum after a frost can be especially dangerous for livestock. Fortunately, prussic acid is relatively short-lived once plants die, with most dissipating after several days as the grass dries down. As a precaution we recommend waiting 10 to 14 days after a frost to graze areas where sorghum species are prevalent. Additionally, before a killing frost sorghum species may regrow. This new growth will be very high in prussic acid and grazing should be avoided until leaves dry down after a killing frost or regrowth reaches 18 to 24 inches tall, which is unlikely at this point in the year. If regrowth occurs, a herbicide application is unlikely to be effective in controlling sorghum species at this point and is not recommended.
