Written by: Dr. Gary Bates, University of Tennessee Forage Specialist
The winter frosts are upon us. The cool weather is usually good for grasses like tall fescue and orchard grass but sorghum, sorghum x Sudan, Sudan grass hybrids and johnsongrass can be toxic after a frost.
The frost causes a release of prussic acid or cyanide. Allowing cattle to graze sorghums that have been frosted can be disastrous. Cyanide prevents oxygen transfer from the blood and can be lethal due to suffocation at the cellular level.
Here are some things you can do to reduce the potential for prussic acid poisoning:
- Don’t graze for two weeks after a non-killing frost.
- Don’t graze after a killing frost until plant material is dry. It’s usually considered safe to feed six to seven days after a killing frost.
- Don’t graze new shoots or tillers emerging, because they are high in prussic acid, until they reach a height of two feet.
- Don’t graze prussic acid-producing plants at night if frost is possible.
- Don’t feed silage cut from a frosted field for six weeks after ensiling.
Prussic acid usually dissipates in 48 hours following a frost but be extra careful in the meantime to avoid poisoning.