Livestock owners need to be aware that frost and freezing weather can result in some forage species becoming toxic with regard to livestock consumption.  The issue is that some forage species contain compounds called cyanogenic glucosides in their cell compartments.  Frost and freezing temperatures damage plant cell membranes, allowing those compounds to mix.  The result is production of hydrogen cyanide, more commonly known as prussic acid.  Grain and forage sorghum, sorghum-sudangrass hybrids and sudangrass are all warm season annuals that have this potential for prussic acid or cyanide poisoning following a frost event.  Some other species that may be in or near pastures and hay fields and that are known to develop toxic levels of prussic acid include Johnsongrass, shattercane, black cherry, elderberry and chokecherry.

The formation of hydrogen cyanide or prussic acid occurs very quickly, within minutes to a couple of hours after even a light frost.  Cyanide is one of the most rapidly acting toxins and with a lethal dose, death can happen quickly, within a 15 to 20 minute time span.  Do not let livestock graze any plants in the sorghum family immediately following a frost event.  However, because prussic acid is actually a gas, it will dissipate with time as the plant dries out with the passage of time.  For this reason, chopping these plants for silage or using them for dry hay production is a better option than grazing after a frost.

After a frost, cyanide is more concentrated in young leaves and tillers than in older leaves or stems.  If grazing is your only option to utilize this forage, take these precautions:

• Do not graze on nights when frost is likely.
• Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days.
• After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of toxic compounds. 
• New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a killing freeze, and then wait another 10 to 14 days before grazing the new growth.
• Use heavy stocking rates and rotational grazing to reduce the risk of animals selectively grazing leaves that can contain high levels of prussic acid.

More information about use and management of frosted forages is available on the Wayne County Extension web site at http://go.osu.edu/agwayne.