Fall is in the air and Jack Frost will strike sooner or later. When he does, questions always arise concerning the dangers of feeding frosted forages. A very few forage species can be extremely toxic soon after a frost.
The warm-season annual grasses in the sorghum family and other closely related species are capable of becoming toxic to livestock after a frost event. Those species contain compounds called cyanogenic glucosides that convert quickly to prussic acid in freeze-damaged plant tissue. Prussic acid is also known as hydrogen cyanide - the very substance of murder mysteries!
The potential toxicity after frost varies by species. Sudangrass varieties are low to intermediate in cyanide poisoning potential, sudangrass hybrids are intermediate, sorghum-sudangrass hybrids and forage sorghums are intermediate to high, and grain sorghum is high to very high and is most likely to be toxic after a frost. Piper sudangrass has low prussic acid poisoning potential. Pearl millet and foxtail millet have very low levels of cyanogenic glucosides and rarely cause toxicity.
Other species that have potential to have toxic levels of prussic acid after frost are Johnsongrass, chokecherry, black cherry, indiangrass, elderberry, and some varieties of birdsfoot trefoil.
Animal symptoms of prussic acid poisoning: Animals can die within minutes if they consume forages such as the sorghum species that contain high concentrations of prussic acid in the plant tissue. The prussic acid is released from the forage and interferes with oxygen transfer in the blood stream of the animal, causing it to die of asphyxiation. Before death, symptoms include excess salivation, difficult breathing, staggering, convulsions, and collapse.
Ruminants are more susceptible to prussic acid poisoning than horses or swine because cud chewing and rumen bacteria help release the cyanide from plant tissue.
Factors that increase prussic acid toxicity: Plants growing under high nitrogen levels or in soils deficient in phosphorus or potassium will be more likely to have high cyanide poisoning potential. After frost damage, cyanide levels will likely be higher in fresh forage as compared with silage or hay. This is because cyanide is a gas and dissipates as the forage is wilted and dried for making silage or dry hay.
Young, rapidly growing plants of species that containing cyanogenic glucosides will have the highest levels of prussic acid. After a frost, cyanide is more concentrated in young leaves than in old leaves or stems. New growth of sorghum species following frost is dangerously high in cyanide. When sorghum species regrow after a drought, the new growth is also dangerously high in cyanide. Pure stands of indiangrass (not common in Ohio and nearby regions) can have lethal levels of cyanide if they are grazed when the plants are less than 8 inches tall.