We have had a beautiful fall so far, but Jack Frost will be visiting us soon. Now is the time to finish harvesting and grazing several forage species that can be extremely toxic soon after a frost. Those include primarily annual grasses in the sorghum family and other closely related species that contain compounds called cyanogenic glucosides, which are converted quickly to prussic acid (i.e. hydrogen cyanide) in freeze-damaged plant tissues.
Other species that can develop toxic levels of prussic acid after frost are Johnsongrass, shattercane, chokecherry, black cherry, indiangrass, and elderberry. It is always a good idea to check areas where wild cherry trees grow after a storm and pick up and discard any fallen limbs to prevent animals from grazing on the leaves and twigs.
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.
Animals can die within minutes if they consume forage with high concentrations of prussic acid. Prussic acid 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.
Plants growing under high nitrogen levels or in soils deficient in phosphorus or potassium will be more likely to have high prussic acid 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 contain cyanogenic glucosides will have the highest levels of prussic acid. After a frost, cyanide is more concentrated in young leaves and tillers than in older leaves or stems. New growth of sorghum species following a non-killing frost is dangerously high in cyanide. Pure stands of indiangrass can have lethal levels of cyanide if they are grazed when the plants are less than 8 inches tall.
The following guidelines will help you avoid danger to your livestock this fall when feeding species with prussic acid poisoning potential:
• Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost, even if it was a light frost.
• 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 hard, killing freeze,then wait another 10 to 14 days before grazing the new growth.
• Don’t allow hungry or stressed animals to graze young growth of species with prussic acid potential.
• Graze or greenchop sudangrass only after it is 15 to 18 inches tall. Sorghum-sudangrass should be 24 to 30 inches tall before grazing. Never graze immature growth or short regrowth following a harvest or grazing (at any time of the year).
• Do not graze wilted plants or plants with young tillers.
• Green-chopping the frost-damaged plants will lower the risk compared with grazing directly, because animals be less likely to selectively graze damaged tissue. However, the forage can still be toxic, so feed greenchop with great caution after a frost.
• Always feed greenchopped forage of species containing cyanogenic glucosides within a few hours, and don’t leave greenchopped forage in wagons or feedbunks overnight.
Hay and silage are safer
Prussic acid content in the plant decreases dramatically during the hay drying process and the forage should be safe once baled as dry hay. The forage can be mowed anytime after a frost if you are making hay. It is very rare for dry hay to contain toxic levels of prussic acid. However, if the hay was not properly cured and dried before baling, it should be tested for prussic acid content before feeding to livestock.
Forage with prussic acid potential that is stored as silage is generally safe to feed. To be extra cautious, wait 5 to 7 days after a frost before chopping for silage. If the plants appear to be drying down quickly after a killing frost, it is safe to ensile sooner after a frost.
Delay feeding silage for 8 weeks after ensiling. If the forage likely contained high levels of cyanide at the time of chopping, hazardous levels of cyanide might remain and the silage should be analyzed before feeding.
What about frosted alfalfa, clover, and grasses?
Other common forages such as alfalfa, clovers, and cool-season perennial grasses do not produce toxic compounds after a frost. However, the risk of bloat is higher when grazing alfalfa, clovers, or other legumes one or two days after a hard frost. The bloat risk is highest when grazing pure legume stands, and least when grazing stands having mostly grass.
The safest management is to wait a few days after a killing frost before grazing pure legume stands until the forage begins to dry from the frost damage. It is also a good idea to make sure animals have some dry hay before being introduced to lush fall pastures that contain significant amounts of legumes.