Heading into November we had a few, light scattered frosts in the area that have generated some questions about forage use after a frost. The two most common questions concern the use of warm season grasses in the sorghum family and grazing alfalfa. The issue with grasses in the sorghum family, which includes sorghum-sudangrass hybrids, sudangrass and Johnsongrass in addition to sorghum, is that they contain cyanogenic glycosides and enzymes that convert those compounds to free cyanide (sometimes called Prussic acid) within their cells. Prussic acid or cyanide is a lethal toxin.
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.
Under normal circumstances the cyanogenic glycosides and the enzymes are held in different locations within the plant cell and don’t come into contact with each other. However, when plant cells are ruptured after being frozen, chopped, wilted or crushed, those cell barriers are broken and cyanide can rapidly form. Cyanide is a gas and it will volatilize and leave the plant tissue but it takes some time, thus the recommendation is not to allow livestock to graze frost damaged forages until several days (three or four) have passed. Generally this refers to a hard frost. In the case of light frosts where the temperature is greater than 28 degrees F, there are publications that say to wait two weeks until grazing. The highest concentration of prussic acid is found in the leaves of immature plants (less than 18 to 24 inches tall) while stalks of mature plants (greater than 30 inches tall) contain the lowest concentration.
Probably the safest and least risk practice of utilizing sorghum species forages after frost is as a dry hay or ensiled forage. By the time the plants are dry enough to bale, the cyanide gas will have volatilized and dissipated from the plant so there is no feeding risk. In the case of an ensiled forage or wet wrapped baleage, the cyanide concentration is greatly reduced during the ensiling process. The general recommendation is not to feed these ensiled or baleage forages until at least four to six weeks after ensiling or wrapping.
Occasionally there are questions about grazing alfalfa after a frost. Anytime a pure or very high percentage legume is grazed, the livestock owner should take precautions to prevent bloating, but in the case of alfalfa, the risk of bloating is increased for a few days after the plants have been exposed to a hard frost of 25 degrees F or lower. Once those plants start to wilt (in the case of a hard killing frost) or several days have passed, the risk of bloat decreases.
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.
For those livestock owners with tall fescue pasture, frost is actually good news because the sugar content within fescue increases. It is part of the reason that tall fescue works well for stockpiled late fall and winter grazing.