We have had some scattered frosts around the state 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.
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 do not allow livestock to graze frost damaged forages until several days (3-4) have passed. Generally this refers to a hard frost. In the case of light frosts where the temperature is greater than 28 F, there are publications that say to wait 2 weeks until grazing. The highest concentration of prussic acid is found in the leaves of immature plants (less than 18-24 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 4-6 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 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.
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.
Finally, I sometimes get questions about late fall management of pastures and when can livestock be allowed to graze the plant down close to soil level. My advice is to take care of that plant and maintain leaf residue until that plant goes dormant. Generally once soil temperatures drop to that 40 degree or lower mark the plant is dormant and remaining leave residue can be grazed off if needed.