– Bill Weiss, Dept. of Animal Science, The Ohio State University
Some summer annual grasses contain compounds that can be converted to cyanide when the plant cells are damaged. The concentrations of these compounds vary among plant species: sorghum contains the highest concentrations, followed by sorghum-sudangrass crosses (sudax), and sudangrass contains the lowest concentrations. The concentrations of these compounds are highest in immature plants and decrease as the plant matures and leaves contain much higher concentrations than do stems. Frost will damage or kill plant cells which allows the formation of cyanide making the forage toxic to ruminants. Ruminants should not be allowed to graze frost-damaged sorghum, sudax and sudangrass. The cyanide that is produced following a frost will eventually dissipate into the air after the killed plant material wilts. This usually requires 5 or 6 days after a frost. If the frost completely kills the plant, grazing can resume 5 or 6 days later, however, if the plants were not completely killed, grazing after a frost has a high risk of causing toxicity because regrowth usually contains high concentrations of toxic compounds. Therefore, grazing of these types of plants should cease once the first frost has occurred and should not commence again until 5 or 6 days following a killing frost.
Frost-damaged sorghum, sudax, and sudangrasses can be made into hay or silage with little or no risk of toxicity. When the plants are wilted enough to make hay, most of the cyanide, which is volatile, will have dissipated. Normal silage making also allows most of the cyanide to dissipate, but the silage should not be fed for at least 3 wk following ensiling. Feeding green-chopped forage that has been frost damaged has a lower risk than grazing because of reduced selection by the animals, however, green-chopped sorghum, sudax, and sudangrass can still be toxic.