– Mark Sulc, OSU Extension Forage Specialist
Sudex is one of the forages that can be toxic when frosted.
I am beginning to get questions about toxicities that can develop after forages are frosted. There is potential for some forage toxicities and other problems that can develop after a frost. Prussic acid poisoning and high nitrates are the main concern with a few specific annual forages and several weed species, but there is also an increased risk of bloat when grazing legumes after a frost.
Nitrate accumulation in frosted forages. Freezing damage slows down metabolism in all plants, and this might result in nitrate accumulation in plants that are still growing, especially grasses like oats and other small grains, millet, and sudangrass. This build-up usually is not hazardous to grazing animals, but greenchop or hay cut right after a freeze can be more dangerous. When in doubt, send in a sample to a forage testing lab and request a nitrate test before grazing or feeding a forage after a frost.
Prussic Acid Toxicity
Several forage and weed species contain compounds called cyanogenic glucosides that are converted quickly to prussic acid (i.e. hydrogen cyanide) in freeze-damaged plant tissues, or under drought conditions. Some labs provide prussic acid testing of forages (see a partial list at the end of this article). Sampling and shipping guidelines should be carefully followed because . . .
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