By Bruce Anderson
When plants freeze, changes occur in their metabolism and composition that can poison grazing livestock if not properly managed.
Sorghum-related plants like cane, sudangrass, shattercane, and milo can be highly toxic for a few days after frost. Freezing breaks plant cell membranes. This breakage allows the chemicals that form prussic acid, which is also called cyanide, to mix together and rapidly release this poisonous compound. Livestock eating recently frozen sorghums can get a sudden, high dose of prussic acid that can be lethal. Fortunately, prussic acid soon turns into a gas and disappears into the air. Delay grazing sorghum for three to five days after a freeze to reduce the risk of poisoining.
Freezing also slows metabolism in all plants. This stress can allow nitrates to accumulate in plants that are still growing, especially grasses like oats, millet, and sudangrass. This build-up usually isn't hazardous to grazing animals, but green chop or hay cut right after a freeze can be more dangerous.
Alfalfa reacts two ways to a hard freeze — one close to 20°F and cold enough to cause plants to wilt. It can cause nitrate levels to increase, but rarely to hazardous levels. Freezing also makes alfalfa more likely to cause bloat for a few days after the frost. Then, several days later, after plants begin to wilt or grow again, alfalfa becomes less likely to cause bloat. Waiting to graze alfalfa until well after a hard freeze is a good, safe management practice.
Frost causes important changes in forages so manage them carefully for safe feed.