Freezing breaks plant cell membranes which allows the chemicals that form prussic acid to mix together and rapidly release a poisonous compound. Livestock which eat recently frozen sorghum can get a sudden, high dose of prussic acid and potentially die. Fortunately, prussic acid soon turns into a gas and disappears into the air. Waiting three to five days to graze these forages should help you avoid potential problems.
Freezing also slows down metabolism in all plants. This stress sometimes permits 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, however feeding green chop or hay cut right after a freeze can be dangerous. Alfalfa reacts two ways to a hard freeze -- one that drops to 20°F and is cold enough to cause plants to wilt. Nitrate levels can 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.
Bruce Anderson
Extension Forage Specialist
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