How Freezes Affect Forages
With temperatures dropping to the low 30s in northwest Nebraska this week, freezes are likely and may create a hazard for livestock feeding on forages.
When plants freeze, changes occur in their metabolism and composition that can poison livestock. Sorghum-related plants, such as cane, sudangrass, shattercane, and milo, can be highly toxic for a few days after a frost. Freezing breaks plant cell membranes, which allows the chemicals that form prussic acid, also called cyanide, to mix together and rapidly release a poisonous compound. Livestock eating recently frozen sorghums can get a sudden, high dose of prussic acid and could die. Fortunately, prussic acid soon turns into a gas and disappears into the air. Waiting three to five days after a freeze before grazing sorghums can greatly reduce the chance of poisoning.
Freezing also slows down metabolism in all plants. This stress can cause 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 that drops down to about 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. 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, safer management practice.