Don’t forget: Forages can become poisonous to livestock after freeze
November 19, 2016
If you haven't experienced a freeze yet this fall, you soon will. And remember, a freeze can cause hazards for using some forages.
When plants freeze, changes occur in their metabolism and composition that can poison livestock. But you can prevent problems.
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 release this poisonous compound rapidly. Livestock eating recently frozen sorghums 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. So wait 3-5 days after a freeze before grazing sorghums; the chance of poisoning then becomes much lower.
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. The build-up isn't usually hazardous to grazing animals.
— Courtesy of UNL