A rancher posted on a facebook group I’m part of that she’d lost 4 cows and one bull to bloat four days after turning them into a frosted alfalfa pasture. What should have been no problem suddenly turned deadly thanks to changes in frosted alfalfa that increase bloat potential. But that’s not the only plant that poses problems after frost. Here Bruce Anderson of University of Nebraska Lincoln reminds us of other dangers and how to avoid them.
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, johnson grass 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 to 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. 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, down close to twenty degrees, 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. So waiting to graze alfalfa until well after a hard freeze is a good, safer management practice.
Frost causes important changes in forages so manage them carefully for safe feed.
Let’s Add a Bit More to Bruce’s List
Does your pasture include wild cherry, black cherry, elderberry or chokecherry trees/bushes? BEWARE! Damaged leaves, whether wilted from cutting or drought, or frosted, have high levels of cyanide and can kill livestock. Animals consuming large amounts of this plant may die in 1 to 60 minutes and show only convulsions and death as signs of poisoning.
If you’ve got these plants in or near your pastures, it’s a good idea to check after a storm and pick up and discard any fallen limbs to prevent animals from grazing on the leaves and twigs.
Plants growing under high nitrogen levels or in soils deficient in phosphorus or potassium will be more likely to have high prussic acid poisoning potential. After frost damage, cyanide levels will likely be higher in fresh forage as compared with silage or hay. This is because cyanide is a gas and dissipates as the forage is wilted and dried for making silage or dry hay.