Managing frosted forages

When we get a frost, forage crops such as sudangrass, sorghum, and sorghum sudangrass hybrids will build up a toxic gas called prussic acid which is poisonous to cattle and sheep. Signs include staggering, labored breathing, spasms, and foaming at the mouth. They can die within an hour if not treated.

There steps you can take to prevent it. If you haven’t had a frost yet, avoid grazing on nights when there’s a chance for frost because the toxin will develop in the plant within hours.

Rebecca Vittetoe is an extension field agronomist at Iowa State University. She also recommends not grazing any wilted plants or new re-growth coming from those plants.

"When it’s developing that new regrowth, that new regrowth is going to have really high potential levels of that prussic acid. We want people to wait to graze until that new growth is at least 18”-20”, or 24”-30” for sorghum sudangrass," says Vittetoe. "Now, sometimes in the fall like in Iowa, if it’s not that tall, once you have that light frost I don’t expect those warm season annuals to grow a whole lot more."

Your next option is to stop grazing the forage and harvest it to feed later.

"If you were to mow it and let it dry, those forages should be safe to feed within 5-6 days after being dried. So, that would be one potential option there," she says. "You could also chop it for silage and then wait the proper time. Usually, we want to delay feeding that silage for at least eight weeks after ensiling. That’s also going to greatly reduce that risk."

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