3 hazards of grazing forages after the first frost

With the calves weaned and corn stalks still standing, we plan to turn the cow herd out on a field of cover crops we planted earlier this year. The mixture includes sudangrass, turnips, rape, radishes, clover, millet and lentils. We think the new feed should distract the mama cows from their babies and lessen the stress that comes with weaning time.

Additionally, this field of cover crops should tide the herd over for a month or so until we are able to graze corn stalk residues after harvest. Pending the amount of snowfall we receive, these forages should last us until the New Year, with some supplemental hay offered on particularly cold and nasty days.

One thing to keep in mind with grazing fall forages is the frost. Last week, we received our first overnight freezing temperature of the season. At that time, cattle could not graze our cover crops because of the hazards that are present with a freeze.

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According to Bruce Anderson, University of Nebraska-Lincoln Extension forage specialist, “When plants freeze, changes occur in their metabolism and composition that can poison livestock. But you can prevent problems.”

Here are three things to know about the effect of freeze on forages:

1. Prussic acid

Anderson writes, “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 to 5 days after a freeze before grazing sorghums; the chance of poisoning then becomes much lower.”

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2. Nitrate build-up

“Freezing also slows down metabolism in all plants,” says Anderson. “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.”

3. Bloat

Anderson warns, “Alfalfa reacts two ways to a hard freeze, down close to 20 degrees F, 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.”

Cover crops and alternative forages are still excellent options in colder weather, but proactive risk management can negate the dangerous hazards. Be aware of the risks, so you can carefully offer safe feed to your cattle this fall and winter.

The opinions of Amanda Radke are not necessarily those of beefmagazine.com or Penton Agriculture.

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