Hazards of grazing after first frost

University of Minnesota Extension, Stacey Caughey

One thing to keep in mind with grazing fall forages is the frost

A lot of calves are starting to get weaned in the county and this is where we start thinking of how to stretch some of our forages before we can graze corn stalks once they come off. Many of us have cover crops that we want to graze to help get us through this month before we turn out onto corn stalks. One thing to keep in mind with grazing fall forages is the frost. We received our first overnight freezing temps this last week, which means that cattle cannot graze our cover crops because of the hazards that are present with a freeze. According to the University of Nebraska-Lincoln Extension forage specialist, Bruce Anderson “When plants freeze, changes occur in their metabolism and composition that can poison livestock. But you can prevent problems”

Prussic Acid

Anderson writes, “sorghum-related plants, like cane, sudangrass, shattercane, and milo can be highly toxic for a few days after a 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. Wait 3-5 days after a freeze before grazing sorghums; the chance of poisoning then becomes much lower”

Nitrate Build-up

Freezing will also slow down the 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 20 degrees F, cold enough to cause the 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 the 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 an excellent option 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.

Please contact Stacey Caughey, Interim Extension Educator for Stearns, Benton and Morrison counties with any questions at 218-330-5737 or butle269@umn.edu

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