How Frost Impacts Forage Quality and Livestock Safety

By Blake Jackson

Frost can create several forage management challenges for livestock producers, particularly in alfalfa and warm-season annual crops.

According to Penn State Extension educators Leanna Duppstadt and Justin Brackenrich, producers should carefully evaluate grazing and harvesting decisions following frost events to avoid livestock health issues and protect forage stands.

Alfalfa and other legumes can present a risk of bloat after frost, especially during the first few days following damage. Fields consisting primarily of legumes pose the greatest concern, while mixed stands containing more grass carry lower risk.

Experts recommend waiting until frost-damaged forage has dried before allowing livestock to graze. Providing animals with dry hay or another safe forage beforehand can also help reduce the risk of bloat.

For producers planning a late-season alfalfa harvest, it is best to wait until after a killing frost, which typically occurs when temperatures remain around 25°F for several hours. Harvesting too early can reduce the plant's carbohydrate reserves needed for winter survival.

Leaving sufficient stubble, ideally around six inches and no less than 3.5 inches, can help trap snow and improve winter protection.

Warm season annuals including sorghum, sudangrass, and millet require additional attention after frost. Stress conditions can lead to elevated nitrate levels, which may remain high even after a killing frost. Forages suspected of containing high nitrate concentrations should be tested before feeding.

Prussic acid poisoning is another serious concern in sorghum species. Frost-damaged plants can release toxic cyanide compounds that may be fatal to livestock.

Producers should avoid grazing these crops immediately after frost and wait seven to ten days following a killing frost before allowing animals access. If regrowth occurs after a light frost, grazing should be delayed until plants reach at least two feet in height or have regrown for two weeks.

Ensiling can help reduce prussic acid levels, though forage should remain in storage for several weeks before feeding. Hay production also lowers risk because much of the toxic compound dissipates during drying.

Careful forage management after frost can help protect livestock health while maintaining forage quality and stand longevity.

FAQs

• How long should livestock stay off alfalfa after a frost?

Wait until the forage has dried, especially during the first 1-5 days after frost damage.

• Why are nitrates a concern after frost?

Plant stress can cause nitrate accumulation, and levels typically do not decrease after a killing frost.

• Which forages are most likely to contain prussic acid?

Sorghum, johnsongrass, shattercane, and sorghum-sudangrass hybrids pose the highest risk.

• How long should producers wait to graze sorghum after a killing frost?

Wait 7-10 days to allow prussic acid levels to decline as the forage dries.

• Does making hay reduce prussic acid risk?

Yes, the drying process can reduce prussic acid concentrations by up to 75%.

Photo Credit: gettyimages-dszc