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Managing frosted forage crops on prevent plant acres

This article was written by Dr. Jared Goplen, UMN Extension crops educator stationed at the regional Extension office in Morris. While northwest Minnesota was able to largely avoid the prevent plant situation that greeted some in central and southern Minnesota, not all forage crops may have escaped unfrosted. 

Temperatures in much of the state have already been below freezing, or will be soon. If you are still planning to graze or mechanically harvest forage on cover crop acres it is important to keep forage species in mind as some species can have toxic effects on animals. 

Is my forage crop safe?

Harvesting sorghum-sudangrass for forage.
There is not an issue with grazing or feeding forage of most frosted forage crops. Alfalfa, clovers, peas, small grains, and common pasture grasses have little to no concern of being toxic to livestock. There may be a slightly increased risk of bloat with frosted legumes but these crops do NOT produce toxic compounds like prussic acid following a frost. Typical bloat management will alleviate any issues with grazing legumes.

Sorghum species are primary concern

If you are planning to graze or harvest sorghum, sorghum sudangrass, or sudangrass alone or as part of a species mixture, special precautions are needed to prevent health concerns for livestock. Risk is greatest immediately following a killing frost. The health concern gradually dissipates as plants dry out. Generally the lowest risk for poisoning is from sudangrass, while grain sorghums have the highest risk. Sorghum sudangrass hybrids and forage sorghums tend to have an intermediate level of risk. New plant growth or regrowth has the highest concentrations of prussic acid for any of these species.


Grazing frosted sorghum species poses the greatest risk to livestock. During frost events, the plant cells rupture, which triggers the production of prussic acid, or hydrocyanic acid. This compound prevents oxygen transport and causes death by asphyxiation when consumed in large quantities. Forage should be safe to graze once plants dry out and prussic acid dissipates, typically within 1-2 weeks after the frost event.

Mechanical Harvest

While prussic acid is also produced when forage is mechanically harvested, the curing process for dry hay, or the wilting and ensiling process of baleage or silage production should alleviate most concerns of poisoning animals. When chopping for silage, waiting at least 5 days after the frost can eliminate most risk of prussic acid poisoning.

Prussic acid poisoning is not a common issue. When livestock are not grazing these forages during or immediately following a frost, there is little concern. Waiting 1-2 weeks following the killing frost before grazing should prevent most issues. If there is a concern with harvested forages, have a forage sample tested to eliminate any concerns with feeding.

More information

For more information on feeding frosted forages visit:
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