by Margaret Smith, Forage Agronomist, Albert Lea Seed

Fall’s first freeze brings concerns about toxicity in some forages, including three major concerns following freezing temperatures in the fall:

1. Prussic acid Poisoning

2. Nitrate toxicity

3. Bloat

Prussic Acid Poisoning

Sorghum and some related plants contain compounds that soon after a freeze are converted to prussic acid (hydrogen cyanide) in their freeze-damaged tissues. These plants, though, vary in their potential toxicity. Plants within this group listed from highest toxicity potential to lowest are listed below.

  • Grain sorghum: high to very high potential
  • Indiangrass: high toxic potential
  • Sorghum-sudangrass hybrids and forage sorghums: intermediate to high potential
  • Sudangrass hybrids: intermediate potential
  • Sudangrass varieties: low to intermediate potential
  • Piper sudangrass: low potential
  • Pearl and foxtail millet: rarely cause toxicity

After freeze damage, prussic acid (cyanide) will be greatest in fresh forage compared to hay or silage.  Cyanide is a gas and dissipate as forage wilts for silage or dries for hay. Cyanide is more concentrated in young leaves and tillers following frost, so regrowth is particularly a concern.

Grazing Management to Avoid Prussic Acid

  • Don’t graze these species after a killing freeze until plants are dried down typically five to seven days.
  • Don’t graze for two weeks after a non-killing frost.
  • If new growth appears at the base of plants following a non-killing frost, wait for a killing freeze, then delay grazing for 10-14 days.

Harvesting Forage with Prussic Acid Potential

Green chop lowers the risk of prussic acid poisoning compared with grazing, as animals can’t selectively choose young leaves. But, hay and silage are safer options. Mow for hay any time after the killing frost. Cyanide will dissipate from the drying forage. For silage, wait five to seven days after the freeze before chopping, then delay feeding the ensiled forage for eight weeks.

Nitrate Poisoning

Frost slows down forage growth. If roots are still taking up nitrogen, nitrate may accumulate in the plant tissues. This can occur in millets, oats and other small grains, and sudangrass. The concentration for grazing is usually not a problem, but can be in green chop or hay cut right after a freeze. Labs that test forages can evaluated the nitrate content if there are any doubts.


Forage legumes are at increased risk of causing bloat one to two days following a hard frost or freeze.

Freeze damage to succulent leaf tissue releases soluble cell contents that can lead to bloating. Wait a few days after a killing frost before grazing pure legume stands. Legumes in mixed species swards are much less likely to cause any bloat problems, due to dilution of legume in the total forage mixture.