The first frost of the autumn generally brings questions centered around three general topics:

  • Toxic prussic acid potential and management of frosted sudangrass and sorghum sudangrass hybrids. 
  • Suspected toxicity of frosted alfalfa to grazing animals. 
  • Post-frost harvest of last alfalfa cutting.

The potential for prussic acid poisoning and management suggestions are related both to the size of the plant when frosted and the extent of frost damage.

Prussic acid, more correctly called hydrocyannic acid (a cyanide-based compound) is formed in sudangrass or sorghum sudangrass hybrids which are severely stressed or frost damaged. The hydrocyannic acid develops within a few hours after the frost and usually dissipates within a few days.

The safest management is to remove cattle and sheep from frosted fields for several days. Livestock can be returned to frost injured sudangrass that is 18 inches or taller and sorghum sudangrass 30 inches or taller after about three or four days.

Meanwhile, frost-injured alfalfa, clovers, and the commonly used perennial cool-season forage grasses do not have the potential to form hydrodynamic acid, are not considered toxic and can be safely grazed or harvested for hay or silage following a frost.

Finally, when it comes to harvesting alfalfa following frost, there is not a simple answer. In general, it will depend whether the frost was a killing frost or not. If you don’t need the forage, it is best for the alfalfa plants to leave them uncut and standing through the winter.

If it was the hard, killing freeze and you need the forage, harvest as soon as possible after the freeze to salvage as much of the nutritive value as possible. The longer the delay, the greater the weathering damage and leaf loss from the standing frosted plants. To improve plant crown insulation over the winter, consider leaving a 5- to 6-inch stubble at this late-season harvest.

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