First Fall Frost Brings Flurry of Forage Questions

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

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

Is frosted alfalfa toxic?

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.

Now that we've had frost, should I harvest the last alfalfa cutting?

There is not a simple answer. In general, it will depend whether the frost was a killing frost or not. A killing frost is not the first light frost of the season; rather, it is a 23 or 24°F freeze that lasts for four to six hours or so.

If the producer does not 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 the producer needs 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" stubble at this late-season harvest.

If the frost was a light, non-killing freeze, the tops of the alfalfa plants will be visibly damaged but will not likely stop the plants' growth for the season. The damaged tops will deteriorate in nutritive quality for the remainder of the autumn, but the plant will still be attempting to regrow from crown buds and will be using stored sugars. The best management for the plant is to allow it to continue to grow using whatever green leaf area it still has until the hard, killing freeze. Then if the producer needs the forage, it can be cut and harvested for hay or silage; or grazed.

Alfalfa plants cut immediately after a partial freeze (non-killing frost) and that experience further normal growing temperatures will start new regrowth from crown buds, using accumulated proteins and carbohydrates that would otherwise be used for over wintering and regrowth the following spring. When these late-recovering plants experience a killing freeze a few days or weeks later, they will be physiologically weaker and more susceptible to winter injury.

Managing frosted Sudangrass

Sudangrass or sorghum-Sudangrass should never be used for horse pasture but can be grazed by cattle and sheep. Prussic acid, more correctly called hydrocyannic acid (a cyanide based compound) is formed in Sudangrass or sorghum-Sudangrass hybrids that 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 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. Producers should be aware that the risk of damaging levels of prussic acid is very unlikely.

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. If the grass was shorter than these heights when frost injured, withhold cattle and sheep for 10 days to two weeks following the frost to avoid problems. Watch for new shoot regrowth, (tillers or "suckers”) on partially frost killed plants. Direct grazing of these fresh new shoots can be toxic too. Where new shoots appear following frost, avoid grazing until two weeks after the killing frost that kills the new shoots.

If in doubt, move the livestock to another type of forage. Livestock can be returned to the sudangrass or sorghum sudangrass fields following a killing frost and appropriate post-frost delay period.

Producers who want to get frosted Sudangrass or sorghums tested for hydrocyannic acid content should first contact a forage or plant tissue analysis laboratory near you and ask first whether they can do the test for you and what they recommend as the proper procedure for collecting, handling and shipping of the sample to the lab. Forage Testing Laboratories PM 1098A can help you locate a laboratory.-- Stephen Barnhart, PhD, Department of Agronomy

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