Fall Frost Can Affect Forage Toxicity for Animals

Fall frost brings seasonal forage questions and ISU Extension forage specialist provides answers.

The first frost of the autumn generally brings a flurry of forage related questions centered on three general topics:
* What is the toxic prussic acid potential and management of frosted sudangrass and sorghum sudangrass hybrids?
* Is frosted alfalfa toxic to grazing animals?
* Now that we've had frost, should I harvest the last alfalfa cutting?
Steve Barnhart, Iowa State University Extension forage agronomist, provides the following answers to these often-asked questions.

Manage frosted sorghum-sudan and sudangrass

The potential for prussic acid poisoning, and the management suggestions for farmers to take, are related both to the size of the plant when it is frosted and the extent of frost damage. Farmers should be aware that the risk of damaging levels of prussic acid is very unlikely.

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. 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. Then 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.

Prussic acid poisoning is not common

Prussic acid poisoning is not a common occurrence. Very few verified cases are reported by veterinarians. Maybe Iowa farmers are just using good management. Consider the recommendations above to be at the low risk or conservative level.

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.

Frost damaged sudangrass or sorghum sudangrass hybrids can be cut and stored as silage. Hydrocyannic acid is dissipated during wilting and partially during the ensiling process. Observe proper ensiling technique, particularly moisture content, when ensiling these crops.

You can have frosted sorghum-sudan tested

Sudangrass and sorghum-sudangrass hybrids are difficult to dry thoroughly enough for safe storage as dry hay. As with wilting and ensiling, most if not all of the hydrocyannic acid is dissipated in the drying process.

If you want to get frosted sudangrass or sorghums tested for hydrocyannic acid content, you should first contact a local forage or plant tissue analysis laboratory and confirm that they can do the test and what they recommend as the proper procedure for collecting, handling and shipping a sample to the lab. See list of laboratories on PM 1098A "Forage Testing Laboratories". Final thought: Sudangrass or sorghum-sudangrass should never be used for horse pasture.

Often-asked question: 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 they can be safely grazed or harvested for hay or silage following a frost. There is probably a slightly higher bloat risk for grazed alfalfa and white clover the first few days after a frost. Follow normal bloat preventing grazing management when grazing alfalfa and clover.

Indiangrass (a perennial, warm-season prairie grass) and birdsfoot trefoil have a low potential to form hydrocyannic acid. Actual problem cases using these forages should be considered extremely rare and of minimal concern.

Should I harvest the last alfalfa cutting after frost?

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-degree freeze that lasts for four to six hours or so.

If the farmer 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 farmer needs the forage, then he should 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.

If the frost were a light, non-killing freeze, the tops of the alfalfa plants will be visibly damaged but will not likely stop growing 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 a hard, killing freeze. Then if the farmer 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 which experience further normal growing temperatures will start new regrowth from crown buds, using accumulated proteins and carbohydrates that would otherwise be used for overwintering 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.

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