Watch for toxicity in frost-damaged forage
The first frost of the autumn generally brings a flurry of forage-related questions. These questions usually center on three general topics:
- 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?
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.
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 as well. 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 a common occurrence. Very few verified cases are reported by veterinarians. Maybe Iowa producers 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.
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.
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. Sudangrass or sorghum-sudangrass should never be used for horse pasture.
Frost-injured alfalfa, clovers and 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. There is 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.