Feeding Facts

The Dangers of Grazing After a Frost

BY JIMMY PARKER

As fall weather gets here and the first frost of fall gets closer, anyone grazing livestock needs to pay attention to what type of grasses your animals are grazing and if there are any dangers associated with grazing those particular plants after a frost or freeze.

Most of the plants we graze are perfectly fine. Bermudagrass and Bahiagrasses tend to be perfectly safe to graze as long as they are available. Fescue can actually get better after a frost than it was before. However, there are some plants, particularly those in the sorghum families, including Johnsongrass, or sorghum sudangrasses that are well-known for producing prussic acid in times of stress and especially after a frost. Prussic acid is a cyanide-based compound that is absorbed into the blood. It binds with hemoglobin and interferes with oxygen transfer. This causes asphyxiation and can kill an animal within minutes.

Some of the symptoms of prussic acid poisoning are excess salivation, difficulty breathing, staggering, convulsions and collapse. These symptoms tend to happen rapidly and ruminants are more susceptible than other species because cud chewing and rumen bacteria increase the release of cyanide.

When plants freeze, changes occur rapidly in their metabolism. Light frosts that stress the plant but do not kill it entirely are associated with prussic acid poisoning. Cyanide compounds are generally highest in immature plants and less prevalent in mature plants. Leaves contain higher levels than stems and plants that are growing under high nitrogen levels or in phosphorus- or potassium-deficient soils will be more likely to have high cyanide levels.

Luckily cyanide is volatile and dissipates as time passes or as the plant dries. Removing the animals for a period of time is recommended. After 10 to 14 days from the last frost, it is generally safe to resume grazing sorghum plants. If that is not an option, there are other things that can be done. Grazing after three or four days if the plants are 20 to 30 inches tall is usually safe. Keep in mind that new growth at this time will be more dangerous than the mature forages and should be avoided for that 10- to 14-day window.

Grazing high risk pastures should be avoided on nights when frost is likely if possible since high levels of prussic acid are produced within hours of a frost. If removal for any significant period of time is not an option, providing good hay and making sure that the animal’s stomachs are full before grazing frost-damaged plants can be helpful in diluting the toxic compounds. These animals should be monitored closely for the first several hours for any signs of prussic acid poisoning.

Alabama farmers typically start seeing frosts sometime in late October and planning now might save you some animals and at least some headaches when that frosty morning arrives. The light frosts can be a problem, but so can a true killing frost. Prussic acid can form then too, and while the cyanide levels tend to drop dramatically after about three days, most experts recommend removing animals from pastures with high-risk plants for five to seven days after a true killing frost just to be safe. Again, if you do not have the option to remove the animals, then provide good hay and maybe some feed to help dilute the cyanide levels in the animals’ diet and watch them closely for a few hours to make certain that you are not seeing signs of prussic acid poisoning.

Most farmers typically wait as long as they can before putting out hay, but sacrificing a roll or two when the risk of frost approaches might be money well spent to cut down on the risk of having problems with prussic acid poisoning.