Warm Season Annuals and Alfalfa after Frost

Warm Season Annuals and Alfalfa after Frost
Oct 06, 2022

By Leanna Duppstadt


Legumes, especially alfalfa still have the potential to cause bloating after a frost. The first 1-5 days are the most crucial depending on the severity of the frost. Pure legume stands are the most problematic and the concern decreases in fields with higher percentages of grasses in the mix. It is best to wait until the forages have dried after frost damage before grazing. Be sure to provide an alternative safe, dry forage before turning animals out onto frost damaged, majority legume, fields. Another option is to mow the field, allow it to dry and let the animals graze the dry hay on the ground.


Anything that interferes with normal plant growth can cause nitrates to accumulate in plants that are still growing. That is because the plants' ability to convert these nitrates into protein is impaired. Forages of concern include warm season annuals such as millet, sorghums, and sudangrass. Nitrate levels will not decrease after a killing frost and forage testing is recommended for forages that are suspected to contain high levels of nitrates.

Prussic Acid

A cyanogenic compound is normally found in sorghum species in a bound, non-toxic form called dhurrin; however, after a killing frost or another source of damage to the plant, a compound also present in these forages called emulsion reacts with the dhurrin and "frees" it, causing a highly toxic, extremely poisonous cyanogenic compound to be produced within the plant. Cyanide interferes with the oxygen-carrying function in the blood, resulting in animal asphyxiation. A concentration of a mere 0.1 percent or greater of dry tissue is considered dangerous and could kill livestock. Ruminants are more susceptible since they consume large amounts of forage and the rumen microbes may aid in the release of the cyanide from the consumed plant tissue.

All species of sorghum contain prussic acid within the vegetative portion of the plant. Sorghum, johnsongrass, and shattercane contain the greatest levels of prussic acid and can still be hazardous as weeds in pure stands of sudangrass or sorghum-sudangrass fields or pastures. Sudangrass contains approximately 40 percent less prussic acid than other sorghums; however, a sorghum x sudangrass hybrid contains a greater level of the toxic compound than sudangrass alone. Improvements in genetic development of forages now allow options for planting varieties of sorghums that contain lower levels of prussic acid, helping to reduce the risk of prussic acid poisoning in livestock.

Another option for incorporating a summer annual pasture or hay crop while reducing the risk of prussic acid poisoning is the use of pearl millet and foxtail millet, which do not contain toxic levels of prussic acid, even after a killing frost. Therefore, it can be grazed any time. Corn is another warm season annual that does not have this issue.

Generally, the greatest levels of prussic acid can be found in the leafier areas of the plant, in particular when the plant is at the boot stage. As plants mature, the prussic acid content declines because the stalk makes up a larger percentage of the plant.

After a killing frost, forage tissues rupture, and cyanide can form. Do not graze on nights when frost is likely as high levels can be produced within hours. Toxic prussic acid does not begin to decline until after the leaves have died. Since cyanide is a gas, it slowly dissipates over time as the plant tissue dries, so to be safe, wait at least 7-10 days after a killing frost to graze or green chop forage. This is roughly the amount of time it will take for the forages to dry. If forages regrow after a non-killing frost, do not graze or feed until the regrowth has reached a minimum of 2 feet in height or 2 weeks, as the regrowth will likely contain high, very toxic levels of prussic acid. Remember that even with overall decreasing prussic acid concentrations, animals may selectively graze young growth that still has high concentrations. Rotational grazing or feeding with other safe pastures, dry hay or ground cereal grains beforehand can reduce the risk by decreasing their overall consumption. Green chop can decrease the risk as well since it cannot be selectively grazed, and the less toxic stems will help to dilute the concentrations.

If chopping for silage is desired, a producer should wait 5-7 days after a frost before harvesting. These forages with a risk of high cyanide levels at the time of chopping should be ensiled for a minimum of 8 weeks before feeding. Storage allows the cyanide gas to escape over time during the fermentation process, but silage should be analyzed before feeding to ensure the toxic compounds have been reduced to a safe level for consumption.

Prussic acid concentrations also decline up to 75% during the hay drying process. Frosted forage can be mowed at any time after a frost and used for hay.

It is important to note that heavy rates of fertilization and drought can also cause high levels of prussic acid accumulation in these forages even months before a killing frost; therefore, precaution should be taken during these conditions as well. Ensiling these forages helps to reduce the risk of toxic levels of prussic acid. Sorghum silage should not be fed any earlier than 3-4 weeks after harvest as a precaution.

If you believe that your livestock are experiencing any symptoms of prussic acid poisoning, which can include labored breathing, excess salivation, staggering, and convulsions, contact a veterinarian immediately. Symptoms appear rapidly, within minutes depending on consumption rate, and treatment needs to be administered quickly. If you are concerned about feeding frosted forages, you can have it tested for cyanide concentration.

For more information or needs on testing for prussic acid or nitrates, contact your local Extension office or questionable forage can be tested for prussic acid (HCN) at the Pennsylvania Department of Agriculture Summerdale Laboratory in Harrisburg (717-787-8808).

Source : psu.edu
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