Skip to Content

The Weekly News Source for Wyoming's Ranchers, Farmers and AgriBusiness Community

Ranchers encouraged to be mindful of impact of freezing on forages

by Wyoming Livestock Roundup

Many producers use fall grazing and fall crops to feed animals for the upcoming winter, but one factor, which could cause serious damage, is freezing temperatures.

“This is the time of year, producers should be careful about freezes and frosts,” says Anowar Islam, University of Wyoming (UW) Plant Sciences associate professor and Extension forage agroecologist.

“Freeze damage is dangerous because plant growth, metabolism and composition are decreased, which can be lethal to livestock,” he adds.

Effects of freezing

“Small grain crops like sorghum, sudangrass and millet have the tendency to produce prussic acid, which is toxic to livestock,” Islam mentions.

He says freezing temperatures break plant membranes and release chemicals from the plant cells, which combine to create prussic acid, also known as hydrogen cyanide.

“If livestock or wildlife were to eat freeze-damaged forages, they could die within one or two hours, due to toxic levels of prussic acid,” Islam states.

Not all plants produce prussic acid after a freeze, but the sorghum plant group has a higher tendency to become toxic.

Some millets, wheat and even corn can produce toxins after exposure to freezing weather, notes Islam.

“Sorghum and related crops are the most dangerous. Producers should be very cautious when feeding livestock, especially after a freeze,” he says.

Cool season grasses

In Wyoming, cool season grasses are predominately grown and are also affected by freezes but are not toxic like sorghum and sudangrass.

Islam mentions any kind of damage, whether it is freezing temperatures or hail damage, can increase a cool season plant’s ability to produce nitrate.

“Low temperatures slow down plant metabolism, and nitrate accumulates in the lower parts of the plant. Animals that graze on damaged plants could suffer from nitrate toxicity,” he notes.

Another affect freezing has on plants is reduced nutritional quality.

“Freezes don’t affect nutritional quality too much, but if there are multiple freezes in one season, plants might lose their leaves, resulting in reduction of quality overall,” according to Islam.

Tolerant crops

There are a few crops that can handle freezing temperatures better, like alfalfa, fescues, orchard grass and bromes.

“Alfalfa might freeze, but most of the time, it will regrow with higher temperatures. Tall fescue is very unique because it has a wax coating on the leaves, so freezes don’t have as much of an affect,” Islam states.

“Even if the fescue is damaged by low temperatures or hail, the damage helps the plant accumulate more carbohydrates, which increase nutritional quality,” he adds.

Most cool season grasses lose quality with high temperatures and start to collect more carbohydrates in the fall to overwinter. The next spring, plants start to regrow, photosynthesize and collect carbohydrates, says Islam.

“Normally, at the peak of summer and in the winter, plant quality declines. The fescues, orchard grass and bromes are freeze tolerant and can even have higher quality compared to other cool season grasses in the fall and winter,” he states.


To avoid prussic acid or nitrate poisoning, Islam recommends waiting three to five days to graze or feed affected crops after a freeze, especially for sorghum-related varieties.

“The toxicity will affect all animals, whether it be livestock, wildlife or domestic animals. If any animal eats affected plants, depending on the level of prussic acid or nitrate at the time of ingestion, the toxins can be deadly,” notes Islam.

The general rule of thumb for prussic acid toxicity is levels below 200 parts per million (ppm) are nothing to worry about, but levels between 200 and 600 ppm pose a high potential for poisoning.

Another solution is delayed grazing for cattle, sheep and goats. For horses, it is not recommended they be fed sorghum or sudangrass because they have low toxicity tolerance, notes Islam.

“After a frost, I recommend producers delay grazing and feeding animals affected forages for at least three to five days. This time period allows toxins to leave the plants, so it is important for producers to have other feed sources and water available,” he states.

Another solution for producers who suspect a crop may have freeze damage is to send a sample to be tested. The sample can be tested for prussic acid and nitrates to give producers the information needed to make management decisions, says Islam.

“After a crop is harvested, it is highly recommended for producers to test for toxins that may be present. If the crop is still growing and there is a frost, producers should wait to harvest, giving the plants time to regrow and get rid of toxins,” he adds.

Islam’s final advice is simply to watch the weather, and when there’s a frost, delay grazing or feeding affected material to livestock.

Heather Loraas is assistant editor of the Wyoming Livestock Roundup and can be reached at

Back to top