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Fall grazing considerations 

by Wyoming Livestock Roundup

UNL provides tips to manage grazing frosted forage

Producers welcome the first frost for its fly-killing ability, but grazing cattle on fall forage comes with a few safety tips. 

In a recent University of Nebraska-Lincoln (UNL) BeefWatch podcast, dated Oct. 1, UNL Beef Extension Educator Troy Walz discusses an article titled “Cautions for Cattle Grazing Frosted Forage,” published in the October UNL BeefWatch Newsletter.

According to Walz, when some forages freeze, grazing cattle may potentially bloat or be affected by deadly toxins. When alfalfa, sorghum and grasses like oat or sudangrass freeze, the plant’s composition may change and potentially poison livestock. 

Mitigating risks, avoiding bloat

“When grazing alfalfa in the fall, bloat remains a potential problem, especially during the first three to five days after alfalfa has been exposed to freezing temperatures,” Walz states. “The risk of bloat will be minimal only after a significant portion – about 50 to 70 percent – of the alfalfaʼs top growth has been frozen and dried.”

After alfalfa begins to wilt or grow again, it becomes less likely to cause bloat, and waiting to graze alfalfa until a hard freeze occurs is a safer management practice.

Walz says, “The fall freeze is often a slow process, with multiple freezes occurring over several weeks. The time it takes to reach the point where 50 to 70 percent of the alfalfaʼs top growth has been frozen and dried will depend on the severity of the freezes and the amount of standing alfalfa.”

“In high-quality forages like alfalfa, clover and fresh, small-grain shoots, frost damage in the plant will rupture cell walls and make protein and minerals more readily available for one to two days,” writes UNL Extension Educator Ben Beckman in the corresponding BeefWatch Newsletter, published on Oct. 6.

“Many cover crop mixes contain some form of clover, ladino and white clover, which can all cause bloat. These readily available proteins and minerals increase gas buildup in the rumen to the point animals cannot eliminate them by eructation – belching – creating bloat,” Beckman states.

Dangers of prussic acid

“When grazing forage or grain sorghums, sorghum-sudangrass hybrids and sudangrass, certain conditions may cause livestock to be poisoned by prussic acid,” Walz notes.  “Frost is one of these conditions, as it can release high concentrations of prussic acid for several days.”

Prussic acid turns into a gas and will disappear into the air. Therefore, it is recommended producers remove cattle before a frost and wait five to seven days after a frost before returning to grazing on any of these forage species.  

According to Walz, “When tops have been frosted, new shoots may regrow at the base of the plants, which can be high in prussic acid. Thus, do not graze frosted sorghum or sudan species until regrowth is 15 to 18 inches tall for sudan species and 24 inches tall for sorghum species, or wait several days after the entire plant and shoots are killed by subsequent frosts.”

“Sorghum species contain the highest amount of prussic acid, while sudangrass generally has the least amount. Pearl and foxtail millet have not caused prussic acid poisoning and prussic acid has not been found in these plants,” he adds.

Sampling for nitrates

Beckman notes the metabolism of grasses slows down following a stressful event such as freezing, which allows nitrates to accumulate in the plant. This leaves grass species especially susceptible to nitrate poisoning risks.

A frost can interfere with plant growth especially in oats, millet and sorghums. If nitrates accumulate in plants which are still growing, the build-up isn’t hazardous to grazing livestock, but if hay is cut right after a freeze or is green chop, it can be more dangerous.

“Nitrates commonly concentrate in the lower portions of plant stems, and waiting five days before haying or chopping and keeping a cutting height of six to eight inches will help mitigate risk,” Beckman continues. “Feeds containing high levels of nitrate aren’t necessarily unusable as long as proper action is taken to minimize risk.”

Walz concludes, “If high nitrates are present in plants when they undergo a total killing freeze, nitrate levels will remain in those plants, and testing the forage is recommended.”

Frost can impact forages, but by mitigating these risks, producers can allow cattle to graze safely.

Melissa Anderson is the editor of the Wyoming Livestock Roundup. Send comments on this article to

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