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Considerations for winter grazing discussed

by Wyoming Livestock Roundup

University of Nebraska-Lincoln (UNL) Extension Specialists Aaron Berger and Troy Walz discussed recommendations for grazing winter forage sources such as alfalfa, sudangrass and sorghum, along with other annual forages, in a UNL BeefWatch podcast, dated Oct. 2. 

Many producers in the region have already seen frost in their fields, but prolonged freezes are to be expected as they venture further into the season. 

“A freeze can affect forage plants and producers should pay attention to these changes when grazing cattle,” says Berger. 

Bloat, toxicity and nitrates are the three key considerations while grazing winter forages, according to Walz. 

Alfalfa and bloat

“We always talk about bloat when we talk about alfalfa,” says Walz. “We especially have to worry about bloat the first three to five days after alfalfa has been exposed to freezing temperatures.” 

Walz explains frost may increase the incidence of bloat by rupturing the plant’s cell walls, which leads to a higher rate of digestion within in the rumen and consequently, increases gas formation. 

“The risk of bloat will only be minimal after we see a significant portion, about 50 to 70 percent, of the alfalfa top or new growth being frozen or dried,” Walz explains. “The fall freeze-down is a slow process, with many freezes over several weeks. The time it takes to reach the 50 to 70 percent point depends on the severity of the freezes and the amount of standing alfalfa.”            

A corresponding BeefWatch newsletter written by UNL Extension Educators Brad Schick and Ben Beckman notes grazing should take place soon after the freeze event to salvage as much of the nutritive value of the forage as possible. The newsletter also shares feed additives, such as nonionic surfactants, poloxalene and ionophores, may help to reduce bloat risk, but require regular consumption by cattle in order to be effective. 

Annual forage toxicity

Frozen sorghums and sudangrass may be a source of prussic acid poisoning. Walz explains grazing forage or grain sorghum, sudangrass or sorghum/sudan hybrid forages may subject livestock to poisoning. 

“We see cyanide production in these frozen plants, and plants that are frozen might release high concentrations of prussic acid for several days after the freeze,” says Walz. “Prussic acid is the highest right after the frost when chemicals inside the plant mix to form cyanide. After the plants wilt, prussic acid released from plant tissues declines.”

Walz explains the sorghum species contain the highest amount of prussic acid, while sudangrasses generally contain less. Prussic acid is often found at its highest levels directly after a frost. 

“We always recommend producers remove cattle right before a frost, if they know one is coming and wait to return to the pasture for five to seven days,” shares Walz. “There will be no more top growth after the tops have frosted, thus the plants regrow from the base, which is also very high in prussic acid.”

Walz recommends not grazing frosted sudan species that have grown to 15 to 18 inches tall or sorghum species which have grown to 24 inches tall. The other option, according to Walz, is to wait to graze until the entire plant is killed by subsequent frosts.

“Often, people ask about prussic acid poisoning with pearl millet and foxtail millet, but prussic acid has not been found in these plants,” Walz says. “However, prussic acid production has the potential to increase with high nitrate levels.” 

Nitrate poisoning

“Anytime we interrupt plant growth with drought, hail or frost, it interferes with normal plant growth and causes nitrates to accumulate in plants that are still growing,” Walz explains. “Plants we typically think about in terms of nitrate accumulation are grasses, oats and millet.” 

“Frost may damage or completely destroy the leaf area of the plant, and a decrease in leaf area limits photosynthetic activity,” notes Walz. “Nitrates absorbed by the roots are not converted into plant protein and instead, they accumulate in the stem and stalk.” 

Nitrate poisoning is often fatal for cattle. In the rumen, nitrate is broken down into nitrite, which is further broken into ammonia and used by rumen microbes to synthesize proteins.

However, when consumed in high concentrations, nitrite is absorbed into the bloodstream and reduces the ability of red blood cells to bind with oxygen, creating an oxygen deficiency in tissues. 

“Before grazing, it is probably worth it to have forage tested after it has frozen,” says Berger. “While challenging, being aware of what is in the pasture and what management options are available makes testing worth it.” 

Averi Hales is the editor for the Wyoming Livestock Roundup. Send comments on this article to

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