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Nebraska Extension educator cautions against grazing frosted forages

by Wyoming Livestock Roundup

Freezing temperatures can elevate risks of bloat, prussic acid and nitrate poisoning in cattle grazing alfalfa and annual forages. The University of Nebraska-Lincoln BeefWatch podcast welcomed Nebraska Extension Educator Troy Walz on Oct. 9 to discuss management practices to mitigate risks associated with cattle grazing frosted forages.

Grazing frosted alfalfa

Producers grazing alfalfa in the fall need to consider bloat, says Walz. Bloat is especially problematic the first three to five days after alfalfa has been exposed to freezing temperatures.

“Usually, after alfalfa has been frozen pretty hard and wilts down, about 50 to 70 percent of the alfalfa top is frozen and dried, and that’s when risk of bloat is going to decrease with alfalfa,” he says.

Prussic acid risks

Cattle grazing summer annual grasses including sorghum, Sudangrass and sorghum-Sudangrass hybrids may be at risk for prussic acid poisoning, Walz says. Prussic acid is hydrogen cyanide and causes problems with cell respiration in animals resulting in death. 

“Frost is one of these conditions causing prussic acid to be an issue with grazing livestock,” he says. 

Frozen plants release high concentrations of prussic acid for several days, says Walz. Typically, after five to seven days, the prussic acid is turned into a gas and is released out of the plant.

“We always recommend if producers are grazing sorghum, Sudangrass or sorghum-Sudangrass hybrids, that after the frost has damaged those plant cells, wait at least five to seven days to graze,” he says. “Ideally, producers might want to even wait until the plant is frosted clear to the ground, because if it is just frosting the tips of those leaves, it’s going to cause some prussic acid issues.”

He says the five to seven day pause in grazing should be implemented after each frost. After the tops of plants are frosted, growth may come back at the base of the plant, and those new shoots will be high in prussic acid.

“We recommend producers do not graze sorghum species until the regrowth is 15 to 18 inches tall and for Sudan species, 24 inches tall,” he says.

Fields with high nitrogen rates and low phosphorus rates will typically also have higher prussic acid concentrations, Walz says.

“Producers may have to worry about nitrates in all of their summer annuals, because frost interferes with normal plant growth which can cause nitrates to accumulate in the plants that are still growing and in grasses like sorghum, Sudangrass, foxtail millet, pearl millet and even oats,” he says. “If nitrates are high when producers undergo a total killing freeze, those nitrates are going to stay in the plant, so nitrate levels are going to remain high.” 

Walz recommends producers test forages for high levels of nitrate concentrations before feeding.

“If levels are high, producers will need to do some mixing of their ration to get the nitrate concentration down to be safe for their livestock,” he says.

Sampling forages

Walz offers a few considerations for producers sampling forages standing in their fields.

“Usually, nitrates are higher in concentration in the lower part of the stock,” he says. “Usually, leaves are lower in nitrates than what we see as we go down the stock, so if producers are going to sample for nitrates, they probably need to sample the bottom eight to 10 inches and then sample 10 inches and above.” 

Even if the plant is four feet tall, producers should sample the bottom 10 inches, the middle two feet and the top foot to give an idea of the concentrations of nitrate, he says. This will give producers an indication of how to graze the plant.

“If producers have really high nitrate concentrations in the bottom part of the stem, they can’t rub that into the ground,” Walz says. “Producers don’t want cattle eating the bottom part of the stock.” 

Walz acknowledges the importance of understanding the risks associated with grazing frosted forage and says mitigating these risks is in producers’ best interest. 

“Even though it’s going to take some time and labor to move cattle around, producers need to mitigate the risk the best they can,” he says.

Kaitlyn Root is an editor for the Wyoming Livestock Roundup. Send comments on this article to

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