Key strategies for Frost-Affected livestock health

By Scout Nelson

As frost begins to appear across parts of the Northern Plains, low temperatures can cause leaf burning and plant stress in many frost-sensitive annual and perennial forage plants. James Rogers, a forage crops production specialist at North Dakota State University Extension, highlights three main frost effects: prussic acid production, nitrate accumulations, and bloat.

“The most concerning of these frost effects is prussic acid which can develop in the sorghum family of plants,” Rogers says. “If cattle are grazing sorghum forages and frost warnings are in the forecast, remove cattle until after a killing frost has completely killed the plant and residual growth has fully dried down.”

Prussic acid is released in sorghum when the plant undergoes stress, such as frost, drought, grazing, or chemical applications. Concentrations are highest in new, rapidly growing leaf tissue, posing a hazard for grazing cattle. Once ingested, prussic acid blocks oxygen delivery to body cells.

Light frost may stop plant growth but not kill the plant crown, allowing new shoots to potentially contain high levels of prussic acid. Fortunately, levels decrease as sorghum dries, typically taking a week or more. After completing dry down, cattle can safely graze.

Frost can also lead to nitrate accumulation in forage crops, such as annual cereals, brassicas, sorghums, and millets. High nitrate levels can occur in hay or crops targeted for late-season grazing, leading to nitrate poisoning in livestock. Testing forage for nitrate levels is crucial, and producers may access free testing through NDSU Extension.

Additionally, frost damage can increase the risk of bloating in ruminants, especially in legumes like alfalfa and clover. Bloat occurs when gas builds up in the rumen, preventing animals from expelling gas. This risk is highest one to seven days after frost.

To mitigate risks, Rogers suggests several management steps:

• Prussic acid: Avoid grazing sorghums after light frost for 7-10 days. Wait until regrowth reaches 18-24 inches or after a killing frost.
• Nitrate accumulation: Test forages, dilute high-nitrate with low-nitrate forages, and monitor cattle health.
• Bloat prevention: Avoid grazing frost-damaged legumes for at least three days and provide supplements like poloxalene blocks

“Fall is a great time of year with the changing of the seasons, but it can also cause negative changes in our forages that can impact livestock health,” Rogers says. “Be aware of these concerns and be prepared to deal with them.”

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