by Elizabeth Cronin, North Dakota State University

As frost begins to appear across parts of the Northern Plains, temperatures have been low enough to cause leaf burning and plant stress in many frost-sensitive annual and perennial forage plants. James Rogers, North Dakota State University (NDSU) Extension forage crops production specialist, 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 can be released in the sorghum family when the plant undergoes a form of stress, such as frost, drought, grazing or chemical applications. When prussic acid develops, concentrations are highest in new, rapidly growing leaf tissue and higher in leaves than stems, creating a hazard for grazing cattle. Once ingested by the animal, prussic acid blocks body cells from receiving oxygen.

A light frost can stop plant growth, but it will not kill the plant crown. The root crown then can still produce new shoots that can contain high levels of prussic acid. Cattle can detoxify a high percentage of low-level prussic acid exposure, but the high concentrations found in fresh regrowth following a stress period greatly increase the risk.

Fortunately, prussic acid levels drop as the sorghums dry. Complete dry-down will usually take a week or more, depending on drying conditions. After the plants have completely dried down, it is safe for cattle to graze out the sorghum as a standing hay crop.

Frost can also cause conditions for nitrate accumulation. Forage crops such as annual cereals (oats, barley, rye), brassicas (turnips, radish), sorghums, and millets can accumulate nitrates, leading to nitrate poisoning in ruminant animals. High-nitrate levels may occur in hay containing these crops or late-planted crops that are targeted for late-season grazing or hay production.

Unlike prussic acid, drying of the forage for hay will not eliminate high levels of nitrate. Ensiling reduces levels by up to 50%, but if the initial nitrate level is high, even a 50% reduction might not be enough to reduce the concentrations to a safe feeding level for all livestock. Testing for nitrate levels is crucial to develop management strategies for feeding. Nitrate testing may be available at no cost to North Dakota producers through NDSU Extension.

Frost damage can also increase the risk of bloat, particularly in legumes. In ruminants, bloat occurs when froth builds up in the rumen, preventing the animal from expelling gas. The gas builds, putting pressure on internal organs.

Frost damage in legumes or other plants with high levels of soluble proteins, such as alfalfa, clovers and vegetative cereal crops, causes a rupturing of plant cells, which then releases soluble proteins. Plant cell damage can also lead to increased levels of potassium, calcium and magnesium. These combined effects lead to increased risk of bloat in ruminants. This risk is highest one to seven days after frost occurs.

To mitigate risk during periods of plant stress, Rogers suggests several management steps based on the issue of concern: