Avoid hidden hazards in fall forages

Late summer is when many livestock producers start looking ahead to fall grazing opportunities. Before cattle are turned onto cover crops or drought-stressed forages, however, it’s important to understand the risk of nitrate toxicity and the management practices that can help prevent it.

Nitrate toxicity can occur quickly in cattle grazing high-nitrate forages, particularly if animals are forced to consume the lower portions of plant stalks where nitrate concentrations are highest.

Early or chronic nitrate toxicity may cause watery eyes, reduced milk production, decreased appetite, weight loss, night blindness and, in some cases, abortion. Acute nitrate toxicity develops rapidly and can be fatal. Symptoms include labored breathing, increased heart rate, weakness, staggering and cyanosis—a bluish discoloration of the eyes, tongue, mouth and vulva caused by lack of oxygen in the blood.

If nitrate toxicity is suspected, contact a veterinarian immediately. While treatment is possible, prognosis can be poor unless intervention occurs quickly. Prevention is the best defense.

Fortunately, nitrate toxicity can often be avoided by understanding how nitrates function in both the rumen and the plant.

Under normal conditions, rumen microbes convert nitrate into nitrite and then into ammonia. The microbes use the ammonia to build protein, which eventually becomes available to the animal when the microbes are digested.

Problems arise when cattle consume excessive amounts of nitrate, and then nitrite accumulates faster than it can be converted to ammonia and protein. The excess nitrite is absorbed into the bloodstream, where it converts hemoglobin—the oxygen-carrying component of blood—into methemoglobin, which cannot transport oxygen effectively. As a result, the animal’s tissues and organs are deprived of oxygen, causing the symptoms associated with nitrate poisoning.

Plants absorb nitrogen from the soil through their roots. During periods of adequate sunlight and active photosynthesis, nitrates are rapidly converted into plant protein. However, plants may accumulate nitrates when they absorb nitrogen faster than they can utilize it. This typically occurs during periods of plant stress, including drought, frost, extended cloudy weather, hail damage, pest pressure, disease or certain herbicide applications. Because of this, cover crops grazed during stressful growing conditions should be evaluated carefully. Annual cereal grains and brassicas, such as turnips and radishes, are particularly prone to nitrate accumulation under certain conditions.

Consider these precautions before grazing cattle on cover crops or other at-risk forages:

•    Test forages for nitrate levels, especially following drought, frost or prolonged periods of cloudy weather.

•    Take extra caution on fields with high proportions of brassicas or cereal grains, high soil nitrate levels or recent heavy fertilizer applications.

•    Avoid grazing fields for three to five days after a drought-breaking rain, as nitrate concentrations often spike during this period.

•    Do not turn hungry cattle into high-risk fields. Animals that are hungry are more likely to overconsume potentially dangerous forage.

•    Graze lightly and encourage cattle to consume the upper portions of plants, where nitrate concentrations are generally lower.

•    Introduce cattle gradually by limiting access to the pasture and feeding additional forage sources.

•    If there is cause for concern, consider ensiling, which can reduce nitrate levels by 40% to 60%.

As a general guideline, nitrate levels below 0.25% (2,500 ppm) are considered safe for all classes of cattle. Labs results may also show nitrate-nitrogen (nitrate-N), and levels below 600 ppm are generally considered safe. Forages with higher nitrates may require restricted feeding or dilution with lower-nitrate feeds. Consult your forage laboratory, nutritionist or veterinarian for specific feeding recommendations.

With proper testing and management, producers can safely graze many cover crops and other forages while minimizing the risk of nitrate toxicity. Visit with the livestock and agronomy experts at your local MFA for more information. 

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