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Six cautions when grazing cover crops

Rebecca Kern for Progressive Forage Published on 08 July 2021

Grazing cover crops can be a cost-effective way to achieve multiple operational goals. Cover crops can provide ground cover to prevent erosion, improve soil health over time and provide nutrient-dense forage for grazing. However, turning cattle out onto cover crops to graze without understanding the risks can lead to a wreck.

I have consulted with a producer who lost cattle to sulfur toxicity. After the incident, that producer understood the value of testing cover crops prior to grazing. Here are six considerations when grazing cover crops:

1. Nitrates

Cover crop mixes typically include several plant species known to accumulate nitrates: brassicas, such as turnips and radishes, or small grain forages, such as wheat, oats, millet or grain sorghum. When cattle consume high-nitrate feeds, microbes in the rumen convert that nitrate to nitrite. The nitrite is a gas, which cattle belch and then inhale. The nitrite then binds to blood hemoglobin preventing oxygen from binding. Levels between 1,400 – 2,100 parts per million (ppm) nitrate nitrogen (NO3-N) can cause spontaneous abortions with no warning signs or symptoms. At levels between 2,100 – 4,000 ppm NO3-N, sudden death may occur and therefore, animals grazing cover crops should be slowly acclimated to consumption of nitrates and offered a low-nitrate roughage to fill up on first. Never allow hungry cattle onto a high-nitrate field.

2. High sulfur

The toxic level of sulfur in a cattle diet is 0.4 ppm, on a dry basis. Brassicas are sulfur accumulators that occasionally test above the maximum tolerable level and are often included in cover crop grazing mixes. When sulfur intake is above the tolerable level, thiamin metabolism is impaired in a condition known as polio-encephalomalacia (PEM). Head pressing, blindness and muscle tremors are all clinical symptoms of PEM, which, untreated, results in death. Ensuring the mix includes many species to avoid overconsumption of brassicas can help prevent PEM issues.

3. Low magnesium

Cover crops tend to have low magnesium. The magnesium requirement for a beef cow is 0.2% of the diet at peak lactation and 0.1% of the diet for growing cattle. When cover crops contain less magnesium than is required, a magnesium deficiency can develop resulting in grass tetany. When cattle develop grass tetany, they stop grazing, become overly alert and appear uncomfortable. Cattle will then begin to stagger until they finally lie down with their head pulled back into a “star gazing position.” Untreated, this condition will result in death. To prevent the development of a magnesium deficiency, many producers grazing cover crops feed a mineral between 8%-12% magnesium to be consumed at a rate of 2.5-4 ounces per head, per day. However, if magnesium concentrations in cover crops is sufficient or high, magnesium toxicity may result from feeding a high-magnesium mineral. Therefore, lab analysis of the forage will be critical for choosing a mineral supplement.

4. Prussic acid

In cover crop mixes, there are species of plants which accumulate hydrogen cyanide, a poisonous gas commonly known as prussic acid. The toxic gas accumulates in leaves under stressful growing conditions. These plants include sorghum grasses, sudan grasses and flax. Stressful growing conditions include drought or frost. Plants grown in drought conditions should be tested for prussic acid prior to grazing because consumption of high levels of the gas results in sudden death. Contrarily, frost typically breaks cell walls allowing the release of the gas and therefore should be safe to graze after four to seven days. However, regrowth after a frost should be tested prior to grazing. The plant is stressed from the previous frost, but the cell walls have not been broken to allow gas to escape.

5. Bloat

A frothy bloat is often attributed to legumes or high-protein grasses. Cover crop mixes high in legumes such as clover, beans or cowpeas may result in some animals overindulging and developing frothy bloat issues. These legume plants are high in soluble protein and sugars, which allows microbes to ferment and grow at a rapid rate, resulting in a high rate of gaseous by-product accumulation. Smaller paddocks and more intensive grazing strategies can decrease the ability of animals to select these forage varieties, thereby reducing bloat risks.

6. Choking

Brassicas such as radishes and turnips may be pulled from the ground to be consumed by cattle grazing cover crops. If they are swallowed without proper mastication, the animal may choke on the large root. This is particularly a concern for young cattle who are inexperienced in grazing brassicas.

Overall, grazing cover crops can be a great way to provide a high plane of nutrition to cattle, prevent soil erosion and improve soil health. While animal health issues are rare while grazing cover crops, they do happen. Lab analysis can help producers evaluate their risk.  end mark

PHOTO: Photo by Mike Dixon.

Rebecca Kern
  • Rebecca Kern

  • Animal Scientist
  • Ward Laboratories
  • Email Rebecca Kern

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