Managing Cattle Health Issues When Grazing Cover Crops
Courtesy of Iowa State University Outreach and Extension
Cover crops can offer excellent feed value when properly incorporated into beef cattle diets. However, common cover crop species sometimes pose potential health issues. These issues are managable, and producers should educate themselves on potential risks and associated mitigation strategies. The focus of this factsheet is to discuss common potential health issues associated with cover crop species of the Midwest.
Small Grains – Cereal Rye, Wheat, Triticale, Oats, and Barley
Cereal rye, for a variety of reasons, is the most commonly used cover crop in the Midwest. When grazed or harvested appropriately, this winter annual can be a very high quality forage for beef cattle. Several other small grains such as wheat, triticale, oats, and barley can also be used for cover crops and livestock forages. These small grains are relatively safe to use for beef cattle feed but can pose some risk for grass tetany, ergot poisoning, and nitrate toxicity.
Grass tetany (or hypomagnesemia) is a magnesium deficiency of grazing cattle. Iowa cattle are most susceptible in the spring when consuming fresh, lush grass. This grass tends to be high in protein, moisture, and potassium but low in magnesium, calcium, and sodium. Spring-calving cows in early lactation have high requirements for magnesium and calcium as they share these electrolytes with calves through their milk. Although most grass tetany cases occur in the spring, it is important to recognize the potential risk with fall growth as well. New growth of cereal rye and other small grains, spring or fall, can contribute to hypomagnesemia of grazing cattle.
Affected cattle will become nervous, agitated, and may experience muscle tremors. Additional clinical signs can include staggering, paddling, convulsions, lack of coordination, grinding of teeth, aggressiveness, excessive vocalization, and blindness. Grass tetany requires emergency therapy and veterinary diagnosis is usually made based on history and clinical signs.
Ergot poisoning is caused by infection of the forage plant with Claviceps purpurea. This fungal organism most commonly affects small grains and grasses. The organism is common in the environment and seems to be most commonly found in plants during the summer months when cool, wet springs are followed by dry, hot summers.
Sclerotia, or ergot bodies, replace seed heads of infected plants and produce alkaloids similar to those produced by the endophyte of infected tall fescue. Ergot alkaloids cause vasoconstriction of peripheral blood vessels and contribute to gangrene of extremities, hyperthermia, heat stress, and decreased milk production. Additionally, cattle can sometimes show neurologic signs such as hyperexcitability and muscle tremors.
The toxic sclerotia develop at approximately the same time as seed heads so the greatest risk would be when grazing or harvesting infected plants that have matured and developed seed heads. Grazing prior to the development of seed heads can help reduce the risk of ergot poisoning. Cereal rye and other small grains, seeded in the fall as cover crops, are often grazed in the fall or early spring before seed head development, thus making this risk relatively low.
Nitrate levels can be a concern with some cover crops, including small grains and brassicas. Nitrates can accumulate in many plant species, especially when nitrogen is readily available but plants are unable to utilize it, such as in drought conditions. When consumed, nitrates are converted to nitrites in the rumen. Nitrites move into the blood and disrupt hemoglobin molecules, making them unable to carry oxygen. Clinical signs include weakness, lethargy, incoordination, increased heart rate, increased respiratory rate, and dark, chocolatecolored blood. Upon observation of these clinical signs, producers should contact a veterinarian for assistance with diagnosis and treatment.
Many plants can accumulate nitrates, including numerous cover crops such as cereal rye, oats, wheat, rape, Sudangrass, and alfalfa. Numerous weeds such as pigweed, Canada thistle, lamb’s quarter, and Johnson grass can also accumulate nitrates. The greatest concentration of nitrates is usually in the lower part of the plant stalk. Fertilization rates affect the available nitrogen, thus wellfertilized crops are more likely to accumulate nitrates.
Drought conditions often cause plants to accumulate nitrates thus increasing the risk for nitrate toxicity. Establishment of cover crops may prove difficult in drought conditions. Depending on weather patterns and available nitrogen in the soil, drought-stressed cover crops may accumulate nitrates. Producers should test drought-stressed cover crop forages to ensure safe nitrate levels.
Tips for grazing small grain cover crops
• To prevent grass tetany, it can be helpful to provide high magnesium mineral mixes during times of high risk grazing on tetany-prone cover crop fields. It is best to begin feeding high magnesium mineral mixes up to two weeks prior to grazing tetany-prone fields. Although they cannot effectively store magnesium, this allows cattle to establish desired intake and homeostasis of magnesium levels.
• To minimize risk of ergot poisoning, producers should graze small grain fields while plants are in the vegetative stages of growth or prior to the development of seed heads. If grazing mature, reproductive plants, producers can scout for sclerotia. If sclerotia are prominent, clipping seed heads may reduce risk of ergot poisoning.
• For high-risk plant species, especially in early growth stages and for plants stressed by poor growing conditions, producers should consider testing for nitrate levels.
Brassicas such as radishes, turnips, and rape are cool season annuals that are often used for cover crops in the Midwest. They can be planted in the late summer for fall grazing. Brassicas are unique plants that present both interesting opportunities and challenges. Plant material includes leaves and bulbs or roots, both of which can be consumed by cattle. Anecdotally, cattle will eat bulbs but may take some time learning to consume the bulbous material found at or below the ground’s surface. Bulbs can become quite large and present at least a mild risk of choking.
Lack of dry matter and fiber On a dry matter basis, bulbs and leaves are extremely nutrient dense. Leaves can contain 18-30 percent crude protein and 55-80 percent, or greater, total digestible
nutrients (TDN). Roots can contain 8-18 percent crude protein and 65-90 percent TDN. In terms of nutrient content, brassica forage is more comparable to concentrate than traditional forages. Brassica plants can be extremely high in moisture, making their feed value much less on an as-fed basis. Leaves and roots both can be upwards of 90 percent moisture. With such great moisture content and conversely so little dry matter, it can be difficult for cattle to eat enough material to meet nutrient requirements and to maintain a healthy rumen environment. Sulfur toxicity
Brassicas tend to be relatively high in sulfur. The maximum tolerable limit for sulfur in beef cattle diets has been suggested to be 0.3 percent sulfur in diets containing greater than 85 percent concentrate, and 0.5 percent in diets containing greater than 40 percent forage. Brassica samples have been known to contain greater than 0.5 percent sulfur, thus posing a risk for sulfur toxicity,
reduced performance, and poorer mineral absorption. Sulfur toxicity can manifest as polioencephalomalacia, sometimes called “polio” or “PEM” for short. This neurologic disease can cause blindness, ataxia, recumbency, seizures, head pressing, coma, and death. More mild clinical signs associated with PEM may include lesser dry matter intake, lesser average daily gain,
and lesser hot carcass weight, as well as trace mineral deficiencies caused by antagonism between sulfur and absorption of numerous trace minerals. Nitrate toxicity Brassicas can also accumulate nitrates and pose a risk for nitrate poisoning. Nitrate levels greater than 1,500 parts per million (ppm) of nitrate-nitrogen are considered potentially toxic. In an Iowa State University study
conducted at the Iowa State Beef Teaching Farm, a cover crop mixture of radishes, turnips, and oats was tested for nutritional values and nitrate levels. Mixed samples of leaves tested in October, November, and December were 1,960, >5,000, and 3,880 ppm of nitrate-nitrogen, respectively. Bulbs tested in October and December were 3,420 and 1,380 ppm nitrate-nitrogen, respectively. This cover crop mix also contained sulfur at levels capable of causing sulfur toxicity. Due to high nitrate and sulfur concentrations, researchers utilized strip grazing to control intake and provided ad libitum access to corn stalk bales. Tips for grazing brassicas • Brassicas are best utilized as forage when planted with grass species as a cover crop mix. The brassicas can offer abundant nutrients and the grasses can offer dry matter and fiber to help balance the overall diet. The grasses can also serve to dilute out the high sulfur levels of the brassicas. • It may also be helpful to offer hay, cornstalk residue, or stockpiled grass in addition to brassicas. The dry matter and fiber of these feeds can help to balance the consumed diet. • Producers should not supplement with high sulfur feeds or use high sulfur water sources while grazing brassicas. • Brassica forage should be introduced gradually, and may be best managed through rotational or strip grazing