Grazing cover crops: Toxicity considerations
Annual cover crops grown in place of fallow can provide high-quality forage during key production periods and may help reduce soil erosion, suppress weeds, and increase soil nutrient profiles. Traditionally grown for agronomic or soil benefits but not harvested, cover crops are being considered for grazing, haying, or planting as annual forages. They are appealing because of the potential for additional revenue from improved cattle performance combined with the benefits of soil stabilization. Those contemplating this decision should know that plants that work well as cover crops may not be suitable for forage or grazing. In fact, some species can be toxic or fatal to livestock. This article describes popular cover crops and the dangers they present for grazing livestock.
Hairy vetch is a nitrogen-fixing plant that works well as a cover crop but is not recommended as a forage crop because of toxicity to cattle and horses. Hairy vetch prompts an allergic reaction with symptoms such as subcutaneous swelling, photosensitization, hair matting, skin sloughing, oral ulcers, cough, alopecia, weakness, loss of appetite, diarrhea, decreased milk production, sporadic abortions, red-tinged urine, and death. The mortality rate for affected animals ranges from 50 to 100%, usually as a result of kidney failure. Grazing is risky at any stage of plant growth. Animals with black pigmented skin such as Angus, Angus cross, or Holstein cattle and black horses, are the most susceptible, but Hereford cattle also may be affected. Also cattle that have been exposed regularly to hairy vetch are at a greater chance of hairy vetch toxicosis. Overall toxicosis rates are fairly low in the cattle population, but if it occurs in your operation can be devastating.
Hairy vetch poisoning has been linked to herd genetics, which may explain why livestock deaths associated with this plant tend to cluster within herds. Unfortunately, there is no genetic test to indicate live- stock sensitivity to hairy vetch. Weigh potential benefits and risks when deciding whether to plant hairy vetch as a forage crop.
Lupin is a good source of protein and energy in livestock feeds for both ruminants and monogastrics, but use is limited to four nontoxic species: narrowflower lupine (Lupinus angustifolius), white lupine (L. albus), European yellow lupine (L. luteus), and tarwi (L. mutabilis). The lupin grain can be fed and is relatively low in starch, which reduces the likelihood of acidosis. Even though lupin grain is high in protein, when feeding to monogastrics, bear in mind that lupins are low in methionine and lysine.
Six lupin species that are particularly toxic to cattle and sheep are silky lupine (L. sericeus), tailcup lupine (L. caudatus), velvet lupine (L. leucophyllus), silvery lupine (L. argenteus), summer lupine (L. formosus), and sulfur lupine (L. sulphureus). These poisonous plants can kill sheep and may cause cleft palates, crooked legs, distorted and malformed spines, and other birth defects when consumed by pregnant cows.
Amaranth is a bushy plant related to pigweed. Species used for grain production include love-lies-bleeding (Amaranthus caudatus), red amaranth (A. cruentus), and Prince-of-Wales feather (A. hypochondriacus). The grain from the amaranth plant is marketed to food processors, breakfast cereal companies, and health food stores. Spiny amaranth or spiny pigweed (Amaranthus spinosus L.), redroot pigweed (Amaranthus retroflexus), and Palmer amaranth (Amaranthus palmeri) are examples of amaranth species that are classified as true weeds and hard to control in pastures and crops. Palmer amaranth is consistently high in nitrate and potentially toxic to cattle. Know which amaranth species you are getting before using it as a forage crop for livestock.
Brassicas – Kale, rapeseed, swede, turnip, canola, mustard
Brassicas provide high-quality, high-protein feed for cattle. Aboveground parts provide 20 to 25% crude protein (CP) with 60 to 80% in vitro digestible dry matter. Roots are 10 to 14% CP and 80 to 85% digestible. Brassicas are ready for grazing about 75 days after planting. Regrowth is possible if not overgrazed. Palatability increases after a freeze.
Brassicas are high in moisture and low in fiber. Other dry feeds should be offered to maintain a functional ruminal environment. They are low in copper, manganese, and zinc. Plan to supplement with a properly balanced mineral to meet cattle requirements. This is especially important for breeding animals.
Maladies associated with improper grazing include polioencephalomalacia, hemolytic anemia (abnormal breakdown of red blood cells, mainly an issue with kale), pulmonary emphysema (a permanent accumulation of air in lungs), bloat (especially with canola), and metabolic problems associated with glucosinolates. Photosensitivity may be observed in sheep. Polioencephalomalacia (PEM), anemia, and emphysema are normally found when the cattle diet consists solely of brassicas. Brassicas should comprise no more than 75% of the total diet. An iodized mineral pack should be offered to counter negative effects of glucosinolates on iodine uptake.
Canola is high in sulfur, increasing the risk of PEM. Test sulfur levels in the canola plant and water source to minimize toxicity concerns. Sulfur may inhibit absorption of minerals and particularly copper and selenium. Provide a trace mineralized salt and mineral supplement to cattle grazing canola.
Grazing flax is not recommended because of the potential for prussic acid poisoning. Avoid grazing green flax straw, in particular, and especially right after a freeze when risk is higher. On the other hand, harvested flax seed is a good high-protein feed. In addition to 35% CP, it offers a unique fatty acid profile, making it desirable as a dietary supplement for horses. After seed harvest, flax straw’s high cellulose and lignin content makes it a poor-quality forage. Despite desirability as feed, flax is not recommended for grazing or haying.
Small grains – Barley, Oats, Rye, Ryegrass, Wheat, Triticale
Rapidly growing, lush grasses can lead to grass tetany in grazing cattle. Grass tetany is more common in the spring but can occur with the cool-season growth of small grains in the fall and winter. High-protein grasses may contribute to bloat.
To manage grass tetany, provide magnesium to lactating cows, preferably a free-choice mineral containing 8 to 12% magnesium. Begin supplementation before turnout, making sure the mineral is palatable to ensure adequate intake. Increasing legumes to 30% of the pasture may reduce risk of grass tetany.
Nitrate toxicity risk increases with heavy nitrogen fertilization of cool-season grasses. Nitrate can accumulate, reaching dangerous levels on cool, cloudy days, during periods of drought or under environmental conditions that slow grass growth and metabolism.
Grazing either yellow or white sweetclover, is not a risk for cattle, however moldy hay puts cattle at risk for sweetclover poisoning. Low-coumarin varieties such as red clover and Banat sweetclover (Melilotus dentate) are exceptions. Yellow (M. officinalis) and white (M. albus) sweetclover varieties contain the most coumarin. Avoid feeding moldy sweetclover hay to cows within 2 weeks of calving to reduce the risk of abortion.
Bloat is another concern when grazing clovers and legumes. Forage with less than 50% clovers is less problematic. Never give hungry animals access to lush clover stands, and provide dry hay to reduce bloat. Clovers are less likely to cause bloat than alfalfa. Annual lespedeza, birdsfoot trefoil, and sainfoin are not known to cause bloat, but certain birdsfoot trefoil species may contain high levels of prussic acid.
Cattle will consume lablab, cowpea, sunhemp, mungbean, and soybean, which are suitable for grazing. Bloat can be an issue as with other legumes, but the risk is fairly low. Sheep are less tolerant of these crops than cattle. A small percentage, usually crossbred sheep, may show photosensitivity around the face and ears while consuming cowpea. This is rare with lablab or soybean. Sheep grazing rain-damaged mature soybean crops may become ill if exposed to the fungus Phomopsis and toxins that cause lupinosis-like symptoms. Lupinosis causes acute liver atrophy and may lead to death.
Medics are legumes that are recommended as forage because of their high biomass production. Black medic is not palatable to cattle, but animals may consume other varieties. These plants may cause bloat in cattle and sheep to a lesser extent. Snail medics are not as risky as barrel and naturalized medics.
Sorghum, Sudans, Millets, and Corn
Sorghums and millets are warm-season, drought-tolerant crops that grow in above-average temperatures. Four main categories are grain sorghum, forage sorghum, sudangrass, and sorghum-sudangrass hybrids. Cattle grazing on any of these are at risk for prussic acid HCN poisoning. Sudangrass, with low levels of HCN, is the least toxic and rarely kills animals. Sorghum-sudangrasses pose an intermediate threat, and sorghum, with the highest levels of HCN, is potentially the most toxic to livestock.
Corn does not raise prussic acid concerns, but cattle are at greater risk for prussic acid poisoning than sheep. Sorghums and corn have been associated with nitrate toxicity. Test forage before grazing or haying.
Graze sorghum when the plant is 18 inches or higher. HCN levels are highest when the plant is young and growing, after a drought breaks, during regrowth, and immediately following a freeze. A rotational system should be used if grazing sorghums during summer. Briefly, turn cattle into the paddock when plants exceed 18 inches. To keep cattle from consuming regrowth, do not graze the paddock for more than 5 days. Although appetizing, new shoots may be high in HCN. HCN levels tend to increase following a light frost and peak after a killing frost. Wait 10 days after a killing frost to graze or feed sorghums and delay grazing sorghum stalks until after a killing freeze. Prussic acid is not a concern with baled hay because HCN evaporates as the hay dries.
Nitrates are the biggest threat when harvesting sorghum for hay. Although curing removes prussic acid, which is volatile, nitrates remain in the hay. Test plants for nitrate before harvest and raise cutting height, leaving the bottom one-third of the stalk where nitrate concentrations are highest. If the test is high, delay harvest to allow plants to grow and metabolize more of the nitrates.
With millet (pearl or foxtail), nitrate toxicity may be a problem but not prussic acid. These grasses are recommended for horses. Sorghum is not recommended because it contains lathyrogenic nitriles such as β-cyanoalanine, cyanogenic glycosides, which may cause cystitis. Symptoms include urinary incontinence, posterior ataxia or incoordination, and less frequently death and late-term abortion. Mold is a problem with high-tonnage crops such as millet or sorghum hay. Poorly cured hay can cause respiratory problems in horses, and cattle may refuse to eat it.
In Mississippi, researchers studying the effects of cattle grazing on corn reported steer gains of 1.9 to 2.0 pounds per day. Experiments involved turning cattle onto corn after ears developed as plants dried for winter feed. Cattle graze selectively in corn fields, which can lead to acidosis and founder. The Mississippi steers also grazed selectively, consuming leaves, tops, ears, and then stalks when that was all that was left. In a similar Canadian study, cows grazed ears, leaves, tops, then the stalk, in that order.
Corn grazing may lead to nitrate toxicity in cattle. To prevent this and to maintain an optimal rumen microbial environment, strip graze the corn and allow cattle to graze for 2 to 3 days per paddock. Reduce daily corn consumption to decrease the risk of acidosis and founder. Strip grazing also increases forage utilization, even if cattle are only grazing stalks.
These recommendations are based on current information. Consult your veterinarian before grazing or feeding any of the plants discussed. To select forage crops suitable for your operation, consult your area livestock extension specialist and veterinarian.
This eUpdate article is a slightly edited excerpt from the K-State Research and Extension Publication MF3244, “Forage Crops Grazing Management: Toxic Plants”. The full publication can be viewed at https://www.bookstore.ksre.ksu.edu/pubs/MF3244.pdf.
Jaymelynn Farney, Southeast Area Beef Systems Specialist
Dale Blasi, Beef Stocker Management Specialist
John Holman, Southwest Research-Extension Center Agronomist