Farms.com Home   News

Grazing Cover Crops: Toxicity Considerations

By Jaymelynn Farney

Southeast Area Beef Systems Specialist


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.

Poisonous Plants

Hairy vetch


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.

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

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

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 S. Watson) 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.

Metabolic disorders

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), nitrate poisoning, 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.

Nitrate toxicity is possible with brassicas. Test forage before turnout to determine if it is safe based on laboratory results. In general, the more mature the plant, the lower the nitrate concentration. If nitrate value is high early in the season, it may be suitable for grazing at a later date. Introduce animals to the brassica diet over 5 to 7 days. Do not turn out hungry animals. Make sure they are full of hay first.

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.

Flax

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.

Legumes

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.

Source:ksu.edu


Trending Video

RECORD National Corn Yield??

Video: RECORD National Corn Yield??

Agricultural and Economic Briefing: USDA Reports, Global Tensions, and Market Reactions

Welcome back to our channel for a detailed update on key developments affecting the agricultural sector and broader economic landscape. Here's what's on the agenda today:

USDA Crop Production and WASDE Reports The USDA is set to release its monthly Crop Production and WASDE reports today at 11:00am CST. These reports will reflect the updated new crop US corn and soybean balance sheets, incorporating data from the June Planted Acreage report which showed a significant increase in corn acreage. While no major adjustments to US corn and soybean yield projections are expected, the focus will be on potential changes to global supply and demand. The reports are anticipated to bear a mostly bearish tone, especially concerning corn prices.

Geopolitical Developments in Ukraine Ukraine's recent detention of a foreign cargo ship on the Danube River, suspected of carrying stolen Ukrainian grain from Russian-occupied Crimea, has escalated tensions. This incident has stirred concerns about potential Russian retaliatory actions during Ukraine's crucial grain export season. Wheat futures saw a sharp rise following the news, highlighting the sensitive interplay between geopolitical events and commodity markets.

US Drought Conditions and Agricultural Impact Recent USDA drought monitor data indicates mixed conditions across the US Corn Belt and High Plains, with many areas receiving beneficial rainfall and experiencing cooler-than-average temperatures. However, conditions have worsened in parts of western Illinois and northeast Missouri. These evolving weather patterns are critical for crop development stages and will continue to influence market dynamics.

US Crop Export Sales Corn export sales showed an increase last week, with Colombia being the largest buyer. However, soybean sales were relatively weak, with the Netherlands as the primary buyer. The varied performance in crop export sales reflects shifting global demand and market conditions, which traders closely monitor for strategic insights.

Economic Indicators and Market Reactions In a surprising economic update, consumer prices declined for the first time in four years last month, with the CPI falling 0.1% in June. This decline, coupled with the slowest annual inflation increase since March 2021, has significantly influenced market expectations, with the probability of the Federal Reserve beginning rate cuts in September now standing at 89%.

Stay Updated: For ongoing insights into these issues affecting agriculture, trade, and economic policies, make sure to subscribe to our channel. We bring you the latest information to help you understand the complexities of agricultural economics and global geopolitical shifts.

Join the Conversation: How do you think the upcoming USDA reports will impact market prices? What are your views on the ongoing geopolitical tensions and their potential effects on agricultural exports? Share your thoughts in the comments below. Your input is vital as we navigate these complex global scenarios.