Some of the commonly used cover crops are poisonous to cattle, some have manageable metabolic issues and some are considered very safe for cattle.

Common metabolic disorders for cattle on cover crops include bloat, grass tetany, nitrate toxicity, prussic acid poisoning, sweetclover poisoning, polioencephalomalacia (PEM) and glucosinolate poisoning.

Poisonous plants

Hairy vetch – Cover crops have several species that are poisonous to cattle and many are listed on the USDA poisonous plants registry – thus not recommended to plant as a forage crop. The first one to be concerned about is hairy vetch.

Hairy vetch is a legume that has been found to be an efficient nitrogen-fixer with tremendous growth potential; however, if you have ever talked with your veterinarian about hairy vetch, you know they identify this plant as a poisonous plant.

The mortality rate of cattle that show symptoms to hairy vetch toxicosis is 50 to 100 percent. Symptoms of poisoning include allergic-type reactions, especially skin photosensitization, weakness and then death.

Black-hided animals are most commonly affected by hairy vetch toxicity; however, other breeds of cattle have been reported to be affected. The incidence of hairy vetch toxicity is very low; however, when it occurs in an operation it can be devastating.

There currently is no known test to determine “safe” levels of the toxin, primarily because there is not a consensus as to what toxin causes the issues. There is speculation of a genetic component that makes certain animals more susceptible to a negative reaction to hairy vetch.

Since we do not know how to determine if the plant is safe to graze, nor do we have an idea of which animals are susceptible to toxicity grazing, hairy vetch is like playing “Russian roulette.” Many producers have grazed hairy vetch for years and never had an issue, yet it has the potential to turn disastrous.

Lupins – Another cover crop that can be toxic to livestock comes from the lupin family. There are four nontoxic species of lupines that are a good source of protein and energy in diets for ruminants and monogastrics; however, there are six species that are particularly toxic to cattle and sheep.

The common names for the toxic lupines include silky lupine, tailcup lupine, velvet lupine, silvery lupine, summer lupine and sulfur lupine.

Birth defects are the most common symptom of lupine toxicity, which occurs while the pregnant cow is consuming toxic plants. Lupines grow naturally in the Western U.S., and these producers know about the potential detrimental effects to their calf crops.

Amaranth – Amaranths also have some species that are safe for cattle and some species that are very toxic. Amaranth plants are a cousin to pigweed and in the grain-producing species, are very safe for human and livestock consumption. Toxic species of amaranth include spiny amaranth, redroot pigweed and Palmer amaranth.

Nitrate toxicity is the primary issue with these plants; however, they can be a weed issue in crop fields, especially from a herbicide-resistance perspective.

Manageable potentially toxic crops

There are several high-quality forages that can be planted as a cover crop and can be safe for cattle to use as a forage crop with a little management.

Brassicas – Brassicas such as kale, rapeseed, swede, turnip, canola and mustard are very high-energy and high-protein feeds (20 to 25 percent crude protein in the leafy portion) that grow rapidly (often ready to graze in less than 75 days) and provide tremendous biomass. However, feeding brassicas come with some restrictions to maintain animal and rumen health.

Brassicas are high in moisture and low in fiber. Cattle have demands for dry feeds, therefore some source of dry forage must be offered with brassicas. This can be done by feeding hay, planting (broadcasting) brassicas into standing stalks or planting brassicas in combination with forage that will grow and die before turning cattle out to graze.

Secondly, brassicas are low in copper, manganese and zinc, which are very important for immune health and reproduction. At the same time, brassicas have high levels of glucosinolates, which interfere with mineral metabolism, especially iodine uptake. This reduction in mineral metabolism and iodine can lead to hoof and eye issues, in addition to reductions in performance.

To minimize glucosinolate issues and to provide enough dry forage for cattle, it is recommended that brassicas comprise no more than 75 percent of the total diet. As well, for high-risk cattle and breeding animals, making sure the cattle consume an iodized mineral is very important, and it might be valuable to include chelated trace minerals to help offset potential reductions in mineral absorption.

A third potential issue with brassicas is nitrate toxicity because brassicas are nitrate accumulators; they, in turn, have very high levels of nitrates. Nitrates will reduce with a longer growing season, so it might be possible to turn out animals later in the season on brassicas. To minimize nitrate issues, adapt the cattle to the high-nitrate feed over a five- to seven-day period, and never turn out hungry animals on a suspect field.

Small grains – Small grains such as barley, oats, rye, ryegrass, wheat and triticale are commonly grazed and are wonderful forages for cattle. Even though they are commonly grazed, there are some potential metabolic issues including bloat, grass tetany and to some degree, nitrate toxicity.

Generally, bloat is most concerning in small grains. Ways to minimize bloat issues are to provide a bloat-block or ionophore, and provide a source of feed for dilution of energy and excess protein.

Grass tetany occurs when plants grow rapidly and have a low concentration of magnesium. Small grains can cause grass tetany in lactating cows, so to manage for this, a producer should graze low-risk animals, such as young calves or pregnant cows, on the lush flush of grass and provide a mineral with at least 12 percent magnesium.

If palatability is an issue, then offer a chelated magnesium to increase bioavailability to the animal.

Legumes – Bloat is the most common metabolic issue with legumes. Once again, producers can manage for bloat issues in legumes. One option is to keep legumes at less than 50 percent of the diet for cattle. Providing a dry feedstuff also reduces legume bloat.

As well, there are certain types of legumes that are lower risk for bloat, such as birdsfoot trefoil, sainfoin, annual lespedeza, lablab, cowpea, sunhemp, mungbean and soybean. Clovers and alfalfa are the highest-risk legumes for bloat. Sweet clover poisoning is also something to be concerned about with sweet clover hays.

Grazing sweet clovers do not result in this issue, but putting up moldy sweet clover hay can cause issues such as abortions.

Sorghums, sudans, millets and corn – This class of warm-season grasses are drought-tolerant, high-yielding, high-quality feeds for cattle, yet they have some metabolic issues that can be managed. The two main issues are nitrate toxicity and prussic acid poisoning.

It is important to realize nitrates and prussic acid are not complementary issues in plants. They are two stand-alone issues and not directly correlated with each other.

Nitrates are a potential issue in all of these summer grasses. Nitrates are increased with drought stress, and nitrates are highest in young plants and at the stalk. To manage for nitrates, allow the plants to mature somewhat, and do not force animals to graze the stalk base.

When haying these summer crops, if nitrates are an issue, raise the cutter bar so the stalk base, where most of the nitrate is accumulated, is not baled. Nitrate values stay fairly static in the dead plant, so you want to delay harvesting until the plant is at a safe nitrate level.

On the other hand, prussic acid is found in sorghums, sorghum-sudans and some millets. Corn, pearl millet, foxtail millet and sudangrass do not raise prussic acid concerns for cattle. Intensive management can be implemented in grazing sorghums and prussic acid-susceptible millets so that we do not kill animals due to hydrogen cyanide.

When prussic acid is a concern, it is recommended to rotational or strip graze these forages. Generally, when the sorghum plant is 24 inches or taller, prussic acid is not a concern, whereas millets are generally safe at 18 inches.

Do not graze prussic acid plants for at least 10 days after a killing freeze, when the toxin is the most potent. Another interesting facet of prussic acid is that this molecule is very volatile – once the plant dies, the killing components of prussic acid will evaporate and become safe for animals to consume after a time (about 10 days).

This is the reason why baled hays are not a prussic acid concern, whereas these summer annual hays need to be tested for nitrates.

Flax – It is not recommended to graze flax because of the potential for prussic acid (cyanide poisoning). The green straw, especially after a freeze, generally is high in prussic acid. The flax seed is a very nutritive grain; however, the straw is high in cellulose and lignin and is a very poor-quality forage.  

PHOTO: Cattle graze a field of mixed cover crops in southern Idaho in early November. The mix contains triticale, forage pea, turnip and daikon radishes planted in mid-August. Photo by Lynn Jaynes.

For a more detailed description of toxic crops, see Forage Crops - Grazing management: toxic plants.