Toxic weeds and trees in pastures are a major concern for many horse owners. As a responsible horse owner, it is good to inspect horse pastures regularly for potentially toxic plants, but also it is important to recognize that it is almost impossible to completely eliminate all toxic plants from pastures.
Even though toxic plants can be found in almost all horse pastures, the likelihood of toxicity is low for most horses. When assessing a horse’s risk for plant toxicity, two big things to be aware of are the palatability of potential toxins and the concentration required to cause toxicity. Many toxic plants are highly unpalatable, so horses will not consume them if given the choice of other forage. Additionally, some plants that are classified as “toxic” may require that the horse eat a large quantity before showing any toxicity symptoms, whereas some plants may be lethal in very small amounts.
As an example, if comparing a holly tree to a yew tree, there is a vast difference in the amount that a horse must consume in order to show toxicity symptoms. Holly trees are technically considered a toxic species, but the average horse would have to eat approximately 3 lb of berries before developing symptoms. Even then, it would likely show relatively mild symptoms, such as gastrointestinal upset and colic. By contrast, a yew tree is highly toxic and can cause death with just a few bites.
Therefore, when evaluating concerns about toxic plants in horse pastures, it is important to assess the risk associated with each apparent species and be most concerned about the ones that can cause problems in small quantities, especially if they are palatable.
What Toxic Plants Are My Horses Likely to Eat, and Why?
When assessing whether a horse is likely to consume a toxic plant, it is important to consider two things: the plants that a horse would be most likely to eat, and what conditions would make the horse likely to consume toxic plants.
Many plants are toxic but are unlikely to be eaten by a horse because they are unpalatable. The most concerning plants are the few that a horse might develop a taste for and voluntarily eat. For example, there are some reports that yew trees might be palatable to horses. Given that they also have a high concentration of toxins, we should be greatly concerned about yew trees near horse pastures.
While horses will typically leave unpalatable plants alone, it is important to recognize when a horse might be more inclined to eat such a plant. Horses may choose to consume an unpalatable toxic plant out of hunger or boredom. Even horses that are a healthy weight may voluntarily eat toxic plants if they are not receiving enough quality forage.
Horses need to consume approximately 2% of their body weight in dry matter every day. For a 1000-lb horse, this equals 20 lb of dry matter. Hay is roughly 85% dry matter, so this horse would need to eat approximately 23.5 lb of hay to feel “full.” In a pasture setting, grass has a much higher water content compared to hay, so a horse that is grazing would have to consume considerably more grass to eat the same 20 lb of dry matter—potentially 60 lb or more per day, depending on the water content of the grass.
Horse owners frequently make the mistake of relying heavily on calorie-dense grains to meet the horse’s energy needs while skimping on forage. A horse that is housed in a poor-quality pasture and provided little supplemental hay is much more likely to eat things it normally would not, even if it is able to maintain its body weight on the grain it is given. Therefore, it is important to recognize that a good nutrition program will help tremendously to prevent the ingestion of toxic weeds and trees.
Toxic Weeds Present in the Southeastern U.S.
When identifying toxic plants, it is beneficial to group them based on the environment where they grow and therefore have the highest prevalence. Some weeds grow in wet or marshy environments, so if you have a pond or marsh area you should inspect for these. Some grow in shady, forested environments, and some grow in open fields. It is important to know what to look for in each environment. This publication is not a comprehensive list, but provides pictures and information for identification of some of the more common toxic plants in the Southeastern U.S.
Water Hemlock
One of the major toxic weeds of the Southeast that would grow in wet environments is water hemlock (Figure 1). It only takes about 2 lb of this plant to be fatal, so it is a potential danger for horses housed near waterways or marshes. It can be recognized by its small white flowers, which grow in clusters. Its toxins act on the central nervous system, and it can cause convulsions and death. The toxin in water hemlock, cicutoxin, is found in the stems and leaves, but it is primarily concentrated in the tuberous roots.
Poison Hemlock
A similar plant that grows in more upland areas is poison hemlock (Figure 2). It is also toxic to horses, but they have to consume more of this compared to water hemlock for it to be lethal. Similar to water hemlock, poison hemlock has small white clusters of flowers. All parts of the plant are poisonous, particularly the leaves. Toxicosis can result in respiratory paralysis followed by cardiac arrest.
Bracken Fern
Bracken fern (Figure 3) is another toxic plant, and this one can grow in a wide variety of environments, including both wetlands and open fields. It has triangular-shaped leaves and can grow 2–4 ft in height. It produces a thiaminase compound, which results in a thiamine deficiency in affected animals. Toxins in this plant can accumulate over time if the horse chronically grazes it, so toxicity might not show up for several months. Symptoms typically are neurological, and include incoordination, a wide stance, and muscle tremors.
Nightshade
Another toxic weed to beware of in pastures or wooded areas would include nightshades (Figure 4), of which there are different species. There are many varieties of nightshade including horse nettle, which is commonly seen in pastures, and jimson weed. The most toxic nightshade variety is Eastern Black Nightshade, which has berries that ripen to a dark-purple to black color. The plant grows in dry areas and produces a small white star-shaped flower. The flowers develop into green berries that ripen to dark-colored berries. Nightshades generally are considered to have very low palatability, though there are some reports of animals selectively grazing nightshades. Toxicity symptoms are often neurological and include depression, muscular weakness, trembling, labored breathing, incoordination, and possible sudden death.
Pokeweed
A toxic weed commonly seen in horse pastures is pokeweed (Figure 5). This plant can grow 4–10 ft in height, and is recognizable by its reddish-colored stem, its alternate green leaves, and its rather large purple berries. It is considered moderately toxic to horses, as its toxicity is concentrated in the root of the plant, which would be less commonly eaten. Toxicity symptoms include colic and diarrhea as well as potential neurological symptoms, such as convulsions.
Perilla Mint
Perilla mint (Figure 6) can be a highly toxic plant to horses if consumed. It is an erect plant that can grow up to 2 ft in height. It has a serrated, alternate leaf that can be either purple or green with a purplish underside. All parts of the plant are toxic, but especially the flowers and seed heads. Toxicosis results in respiratory distress and potential death.
Bitter Sneezeweed
Bitter sneezeweed (Figure 7) is a prevalent weed in the summer months and can be recognized by its distinctive yellow flowers. Toxicosis in horses is rare because of its extremely unpalatable nature—horses typically will not consume bitter sneezeweed unless they have no other choice. Symptoms of toxicity in animals include digestive symptoms (such as salivation and diarrhea), muscle tremors, and inhalation pneumonia.
Buttercups
Another yellow-flowered toxic plant is the buttercup (Figure 8). A toxin found in the leaves and stems of buttercups can cause significant blistering and irritation to a horse’s mouth and gastrointestinal tract. Symptoms can range from mouth blisters, excessive salivation, and mild colic to more severe symptoms such as hemorrhaging, convulsions, and death. They tend to flower earlier than bitter sneezeweed, with flowers appearing in early spring. They are most prevalent in overgrazed pastures, and they prefer wet soils.
White Snakeroot
White snakeroot (Figure 9) grows in woods and thickets and can tolerate a variety of growing conditions, including full sun and shade in southern areas such as Georgia. It is most commonly found near woodlands and streams. It grows approximately 3 ft tall (with a height range of 1–5 feet) and can be recognized by its coarsely toothed leaves and small, fuzzy white flowers that appear in late summer to early fall. The leaves and stems can be highly toxic to horses, resulting in neurological symptoms such as trembling, weakness, stiffness, tremors, and potentially death.
Milkweed
There are many varieties of milkweed (Figure 10) that can be toxic to horses. Milkweed can be recognized by the presence of a milky sap when leaves or stems are broken. Milkweed can be toxic, and even fatal, in very small quantities (approximately 1 lb for a 1000 lb horse). Symptoms can be neurological or cardiac-related and include excessive salivation, incoordination, convulsions, depression and lethargy, cardiac irregularity, colic, and death. It can be present in both wet areas and dry, upland pastures.
Johnsongrass
Johnsongrass (Figure 11) is considered to have somewhat low toxicity in horses, but should still be considered a toxic weed and removed from horse pastures because of its potential to accumulate cyanide. Care should be taken to avoid feeding hay with high concentrations of Johnsongrass present. Johnsongrass is part of the sorghum family, and this group of plants can produce cyanide under stressed conditions (such as drought). In horses, Johnsongrass poisoning can involve neurological symptoms and teratogenesis (damage to a fetus). Neurological symptoms include weakness, incoordination, dribbling urine, difficulty backing, and progressive paralysis. The amount needed to result in equine toxicity is not established, but toxicosis is often the result of continued consumption for multiple weeks.
Preventing Ingestion of Toxic Weeds
When assessing your horse’s risk for toxic plants, please recognize that it is nearly impossible to remove all potential toxins from your horse’s environment. The better strategy is to make sure that your horse is on a good nutritional program. Most toxic plants are not very palatable, so horses tend to leave them alone unless they are hungry or have nothing else to eat.
Remember too that a horse tends to eat about 2% of its body weight in dry matter each day. Even if you are feeding your horse a good quality grain and it is holding its weight fine, if it is going for long parts of the day with no forage, it will be prone to grazing plants that it would ordinarily leave alone. Make sure your horse has access to good quality forage throughout the day to help prevent them from sampling plants that might be toxic.
Practice good pasture management strategies. Weeds are much less likely to grow in pastures that have a good stand of grass established.
Herbicides can be used to effectively control many toxic weeds if they are applied at the correct time and in the correct concentration. A local county Extension agent can assist you in determining when to apply herbicides and what herbicides might be most effective for the weeds you are trying to control.
Trees in Horse Pastures
Horse owners often ask which trees can be safely planted in horse pastures. Trees provide both shade and a windbreak for pastured horses, as well as a privacy screen for owners. Because of these benefits, horse owners often want trees in or around their pastures. Trees can be incorporated into pastures safely with a few good management strategies.
It is important to choose your trees carefully, if possible, and avoid planting toxic trees around horse pastures. Often, trees are already present in pastures, and the horse owner must decide whether to leave the trees or remove them. If there already are toxic trees present, consider either removing them or fencing them off from the horses.
Major toxic trees listed here include: yew, black locust, horse chestnut or buckeye, red maple, box elder maple, cyanide-producing trees, oak trees, persimmon trees, and many ornamentals.
Yew
As mentioned earlier in this publication, yew trees (Figure 12) are highly toxic to horses and should always be removed. The yew is an ornamental evergreen shrub or tree with red berries. The needles and seeds are toxic to horses, with as little as 0.5 lb being lethal to a horse. Death occurs quickly and involves both gastrointestinal and neurological symptoms.
Black Locust
All visible parts of the black locust tree (Figure 13) are toxic to horses and can result in death if eaten in sufficient quantity. Symptoms include gastrointestinal distress and cardiac symptoms. Horses typically will not consume these trees unless they do not have adequate forage.
Horse Chestnut
The horse chestnut (Figure 14), also called a buckeye tree, is easily recognized by its unique nut. It is typically found in moist soils in wooded areas. The leaves, sprouts, and nuts of the buckeye can be toxic to horses and result in colic and muscular tremors and spasms.
Red Maple
Although the exact toxin has not been established, red maple trees (Figure 15) are similarly dangerous to horses as the cyanide-producing trees listed below. Fresh leaves are not considered toxic, but wilted leaves can be highly toxic to horses and can result in severe anemia and death. Trees should be removed or fenced off so that falling leaves cannot be reached by animals.
Boxelder Maple
Boxelder maple trees (Figure 16) can be highly toxic to horses that consume the seeds from female trees. Horses may develop seasonal pasture myopathy, which can be fatal in 90% of cases. Toxins from these seeds block fat production and cause the breakdown of respiratory and muscle cells.
Cyanide-Producing Trees
Cyanide-producing trees that horses should be kept away from include cherry, peach, and plum trees (Figure 17). If removing these trees is not an option, recognize that the most toxic part of these plants is the wilted leaves. Fence them off well beyond the drip line of the trees so that horses can’t reach fallen leaves. Cyanide interferes with oxygen transport in the body, resulting in death from suffocation.
Oak
Oak trees (Figure 18) can also be considered toxic to horses, particularly green acorns and new leaves. These are considered moderately toxic and can result in poor appetite and weight loss, urinary issues, and potential death if nuts or young leaves are consumed in sufficient quantities.
Persimmon
Persimmons (Figure 19) are another tree that can be toxic if horses decide to eat the persimmon fruit—not because the fruit itself is toxic, but because it can cause a phytobezoar, which is a blockage in the gastrointestinal tract caused by undigested plant materials. If persimmon trees are present in horse pastures, take care to ensure that horses are not able to eat the fruit of the tree.
Ornamental Trees
Less common trees that are considered toxic include the Russian olive, Kentucky coffee tree, and golden chain tree. Many ornamentals, including common landscaping plants like rhododendrons (azaleas) and lantana, are considered toxic. In general, before doing any landscaping around your pasture or barn, double-check to ensure that what you are planting won’t harm your horse—many commonly used landscaping plants are problematic for horses.
Conclusion
This publication should serve as an overview of the most common toxic plants that might be found in horse pastures as well as risk factors that might lead to toxicosis. Local county Extension agents can assist in identifying potentially toxic weeds and trees as well as provide information on herbicide control of weeds. It is important to recognize that good pasture management and a sound nutritional program, including access to good-quality forage, are among the best ways to prevent pasture toxicities in horses.
Status and Revision History
In Review on Mar 11, 2025
Published on Mar 13, 2025