Poison plants to horses

Nine poisonous plants horses should avoid

With some plants and trees being poisonous, and sometimes fatal, to horses, it’s important you know their names, can recognise them, and are aware of the places they may grow, so you can keep your horse safe. Here are the nine most common poisonous plants to watch out for…


While ragwort has a bitter taste and is rarely eaten by horses when it is growing, when it is wilted or dried it becomes more palatable. This plant contains toxins that result in liver failure and even death, so hay should not be made from fields containing ragwort. Eating just 1-5kg of the stuff over a horse’s life time may be fatal.

Ragwort thrives on poor grazing and wasteland, and each plant produces thousands of seeds that are dispersed widely by the wind. Local authorities have legal power to order land owners to clear land containing the weed, and a good guide to identifying it is available on the Defra website (https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69265/pb4192-injurious-weeds.pdf).

It takes two years to fully grow and flower, from a dense rosette of leaves in the first year to producing bright yellow flowers on 30-100cm woody stems in the second. Ragwort thrives on poor grazing and wasteland, but each plant produces thousands of seeds that are dispersed widely by the wind.

Ragwort should be controlled by good pasture management, the use of herbicides or manual control when it should be uprooted, removed, and burned. Spray it when the plant is at the rosette stage and don’t wait for the stem to appear. Mowing and cutting ragwort isn’t a good idea as it will make it grow back more quickly.


Horses will not normally eat fresh foxglove but it is more palatable in hay and just 100g could prove fatal. Symptoms of foxglove poisoning include, contracted pupils, convulsions, breathing difficulties and death after only a few hours.

Deadly nightshade

Despite its name, poisoning from nightshade is not normally fatal to horses but can cause unconsciousness, dilation of the pupils and convulsions.


Buttercups are poisonous to horses if eaten fresh, but a horse would need to eat large amounts to die from eating them.

Seek professional advice on spraying to remove from grazing areas. Dried buttercups are harmless in hay.


Oak trees pose a particular threat to horses when they drop their acorns in the autumn. Acorns are relished by many horses and can lead to severe colic and poisoning if eaten in large quantities.

Collect the acorns up, or move horses to a place without oak in the autumn.


Is common in gardens, and the fallen leaves and berries are as lethal to your horse as the fresh plant – so be careful of fallen leaves and berries being blown into your field, even if the hedges are fenced off.

Just 0.5kg can be fatal, with the horse falling into an insensitive state similar to sleep.


Is also common in gardens so be careful of neighbours hedges and the possibility of people dumping cuttings in the field.

Box privet is the most dangerous, as eating even small quantities can kill a horse.


Very small quantities of this are highly toxic to horses, causing death by failure of the respiratory system.

Sycamore, maple and other acers

This is known as seasonal as it is thought that the helicopter seeds in autumn, and the saplings in spring, contain Hypoglycin-A that causes atypical myopathy in horses. Find out more about atypical myopathy here.

Symptoms include muscular stiffness, reluctance to walk, muscle tremors, sweating, depression, high heart rate, dark urine (reddish in colour). Your horse may appear weak and may have difficulty standing, breathing difficulties, but may still want to eat. Call your vet as quickly as you can.

Find out how to prevent your horse from meeting these poisonous plants here.

Poisonous Weeds in Horse Pastures


Hungry horses do not heed warnings about avoiding poisonous plants. There is widespread belief that instinct protects animals, but this is not always true. Therefore, it is up to horse owners to prevent plant poisonings. The best way to do this is to become familiar with identification of poisonous plants and pasture best management practices.

What makes a plant poisonous? There are several different chemical compounds capable of poisoning that can be found in a variety of plants. The chemicals range from the alkaloids, found in the nightshade family, to the glycosides, present in wild cherry and Sudan grass. The effects of plant toxins can range from mild irritation and weight loss to colic and possibly even death. Plant poisoning can often be difficult to diagnose, as it can resemble other physiological problems. Depending on the degree of plant toxicity, poisoning can occur due to a single contact (or ingestion) or long term repeated contact with a plant.

The degree of danger a poisonous plant represents is a function of the plant’s prevalence, toxicity, and palatability. If good quality forage is plentiful in the pasture, horses will avoid most poisonous plants. In the absence of good quality forage, such as during periods of drought or when pastures are overgrazed, animals may begin to investigate undesirable plants available in the pasture. Fortunately, many poisonous plants are not palatable and horses will only eat them if adequate forage is not available.

Inspect Pastures

A primary way to avoid poisoning from plants is to walk the pastures and inspect for poisonous plants. If poisonous plants are present, they should be removed if possible (i.e. herbicide application, hand digging, or mowing) or excluded via fencing. Do not forget to inspect three or four feet beyond the fence line of the pasture, since many horses will stretch beyond the fence for forage. Walking the pastures will also give you an opportunity to evaluate the productivity of the pasture. Another thing to look for is the density of desirable forage species. Is adequate forage present for the horses, or does the pasture contain many bare areas (more than 30% of the field) without extra hay provided? Are there more weeds present than forage species? Also check the hedgerows surrounding the pasture. Many hedgerows contain wild cherry, maple, and black locust trees or seedlings which can be poisonous. Do not allow broken branches of these species to remain in the pasture, and avoid throwing shrub and tree prunings into the pasture. As listed below, many common shrubs and trees can be dangerous to horses.

Pasture Management

The best defense against poisonous plants is to promote good stands of desirable grass and legume species through a sound pasture management program. Pasture management should include soil testing, liming and fertilizing, good grazing management, mowing, and dragging (see FS368, “Establishing and Managing Horse Pastures”). Weed, insect, and disease control may also be required for pastures. A healthy, productive pasture will resist invasion from most poisonous weeds and provide good quality forage. When pastures are overgrazed, horses will eat the grass and legume species down to the soil, which allows weeds to take over.

Any plant that is not wanted in the pasture is a weed, and many weeds can be eliminated with good pasture management. Weeds can be controlled mechanically by mowing, culturally with good grazing management, or chemically with herbicides. Most poisonous plants are broadleaf plants or woody species. For general broadleaf weed control in pastures, the best results are obtained when weeds are actively growing. Dicamba, 2,4-D, or a combination of dicamba and 2,4-D may be sprayed in permanent pastures to control many annual and perennial broadleaf weeds while not affecting the grasses. However, dicamba and 2,4-D will kill or severely injure most legumes (i.e., alfalfa and clover). When using these herbicides, extra precautions should be used to prevent drift (airborne herbicide spreading somewhere other than the field being sprayed) and labeled information, specifically the grazing restrictions, must be followed.

Common Toxic Plants Found In or Near Horse Pastures

Tall buttercup. (Photo courtesy Sarah Ralston.)

Buttercups: The buttercup species (Ranunculus species) includes several annual and perennial plants which are commonly found in overgrazed horse pastures. Buttercup causes oral irritation when chewed, and horses rarely consume the plant because it is unpalatable. The toxic component is in the fresh leaves and flowers, but they lose toxicity when dried for hay. Symptoms of buttercup poisoning include increased salivation, decreased appetite, colic and diarrhea. In severe cases, poisoning may lead to convulsions and death. As long as horses have access to adequate pasture or hay, it is unlikely that they will eat buttercups.

Jimsonweed. (Photo courtesy Carey Williams.)

Jimsonweed: Jimsonweed (Datura stramonium) is a worldwide nuisance. Other common names include Jamestown weed, thorn apple, downy thornapple, devil’s trumpet, angel’s trumpet, mad apple, stink weed and tolguacha. It is an annual plant, growing up to 5 feet tall in agricultural fields and overgrazed pastures. It can be recognized by its distinctive tree-like shape, white or purple trumpet-like flowers and prickly seed capsules. All parts of the jimsonweed plant are poisonous to horses and humans; toxicity is caused by tropane alkaloids. Symptoms of poisoning in horses include a weak, rapid pulse, dilated pupils, dry mouth, incoordination, diarrhea, convulsions, coma, and sometimes death. Jimsonweed has a foul odor and taste, and horses rarely consume it if they have other quality forage.

Horse Nettle. (Photo courtesy Carey Williams.)

Nightshade Family (including Horse Nettle): The nightshade family (Solanum species) contains many toxic plants, including horse nettle, black nightshade, bittersweet nightshade, some species of groundcherry, and even tomatoes and potatoes. They all contain a glycoalkaloid called solanine in the leaves, shoots, and unripe (green) berries. The plant affects the central nervous system and the gastrointestinal tract. Horses generally do not eat these plants unless they are very hungry and no other feed source is present. Toxicity is highest in green berries, followed by red or black berries, leaves, stems and roots. It is estimated that one to ten pounds of ingested plant material is fatal for horses. Some symptoms of solanine poisoning include dilation of pupils, diarrhea, loss of appetite, and loss of muscular coordination. Some other signs of poisoning are a sudden state of depression, apparent hallucinations, and convulsions.

Pokeweed. (Photo courtesy Sarah Ralston.)

Pokeweed: Pokeweed (Phytolacca americana) is a commonly found weed in horse pastures and around fence lines. It grows erect, resembling a tree, and can reach up to 10 feet in height. The stem is often purple or red in color and can reach a diameter of 4 inches. The leaves are long and elliptical, growing between 12 and 20 inches long. Pokeweed produces clusters of green berries, which mature to a dark purple color. The roots are the most toxic part of the plant, but horses can also be poisoned from the leaves and stems. A toxic compound, called phytolaccotoxin, can cause a burning sensation in the mouth, low grade chronic colic, and diarrhea. Pokeweed is not very palatable, so horses with access to plenty of good quality forage should avoid it.

Japanese Yew. (Photo courtesy Carey Williams.)

Japanese Yew: Japanese yew (Taxus cuspidata) is an ornamental plant that is EXTREMELY toxic to mammals. It is grown as an ornamental shrub or hedge and has small evergreen leaves with bright red berries. Yew leaves have been reported to be palatable to horses, and as little as a mouthful (about 0.1% body weight of leaves) can be fatal within 30 minutes due to respiratory or cardiac collapse. Symptoms are rarely observed, as animals often die very quickly after ingesting this plant. Since it is a very common ornamental shrub, it is especially important to make sure neighbors do not throw yard clippings into your pastures.

Wild Cherry Branch. (Photo courtesy Carey Williams.)

Wild Cherry: The entire Prunus genus (includes cherries, pears, and peaches) is toxic to horses and other livestock. The seeds, foliage, and bark produce hydrogen cyanide, a deadly compound. Leaves are most dangerous when wilted because the percentage of cyanide increases and leaves accumulate sugars as they wilt, but fresh leaves can also cause toxicity if enough are consumed. Two and a half pounds of black cherry leaves would be toxic to a 1,000-pound horse. Drought stress also increases cyanide levels. It is important to note that the small suckers that grow from the base of a cherry tree, even from cut stumps, contain high levels of cyanide.

Black Walnut Shavings (dark) in regular pine shavings. (Photo courtesy Krishona Martinson, University of Minnesota.)

Black Walnut: The bark, woods, nuts, and roots of the black walnut tree (Juglans nigra) contains a toxic compound. There is conflicting research on exactly what the compound is; it was previously thought to be juglone, however juglone did not cause symptoms in experiments. Horses are primarily exposed through black walnut shavings mixed in with other shavings as bedding. Symptoms of exposure include depression, lethargy, laminitis, swelling of the lower limbs, and increased temperature, pulse, respiration rate, abdominal sounds, digital pulse, and hoof temperature. Symptoms usually disappear within hours after the horse is removed from the shavings; however laminitis can present further problems. Since the bark and nut hulls from the black walnut are toxic, these trees should be removed from horse pastures as a precaution.

Red Maples Leaves. (Photo courtesy Krishona Martinson, University of Minnesota.)

Maple Trees: Maple (Acer species) leaves are highly toxic. However, usually this is when they are in the stressed state prior to dying (e.g. leaves on a fallen tree limb lying in a pasture or during the fall). Similar to wild cherry, the leaves are sweeter and more palatable when they are wilted. Fallen and dead leaves remain toxic for about a month and cause severe kidney damage if ingested in large quantities. It is estimated that an adult horse needs to consume 1.5 pounds of leaves or more to become poisoned. Symptoms of toxicity include depression, lethargy, increased rate and depth of breathing, increased heart rate, jaundice, dark brown urine, coma, and death.

Poison Hemlock. (Photo courtesy Laura Gladney.)

Poison Hemlock and Water Hemlock: Poison hemlock (Conium maculatum) and water hemlock (Cicuta species) are both very toxic to horses. They are often found in moist areas and exude an unpleasant, parsnip-like odor when cut. While they look similar at first glance, they are different plants and have different toxic properties. All parts of the poison hemlock are toxic, and toxicity increases throughout the growing season, especially in the roots, which resemble parsnips. A horse must consume about 4 to 5 pounds of poison hemlock for the dose to be lethal. However, water hemlock is much more toxic, and even 8 ounces can be fatal to a horse. Toxicity of water hemlock decreases throughout the growing season; however, the roots remain highly toxic year-round. Both plants affect the central nervous system, and nervousness, trembling, and incoordination may be observed. Horses suffering from water hemlock poisoning typically become violent, with muscle tremors and convulsions. The plants can be differentiated by examining leaf shape: poison hemlock has many small fern-like leaves, and waterhemlock has large serrated leaves.

Alsike Clover (left) compared to red clover (right). (Photo courtesy Sarah Ralston.)

Alsike Clover: Alsike clover (Trifolium hybridum) is found most frequently in Canada, but has been included in some pasture mixes in the U.S. It grows to a height of 15 to 30 inches, and has a small ½-inch diameter pink flower that forms at the ends of secondary branches from the main stem. It should be differentiated from the non-toxic red and white clovers, which have a larger flower, hairy stems and leaves, and a white inverted “V” on the leaf. Alsike clover is known to cause two syndromes: photosensitization (short-term exposure) and “big liver syndrome” (long-term exposure) when infected with a mold. There is also a potential for nitrate poisoning. The more common and acute lesions related to photosensitization are characterized by reddening of the skin exposed to sunlight (especially on white markings), followed by either superficial or deep, dry necrosis of the skin and swelling and discharge, resulting in crusty inflamed areas. If the exposure is prolonged, the alkaloid toxin can cause acute liver failure, which can be fatal.

Rhododendron. (Photo courtesy Stan C. Hockanson, University of Minnesota.)

Rhododendron/Azalea/Mountain Laurel: Plants in the Rhododendron genus, such as azaleas, and other plants in the Ericaceae family, such as mountain laurel, are commonly planted as ornamentals for their colorful and attractive flowers. However, all parts of these plants contain glycosides called grayanotoxins which adversely affect the stomach, intestine, and cardiovascular system. The early symptoms are salivation, diarrhea, colic, and muscle tremors. Later, abnormal heart rate or rhythm may occur. If large enough quantities are eaten, this plant can cause death.

Some other toxic plants found in New Jersey include:

These plants can often be easily identified using the online resources listed below. Many of the sites contain images to aid in identification. If you suspect a plant may be poisonous and need assistance identifying the plant, contact your county agricultural agent. The phone number for your Rutgers Cooperative Extension county agricultural agent may be found in the blue pages of your telephone directory, under County Government (or visit njaes.rutgers.edu/county).

References and Further Reading

  1. Guide to Poisonous Plants. southcampus.colostate.edu/poisonous_plants. This site allows you to search for key words, such as symptoms, in the plants’ descriptions. It also contains several guides with more detailed information. Click the “Search” link to get started.
  2. Poisonous Plants of Veterinary Importance. research.vet.upenn.edu/poisonousplants. This site, maintained by the University of Pennsylvania, contains numerous images of various poisonous plants.
  3. Plants Poisonous or Harmful to Horses in the North Central United States. shop-secure.extension.umn.edu/PublicationDetail.aspx?ID=1923.
  4. Virginia Tech Weed Identification Guide. oak.ppws.vt.edu/~flessner/weedguide/. This site contains photographs and detailed descriptions of hundreds of weeds found in Virginia. Not all plants listed are poisonous to horses.
  5. Meade J.A., Washer R.L., Mohr D.M. Wild Cherry and Livestock (FS152). Cooperative Extension Service, Rutgers, The State University of New Jersey. njaes.rutgers.edu/pubs/publication.php?pid=FS1152.
  6. Burger S.M., Knight A.P. 1996. Horse Owner’s Field Guide to Toxic Plants. Breakthrough Publications Inc., Ossining, NY.

Main photos by Carey Williams

May 2013

Copyright © 2020 Rutgers, The State University of New Jersey. All rights reserved.

For more information: njaes.rutgers.edu.

Cooperating Agencies: Rutgers, The State University of New Jersey, U.S. Department of Agriculture, and County Boards of Chosen Freeholders. Rutgers Cooperative Extension, a unit of the Rutgers New Jersey Agricultural Experiment Station, is an equal opportunity program provider and employer.

Take a stroll through any pasture, and there among the grasses you’ll find any number of different plants. Small vines, broad-leafed weeds, some wildflowers you recognize–some you don’t. And, as disquieting as it may be to contemplate, the chances are pretty good that at least some are toxic to horses. Hundreds of poisonous plants grow in North America, and many are extremely common. “I defy anyone to tell me they have a pasture with zero poisonous plants,” says Jeffery Hall, DVM, PhD, a toxicologist at Utah State University.

Tansy ragwort can be seen in this pasture. |

The good news, of course, is that the vast majority of those plants pose little threat to horses. For one thing, most of them are unpalatable, and horses who are filling up on quality forage aren’t likely to spend a lot of time grazing on the few bitter leaves populating their pasture. Another factor that protects horses is their size–a 1,000-pound animal has to consume significantly higher quantities of most toxins than a smaller animal does to feel any effects. So, for the most part, as long as your horses are healthy and your pasture is in good shape, you have little to worry about.

However, some plants are cause for concern either because even a curious nibble can spell doom or because repeated browsing over weeks or months can lead to serious illness and death. All are worth getting to know by sight–not only so you can eliminate them from your horsekeeping areas, but also so that you can avoid encounters with them in the woods, on the roadsides and along the waterways where you ride. According to Anthony Knight, BVSc, MRCVS, plant toxicologist from Colorado State University, these 10 plants are those most dangerous to horses in the United States:

Bracken fern(Pteridum aquilinum)
Also known as: brake fern, eagle fern
ID:A perennial fern with triangular leaves that can reach two to three feet high. Grows in clumps in woodlands and moist open areas.
Range: Coast to coast, except for the Mediterranean and desert climates of Southern California and the Southwest.
The danger: Bracken fern contains thiaminase, which inhibits absorption of thiamin, which is vitamin B1. Thiamin is necessary to nerve function, and deficiencies can lead to neurological impairment. The relative toxicity of individual leaves is low–horses must consume hundreds of pounds to experience ill effects. However, bracken fern is unique among the toxic plants in that some horses seem to develop a taste for it and will seek it out even when other forages are available.
Signs: Signs are related to neural dysfunctions resulting from vitamin B1 deficiency and can include depression, incoordination and blindness.
What to do: Large doses of thiamin over the course of a week or two can aid in the recovery of horses whose bracken consumption is discovered before the neurological signs are severe.

Hemlock (Conium maculatum)
Also known as: poison hemlock, spotted hemlock
ID: A multistemmed perennial weed with toothed, fernlike leaves and clusters of small white flowers. The stems have purple spots, which are most evident near the base of the plant.
Range: Grows wild along roadsides and other open uncultivated areas throughout North America.
The danger: Hemlock leaves, stems and seeds contain several potent neurotoxins that affect both the central and peripheral nervous systems. Four to five pounds is a lethal dose for a horse. Most animals will avoid the plant.
Signs: Signs appear within an hour or two of consumption, starting with nervousness, tremors and incoordination, progressing to depression and diminished heart and respiratory rates and possibly colic. Death results from respiratory failure.
What to do: There is no treatment, but if smaller doses were consumed, animals may recover with supportive care.

Tansy ragwort (Senecio spp.)
Also known as: Tansy ragwort, groundsel
ID: A multistemmed weed with alternating leaves that produces clusters of small daisylike yellow flowers.
Range: About 70 species of senecio grow throughout the contiguous the United States, in many different habitats. Many are common in pastures and along roadsides.
The danger: Levels of toxicity vary among different members of the species, but all are thought to contain at least some concentration of pyrrolizidine alkaloids, which inhibit cell division, especially in the liver. Damage to the liver is cumulative and irreversible, and most horses succumb to chronic exposure over time, after consuming between 50 and 150 pounds, in total.

Signs: Often, there is no evidence of consumption until signs of liver failure begin to appear: photosensitization, diminished appetite and weight loss, progressing to depression, incoordination and jaundice.
What to do: There is no treatment for advanced stages of liver disease due to this toxin.

Johnsongrass/Sudan grass (Sorghum spp.)
ID: Both johnsongrass and Sudan grass are coarse-stemmed grasses with broad, veined leaves that can grow to six feet in height. Both produce large, multibranched seed heads.
Range: Johnsongrass is a wild grass native to the southern climates, where it grows along roadways and other uncultivated open areas. A close relative, Sudan grass, and its hybrids are cultivated throughout the United States as a forage crop.
The danger: The leaves and stems of johnsongrass and Sudan grass contain a cyanide compound, which when metabolized inhibits the body’s ability to absorb oxygen, in effect suffocating the animal; young shoots of johnsongrass contain the highest concentration of the toxin. Because horses do not metabolize the cyanide compound as efficiently as ruminant animals do, grazing healthy adult plants is unlikely to harm them, but circumstances that injure the plant–wilting, trampling, frost–can chemically liberate the cyanide within the leaves, rendering them dangerous to all species. Cultivated hybrids of Sudan grass typically contain less cyanide, if any. Both species can also accumulate toxic levels of nitrates if overfertilized. Cyanide concentration drops to safe levels when the grasses are cured for hay, but nitrates, if present, do not.
Signs: Signs are consistent with cyanide poisoning. The first indication is rapid breathing, which progresses to tremors, frequent urination and defecation, gasping and convulsions.
What to do: Supportive drug therapy can offset the effects of less severe cyanide poisoning.

Locoweed (Astragalus spp. or Oxytropis spp.)
Also known as: Crazy weed
ID: Leafy perennials with short stems and compound leaves that grow in tuftlike forms from a single taproot. Some species may be covered with silvery hairs. The flowers, often white or purple, are borne on leafless stalks.
Range: Different species of locoweed–spotted or blue, wooly, purple, Lambert’s, two-grooved milk vetch, white-point–grow in varied terrains throughout the West and Southwest, often in dry, sandy soil.
The danger: All toxic species of locoweed contain swainsonine, an alkaloid that inhibits the production of the enzyme necessary for saccharaide metabolism, and the resulting sugar buildup disrupts the function of brain cells.
Signs: Strange behavior is usually the first evidence noticed; horses may bob their heads, adopt exaggerated, high-stepping gaits or stagger and fall.
What to do: There is no treatment for advanced locoism, and its effects are irreversible. Horses with less severe poisoning may recover when access to the weed is removed.

Photo Copyright ASPCA

Oleander(Nerium oleander)
Also known as: Rose laurel, adelfa, rosenlorbeer
ID: An evergreen shrub that can reach the size of a small tree, oleander has elongated, thick leathery leaves that can grow to three to 10 inches long. The flowers, which grow in large clusters at the end of branches, are one to three inches in diameter and can be white, pink or red.
Range: Hardy only in hot climates, oleander is used extensively in landscaping across the southern United States, from California to Florida. It is also grown as a potted plant in northern areas.
The danger: All parts of the plant contain the toxins oleandrin and neriin, which disrupt the beating of the heart. The leaves remain toxic when dried. About 30 to 40 leaves can be deadly to a horse.
Signs: Effects are usually seen several hours after ingestion and last over 24 hours. Signs include colic, difficulty breathing, tremors, recumbency and an irregular heart rate. The pulse may be either slowed or accelerated.
What to do: Horses can survive if treated early with supportive care, such as the administration of activated charcoal to inhibit further toxin absorption and the use of anti-arrhythmic drugs to stabilize the heart.

Red maple trees(Acer rubrum)
ID: A medium-sized tree with leaves that are green in the spring and summer, with shallow notches, bright red stems and a whitish underside; in fall, the leaves turn bright red. The bark is smooth and pale gray on young trees, and becomes dark and broken on older trees.
Range: The native range is eastern North America, from Canada to Florida and west to Minnesota and eastern Texas, but ornamental specimens have been planted all over the country.
The danger: Ingestion of fresh, growing red maple leaves seems to do little or no harm, but when the leaves wilt they become extremely toxic to horses. Access to wilted leaves is most common after storms, which may cause branches to fall into pastures, or in the autumn when the leaves fall and are blown into grazing areas. The toxins in wilted red maple leaves cause the red blood cells to break down so that the blood can no longer carry oxygen; the kidneys, liver and other organs may also be damaged. As little as a pound or two of leaves can be fatal.
Signs: Depending on how many leaves were eaten, signs can appear within a few hours or as long as four or five days after consumption. Signs include lethargy; refusal to eat; dark red-brown or black urine; pale yellowish gums and mucous membranes at first, advancing to dark muddy brown; increased respiratory rate; rapid heart rate; dehydration.
What to do: The only treatment is the administration of large amounts of intravenous fluids and possibly blood transfusions. Recovery depends on how many leaves were consumed and how promptly the horse receives care. (Read about one horse’s recovery in Red Maple Leaf Poisoning Scare.)
Special note: Research indicates that the leaves of at least two related species–the silver and sugar maples–may contain the same toxic elements as red maples, but in less toxic amounts.

Water hemlock (Cicuta spp.)
Also known as: Spotted water hemlock
ID: A perennial weed with erect hairless stems that can grow to six feet from clusters of fleshy roots. The stems are hollow and branching, thicker at the base. Leaves are elongated and toothed, and the small white flowers form flat, umbrella-shaped clusters at the ends of branches.
Range: Water hemlock grows throughout the contiguous United States and is most likely to be found in marshy areas of meadows and along streams and irrigation ditches.
The danger: Water hemlock is considered one of the most toxic plants in the United States. All parts of the plant contain a cicutoxin alkaloid that affects the central nervous system, but the toxin is most concentrated in the root. Because cattle are more likely to pull up and consume the root, that species is considered most at risk of poisoning, but horses have also been known to browse the plant; less than a pound of the leaves and stems can be fatal. The toxin levels in the leaves and stems diminish as the plant ages during the growing season, and additional amounts of toxin are lost when the plant is dried, but water hemlock is never considered safe for consumption. Most animals will avoid the plant.
Signs: The toxins affect neurons primarily within the brain, causing various signs, including excessive salivation, dilated pupils and nervousness, progressing rapidly to difficult breathing, degeneration of the heart and skeletal muscles, seizures and convulsions; death usually results from respiratory paralysis. Signs of poisoning appear within an hour of ingestion, and death typically follows within two to three hours.
What to do: Supportive care initiated before the convulsions begin can offset the worst effects of the seizures, but horses who survive are likely to have experienced permanent damage to the heart and skeletal muscles.

Yellow star thistle/Russian knapweed (Centauria spp.)
Also known as: Barnaby’s thistle
ID:Yellow star thistle is an annual weed that branches out from a single base stem to form a spherical plant up to three feet tall; its round yellow flowers are surrounded by stiff spines 1/2 to 3/4 of an inch long. Russian knapweed spreads via a creeping root system; its erect, stiff stems grow two to three feet high and are covered with gray hairs, and its thistlelike flowers range from purple to white; Russian knapweed has no spines or prickles.
Range: Both plants appear throughout the Western United States, approximately from Missouri to California, and from Mexico northward, almost to Canada. They appear as weeds along roadsides, in cultivated fields and pastures.
The danger: Both plants contain a toxic agent that has a neurological effect on the brain that inhibits the nerves and control chewing. The poisoning is chronic in nature; to receive a toxic dose, horses must consume 50 to 200 percent of their body weight over 30 to 90 days.
Signs: Affected horses may appear to have tense or clenched facial muscles, and they are unable to bite or chew their food effectively. Weight loss is also common.
What to do: There is no treatment, and any neural damage is permanent. Euthanasia is recommended if the horse is too debilitated to eat.

Yew (Taxus spp.)
ID: A woody evergreen shrub with closely spaced, flat, needlelike leaves a half-inch to one inch long. Berries are bright red or yellow, soft and juicy with a hole in the end, where the dark seed is visible.
Range: Western yew and American yew are native to the West Coast and to the Eastern and central United States, respectively, but these two species along with the Japanese and English yews are commonly planted as ornamentals nationwide.
The danger: All parts of the yew plant, except for the fleshy portion of the berries, contain taxine, an alkaloid that causes respiratory and cardiac collapse. The leaves remain toxic even after dried. A single mouthful can be deadly to a horse within minutes.
Signs: Sudden death is the most typical sign of yew ingestion. Animals found alive may be trembling and colicky, with difficulty breathing and a slowed heart rate.
What to do: There is no treatment for yew poisoning. Avoidance is critical; most yew poisonings occur when trimmings are thrown into a pasture after a pruning.

EQUUS thanks Anthony Knight, BVSc, MRCVS, and Jill Richardson, DVM, for their assistance in the preparation of this article. For more information, visit Knight’s website, Guide to Poisonous Plants.

This article originally appeared in the June 2004 issue of EQUUS magazine.

10 Plants Toxic to Horses

Note: This is not an all-inclusive list of plants toxic to horses. Rather, it’s a list of plants commonly found and also highly toxic to horses.


Maple (Acer spp.)

Where it’s found

Thirteen species of maple trees are found throughout the Northern Hemisphere, with a larger distribution in the eastern United States and Canada. The red maple (Acer rubrum) is among the most common, as are the sugar maple (Acer saccharum), silver maple (Acer saccharinum), and box elder (Acer negundo). Only a few species have been associated with the development of clinical signs.

What is the toxin and how does it work?

The toxin is unknown but it damages the red blood cells, making them unable to carry oxygen.

Threat to horses

Most of the case reports and experimental studies are specific to the red maple tree. Other species, especially hybrid species with Acer rubrum in the lineage, may be associated with intoxication. Silver and sugar maples also have been implicated by some research scientists; box elder has not.

  • Ingestion of dry or wilted leaves causes signs; ingestion of fresh leaves does not.
  • Dry and wilted leaves may remain toxic for up to four weeks, but generally do not retain their toxicity over the winter.
  • Generally, leaves dropped after Sept. 15 are considered more toxic, but wilted leaves from branches dropped during summer storms may be just as harmful.
  • It takes about 1.5 lbs to 2 lbs of dried or wilted leaves per 1,000 lbs of a horse’s body weight to cause clinical signs.
  • All organ systems in a horse’s body are affected by the blood cells’ lack of oxygen. The kidneys and liver may be harmed by the red blood cell breakdown products.


These can occur as early as a few hours after ingestion or be delayed for four to five days. Depression, lethargy, and anorexia usually occur first and are followed by reddish-brown urine and pale yellowish gums and mucous membranes. Later signs include dark-brown muddy gums and mucous membranes, difficulty breathing, inability to rise, and death.


Use activated charcoal and mineral oil to decontaminate. Aggressive IV fluids to correct dehydration and protect the kidneys, blood transfusions, ascorbic acid (Vitamin C), nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroids may all be necessary.


Good if animals are treated before signs begin. Once evidence of red blood cell damage occurs, aggressive in-hospital treatment will be needed for survival.


Foxglove (Digitalis spp.), oleander (Nerium oleander), rhododendron (Rhododendron spp.)

Where they’re found

Each of these plants is found to some extent throughout the United States. Different cultivars of foxglove and rhododendron grow and overwinter in just about every state. Oleander is not hardy enough to overwinter in Northern climates but is often found as a houseplant or ornamental container-grown plant.

Foxglove, oleander, and rhododendron contain toxins known as cardenolides or cardiac glycosides. Cardenolides interfere with the electrical conductivity of the heart, resulting in irregularities in heart rate and rhythm.

  • Ingestion of any of these plants is associated with death in horses.
  • Cardenolide concentrations are found in all parts of the plant but are highest in the fruit, flowers, and immature leaves. Dried leaves retain their toxicity.
  • Oleander: ingestion of 30 to 40 oleander leaves is deadly.
  • Foxglove: estimated that ingestion of 100 – 120 grams (3-4 ounces) fresh leaves results in clinical signs and death.
  • Rhododendron: toxic dose in horses is not well established but ingestion of 1-2 pounds of green leaves has resulted in signs.

Signs generally begin just a few hours after ingestion, and most horses are simply found dead. Other early signs include weakness; edema of the head, neck, and eyes; and a slow heart rate that progresses to irregularity. Seizures and inability to rise often occur before death.

Rapid development of illness and signs generally make treatment impossible. Veterinarians can use activated charcoal and mineral oil to decontaminate if done so early after ingestion. Other drugs such as atropine and lidocaine that focus on specific cardiac conduction abnormalities may be useful in hospitalized cases. Digoxin-specific Fab fragments have been used successfully in small animals but are cost-prohibitive in horses.

Very poor once signs have developed. Early and aggressive therapy before signs improves the prognosis.


Bracken fern (Pteridium aquilinum)

Where is it found?

Bracken fern is found throughout the United States in open pastures and woodlands. It prefers moist, acidic soils.

Bracken fern contains a type I thiaminase enzyme. It works by degrading or destroying thiamine (Vitamin B1) and creating a thiamine analog (fake thiamine) that interferes with nerve function and other bodily processes.

  • Thiamine is necessary for nerve function. The primary problem with low thiamine in horses is the development of neurological disease.
  • Both fresh and dried bracken fern is toxic if ingested.
  • Some horses develop a taste for bracken fern and seek it out in the pasture and hay.
  • Horses must consume large amounts of bracken fern for days to weeks before signs develop. If the plant composes 20 percent to 25 percent of their diet, signs develop in about three weeks. If it composes 100 percent of their diet, signs occur in seven to 10 days.

Signs are related to neurological dysfunction and include depression, blindness, gait abnormalities, muscle twitching, and seizures.

Administer IV or IM thiamine for days to weeks. Other treatment is primarily supportive and includes nonsteroidal anti-inflammatory agents, IV fluids, and drugs to prevent seizures.

Generally very good if treatment is begun before neurological problems develop. The onset of seizures and blindness is associated with a poor prognosis.


Black walnut (Juglans nigra)

Where it is found

Black walnut trees have been cultivated in the United States since 1868. They are commonly found in the eastern half of the United States except the northernmost border.

The toxin is unknown. Many believe that juglone, present in black walnut roots and leaves, is the culprit, but scientists are unable to reproduce toxicosis by oral or dermal exposure to juglone.

Black walnut shavings are harmful if ingested; leaves, bark, flowers, and nuts are not.

  • Black walnut shavings, often purchased from furniture manufacturers, should not be used as bedding for horses. Examine new bedding that comes from an unknown source for the presence of black walnut shavings, which are much blacker in color than pine shavings.
  • Horses placed on bedding composed of as little as 20 percent fresh black walnut shavings made from either new or old wood develop laminitis (founder) within just a few hours.
  • Early removal of the horse from the bedding generally results in cessation of signs, but laminitis may continue unabated.
  • Early: Depression, limb edema, stiff gait, laminitis
  • Mid: Colic, increased body temperature
  • Late: Rotation of coffin bone (severe laminitis)

Removal of the horses from the shavings as soon as signs are noticed often stops the progression of laminitis. Wash the horse’s feet and limbs with cold water to remove any remaining shavings and help decrease signs of laminitis. Further treatment is based on the signs and generally includes an NSAID—such as flunixin or phenylbutazone—mineral oil, and good farrier care.

Generally very good if horse is removed within a few hours of exposure. Once laminitis develops, the prognosis for a full recovery decreases.


Tansy ragwort (Senecio spp.)

Where it is found?

More than 70 different species of Senecio are present in the United States (including Oregon). This daisy-like weed is found in hay fields, pastures, ditches, and other unimproved areas. Due to weather conditions and the lack of the tansy’s insect predators, tansy growth in Western Oregon has increased in the past few years. Tansy is also toxic to cattle and can cause abortion in both horses and cattle.

Senecio species plants contain pyrrolizidine alkaloids, which are metabolized to pyrroles in the liver. Pyrroles inhibit cellular division, resulting in production of abnormal liver cells (megalocytes). As the megalocytes die, they are replaced with fibrotic tissue. Not all Senecio species have the same amount of toxin but all contain at least some concentration of pyrrolizidine alkaloids and all are considered harmful.

  • Both fresh and dried plants are toxic if ingested.
  • The plant is not particularly palatable and most cases occur on overgrazed pastures or in the spring when green grass is scarce.
  • Pyrrolizidine alkaldoids are most harmful to a horse’s liver.
  • Damage to the liver accumulates over time and is irreversible.

Generally aren’t present until the liver has failed. Once the liver fails, anorexia, weight loss, photosensitization, depression, blindness, unusual behaviors, and jaundice swiftly follow. Signs generally develop after a total ingestion of 50 lbs to 150 lbs, or about 1 percent to 5 percent of a horse’s body weight, for several weeks.

There is no treatment once signs are present. If ingestion is suspected and complete liver failure has not developed, supportive care is recommended, but the horse may never return to its previous healthy state. IV fluids, electrolytes, glucose, and B vitamins are useful, as is protecting the horse from the sun.

Very poor to death once liver failure has occurred. Poor for cases of suspected ingestion that are caught earlier.


Information on how to control problem weeds like tansy can be found in the Pacific Northwest Weed Management Handbook.


Yew (Taxus spp.)

Where it’s found

Western yew is common along the West Coast, and eastern yew is common along the East Coast and parts of the central United States. Japanese yew is found throughout much of the United States.

Yew contains the toxic alkaloid taxine, which primarily affects the heart and respiratory system.

  • All parts of the plant except the fleshy portion of the red berries are poisonous.
  • The leaves are toxic even when dried.
  • Horses are often poisoned from ingesting discarded yew cuttings found in their pasture or from eating yew-made barn decorations such as wreaths and swags.
  • Ingestion of even a small amount results in signs. Depending on the individual horse, it takes anywhere from a few mouthfuls to 1 lb of leaves per 1,000 lbs of a horse’s body weight to cause death.

Sudden death is the most common sign. Other signs occur within an hour after ingestion. These signs include a slowed heart rate, difficulty breathing, trembling, lack of coordination, impaired movement, and inability to rise. Death generally follows 15 to 30 minutes after the onset of signs.

Rapid development of illness and signs generally make treatment impossible. Veterinarians can use activated charcoal to decontaminate if done so early after ingestion. Other drugs such as atropine and lidocaine that focus on specific cardiac conduction abnormalities may be useful in horses that didn’t ingest a lethal dose.

Very poor to fatal. Sudden death is the normal outcome.

Water hemlock (Cicuta spp.)

Water hemlock is often referred to as the most violently toxic plant in the United States. While found throughout the nation, the plant is less common in the Gulf Coast states. As its name suggests, water hemlock prefers wet areas, such as irrigation ditches, marshes, damp areas in pastures, and riverbanks.

Water hemlock contains the toxins cicutoxin and cicutol, which affect the neurons in the brain and central nervous system.

  • The plant’s roots contain the highest concentration of toxins, however horses rarely ingest these. It’s more likely that horses would eat the leaves and stems, which are also poisonous, when grazing in low-lying, moist areas.
  • The amount of toxins in the leaves and stems decreases as the plant ages and further decreases as the plant dries.
  • It takes about 0.2 to 2 lbs hemlock root per 1,000 lbs of a horse’s body weight to cause death.

These usually occur within an hour after ingestion. Because the toxins target the central nervous system, signs generally include agitation, nervousness, twitching, and seizures. Other signs include excessive salivation, dilated pupils, skeletal muscle weakness, cardiac abnormalities, difficult breathing, and death from respiratory paralysis.

Veterinarians can use activated charcoal to decontaminate if the horse isn’t experiencing seizures and it can safely be accomplished. In the case of seizures, veterinarians can use pentobarbital to slow the activity of the horse’s brain and nervous system.

Very poor to fatal once signs have developed. Most horses are found dead within two to three hours after ingestion. Horses alive after eight hours of developing clinical signs are likely to recover, but usually have some residual cardiac and skeletal muscle damage.

Poison hemlock (Conium maculatum)

Poison hemlock is found throughout the United States except for northern Minnesota, North and South Dakota, and eastern Montana. The plant grows in ditches, uncultivated areas, and near water such as swamps and lowlands. Poison hemlock looks similar to water hemlock. The only way to truly tell the difference between the two is by their roots.

Poison hemlock contains a number of alkaloid compounds including coniine, N-methyl coniine, and gamma-coniine. (Alkaloids are neurotoxins that affect both the central and peripheral nervous systems.)

  • The poisonous parts of the plant are the leaves, stems, and seeds. These parts increase in toxicity as the plant matures, especially with seeds, and decrease as the plant dies.
  • Most horses will avoid eating this plant; however, it can contaminate hay.
  • It takes about 2 to 8 lbs of plant material per 1,000 lbs of a horse’s body weight to cause death.

Signs generally occur within one to two hours after ingestion. Nervousness, tremors, twitching, lack of coordination, inability to rise, depression, and decreased heart and respiratory rates are common signs. Seizures occur in rare cases.

There’s no specific treatment once signs appear, but supportive care is recommended. Veterinarians can use activated charcoal to decontaminate if the horse isn’t presenting with neurological signs and it can safely be accomplished.

Very poor to fatal. Many horses die from respiratory failure. However, those that survive the acute ingestion usually recover with no residual effects.


Small flower buttercup (Ranunculus abortivus) and tall buttercup (Ranunculus acris)

Where they’re found

The plants are found throughout most of the United States; however, they’re rarely found in Western Minnesota, North and South Dakota, and Eastern Montana. Most are weeds found in overgrazed pastures, meadows, and fields. A few varieties are grown as ornamental plants.

The plants contain the chemical ranunculin, which, when crushed or chewed, becomes the toxin protoanemonin. Protoanemonin is a bitter-tasting oil that irritates the mucous membranes of the gastrointestinal tract.

  • The plants generally don’t pose a serious threat because the toxin’s bitter taste and ability to cause mouth blisters limits ingestion. However, poisoning can occur in overgrazed pastures where there are little to no other plants for horses to consume.
  • The plants are most harmful when eaten fresh in the pasture or field.
  • The concentration of toxin varies depending on the species and growth stage of the plant. The flower part contains the highest amount of toxin.
  • Dried plant material and contaminated hay aren’t normally toxic because the protoanemonin converts to a nontoxic, nonirritating anemonin.

These can occur as early as a few hours after ingestion or be delayed up to a day or two, depending on the amount ingested. Nose, lips, face, and skin may blister or swell after direct contact with plant. Blisters in the mouth, oropharynx, and esophagus also are common. Other signs include excessive salivation, an irritated gastrointestinal tract, colic, and bloody diarrhea. Tremors, seizures, and paralysis occur in rare cases.

Removal of the horse from the pasture or field as soon as signs are noticed. Veterinarians may use activated charcoal to decontaminate; however, it should be used cautiously due to gastrointestinal tract irritation from the plants’ toxins. Supportive care including fluid therapy, gastrointestinal protectants, and analgesics are recommended.

Excellent with appropriate care. No residual effects have been noted.

Yellow star thistle and Russian knapweed (Centauria spp.)

Centauria spp. is primarily found west of the Mississippi River between the Mexican and Canadian borders. It’s present as a weed in ditches, pastures, and fields.

The toxin is unknown but suspected to be repin, a sesquiterpene lactone. It affects the neurons in the brain and central nervous system that control chewing. It commonly results in nigropallidal encephalomalacia or the “chewing disease.”

  • Centauria spp. becomes dangerous when ingested in large amounts for a long period of time. Chronic ingestion of 60 percent to 200 percent of body weight for 30 to 60 days results in the onset of signs.
  • Centauria spp. also becomes a problem when it contaminates dry hay or overtakes a grazing pasture.
  • Only horses are affected by Centauria spp.; even mules and burros do not seem to develop signs when exposed to large amounts.

Signs don’t occur until the horse has ingested the plant for one to two months. At that time, the horse may experience drowsiness and frequent yawning. Horses will lose the ability to adequately chew food or drink fluids and will become anorexic and dehydrated. Other signs include involuntary facial tics, such as the twitching of the lips and tongue, and clenched facial muscles.

There is no treatment once signs are present. The damage to the neurons is irreversible.

Very poor to fatal due to the permanent damage to neurons. Once a horse loses the ability to swallow, it will not return despite aggressive therapy.


Ground Ivy (Glechoma hedera)


Also known as Creeping Charlie or Creeping Jenny, ground ivy may become weedy or invasive in some regions or habitats.

Horses may experience toxicosis if they consume alfalfa hay containing 30% or more ground ivy or if they feed exclusively on ground ivy. No other forms of livestock are known to be affected. Livestock in general tend to avoid ground ivy because of its bitter taste.

Symptoms of toxicity include sweating, salivation, labored breathing, pupil dilation and occasionally signs of pulmonary edema.

Updated: 2017-05-01 07:00:00

Author: Lynn Hovda, RPH, DVM, MS, DACVIM. Courtesy of Pet Poison Helpline, (855) 764-7661, plus additional information from USDA.

As praiseworthy as trees are, there are a few situations where horses and trees don’t mix. In some cases, fruit- or nut-bearing trees contribute to colics when horses gorge on their produce. In others, falling branches or uprooted trees injure nearby horses. But the gravest dangers arise with the few tree species that are toxic enough to sicken or kill horses.

Of the non-ornamental native trees, the most deserving of the skull-and-crossbones warning are those that produce cyanide in their wilted leaves. Cyanide suffocates animals by blocking oxygen transport via the red blood cells. The red maple (Acer rubrum) is one such tree whose leaves are harmless most of the year until wind damage or seasonal change causes them to fall from the tree and wilt. Red maple leaves have serrated edges and can turn either red or yellow in ghe fall. “There are other trees that shed red leaves in the fall, but the red maple has some distinctive features,” says Anthony Knight, BVSc, MRCVS, who specializes in toxic trees and plants at Colorado State University. “The underside of the red maple leaf tends to be silvery in color.” Signs of poisoning, including lethargy, discolored urine and darkened gums, may not appear for four days.

Equally toxic are cherry (black cherry, chokecherry, and fire cherry) peach and plum trees, all members of the Prunus species. These leaves also produce cyanide when wilted, affecting horses within a few hours of ingestion.

To be safe, remove these deadly trees or relocate horses away from pastures or paddocks bordered by or containing them. In general, horses are not likely to eat leaves or any other tree parts unless they are quite hungry. However, when curiosity or boredom spurs exploratory bites, the horse may ingest enough of the deadlier species to do harm.

The following trees have no place in horsekeeping areas because of their toxicity or potential for causing digestive distress. They are listed in order of the risk they pose to horses, starting with the most hazardous:

Yew (taxus sp.)
Oleander (nerium oleander)
Red Maple (Acer rubrum)
Cherry trees and relatives (prunus sp.)
Red Maple (Acer rubrum)
Cherry trees and relatives (prunus sp.)
Black Walnut (juglans nigra)
Black Locust (robinia pseudoacacia)
Horse Chestnut, Buckeyes (aesculus hippocastanum)
Oak trees, acorns(quercus sp.)
Russian olive, also known as oleaster (elaegnus angustifolia)

For more information on toxic trees, including detailed descriptions and photographs, visit the Colorado State University website

This article first appeared in the March 2001 issue of EQUUS magazine

The Prettiest, Most Deadly Time of Year

With all the ghouls and ghosts ready to play their tricks this Halloween, it’s easy to forget there’s a much more sinister threat lurking high in the shadows. For your horse, it’s the small, poisonous bombs drifting slowly from the trees that we all need to pay attention to. Falling leaves can pose a potentially deadly threat to our horses. To help us all stay safe, here’s a list of trees that are highly toxic to horses.

Red Maple (Acer rubrum)

For these trees, poisoning usually occurs in late summer and autumn, when their leaves fall and drift onto pasture areas. Red maple leaves are HIGHLY TOXIC to horses. While ingestion of 1.5 lbs is toxic, digesting 3 lbs or more is fatal. Essentially what happens is that the chemicals in Red Maple leaves cause massive destruction of a horse’s red blood cells. Symptoms of poisoning include breathing difficulty, jaundice, urine that is dark brown in appearance, increased heart and respiratory rates, and lethargy. Between 50% and 75% of affected horses will die or have to be euthanized. The best thing to do is check areas where your horse frequently roams. Do not put leaves in their hay and make sure there are none within reach of their pasture area. If your horse does accidentally swallow Red Maple leaves or if they’re showing symptoms of poisoning, call your vet immediately.

Oak (Quercus species)

Oak trees in or near horse pastures don’t need to be cut down, but their branches should be kept out of the reach of any horses. The best thing to do is fence out areas where wilted oak leaves and acorns are plentiful, especially if forage is scarce. Oak leaves and acorns are poisonous to horses in large amounts due to their toxin tannic acid. This chemical can cause kidney damage and gastroenteritis. The Symptoms of Oak poisoning include lack of appetite, depression, constipation, diarrhea (which may contain blood), blood in urine, and colic.

Cherry and Plum Trees (Prunus species)

Cherry trees, plum trees and their relatives actually produce cyanide-containing compounds. That’s literally the same deadly poison used in any great Cold War spy movie. This toxin is found in the leaves, the fruit itself, and their pits. The plants are most toxic when drought or frost stresses them. Symptoms to be aware of include anxiety, weakness, heavy breathing, flared nostrils, and convulsion which all can eventually lead to death.

Just 1/4th of a pound of these leaves is so fatal, it can kill a 1,000 lb horse. Once the plant material is chewed and exposed to the acid within the horse’s stomach, hydrogen cyanide is released and rapidly absorbed into the horse’s bloodstream. Essentially, this chemical prevents living cells from being able to receive oxygen. As a result, an affected horse’s blood is bright cherry red because it’s overloaded with oxygen that cannot be used.

Black Walnut (Juglans nigra)

Your horse can be exposed to Black Walnuts from trees that grow around their pasture land or, more commonly, from shavings used in their stall bedding. Shavings with as little as 20% black walnut content are toxic to your horse. The effects of Black Walnut poisoning happen incredibly quickly, usually within 24 hours after your horse has been exposed. Fortunately, Black Walnut toxicity is usually nonlethal if you can get your horse the proper treatment. Signs to look for with Black Walnut toxicity include laminitis (which will worsen with continued exposure), reluctance to move, increased temperature and heart rate, difficulty breathing, digital pulse, limb edema, and increased gut sounds. If you suspect your horse has been poisoned by Black Walnuts, remove their stall shavings immediately. Cooling their legs and hooves with a hose can help make your horse more comfortable. If caught relatively quickly, your horse shouldn’t suffer any permanent effects. In cases of severe laminitis and edema, consult your veterinarian.

Raked Leaves

That’s right, any pile of leaves can cause injury if ingested by your horse. Horses love the taste and smell of recently fallen leaves. The problem is that leaves are dense and can compact in your horse’s digestive system and cause compaction colic. Make sure no one is disposing their raked leaves into your pasture.

Horses are much more likely to munch down on wilting leaves if they’re not getting enough of the proper nourishment. Be sure to feed your horses plenty of premium quality forage and make sure they’re drinking adequate amounts of water. To help ensure you’re giving your horse the proper amount of forage, we’ve introduced a new and improved Feed Calculator to Standlee’s new website. Try it out for yourself by clicking the button below.


The Buttercup causes oral irritation when chewed, and horses rarely consume the plant because it is unpalatable.

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

There are numerous poisonous plants in horse pastures all over the United States. The good news is that most are unpalatable and horses tend to selectively graze around them and never consume them. However, in pastures that are overgrazed, horses might start to nibble at some of these toxic plants. Also, if these plants are in hay fields and get mixed into hay bales, it becomes more difficult for horses to eat around them.

The adverse effects to the horse range widely with the amount of ingestion and often the environmental conditions the plant was grown under (drought typically increases the poison concentration in some plants). Other factors that can affect toxin level include stage of growth, season and fertilization, part of the plant eaten, as well as boredom, age and general health of the horse.

While the effect on the horse will depend on the amount ingested and the amount of toxin accumulated in the plant, most toxins primarily attack the major systems in the horse, such as the respiratory, cardiac and digestive systems.

Weeds can be classified as annuals or perennials. Annual plants are plants with a life cycle that lasts only one year. They grow from seed, bloom, produce seeds and die in one growing season. Perennials are plants that persist for many growing seasons. Generally, the top portion of the plant dies back each winter and regrows the following spring from the same root system.

A Few Common Toxic Plants
Found In or Near Horse Pastures

Buttercups: The Buttercup causes oral irritation when chewed, and horses rarely consume the plant because it is unpalatable. The toxic component is in the fresh leaves and flowers, but they lose toxicity when dried for hay. Clinical signs of buttercup poisoning include increased salivation, decreased appetite, colic and diarrhea.

Jimsonweed: Jimsonweed can be recognized by its distinctive tree-like shape, white or purple trumpet-like flowers and prickly seed capsules. All parts of the jimsonweed plant are poisonous to horses and humans. Clinical signs of poisoning in horses include a weak, rapid pulse, dilated pupils, dry mouth, incoordination, diarrhea, convulsions, coma and sometimes death. Jimsonweed has a foul odor and taste, and horses rarely consume it if they have other quality forage.

Jimsonweed can be recognized by its distinctive tree-like shape, white or purple trumpet-like flowers and prickly seed capsules.

Bruce Ackley, The Ohio State University, Bugwood.org

Nightshade Family: The nightshade family contains many toxic plants, including horse nettle, black nightshade, bittersweet nightshade, some species of groundcherry, and even tomatoes and potatoes. The plant affects the central nervous system and the gastrointestinal tract. It is estimated that one to 10 pounds of ingested plant material is fatal for horses. Some clinical signs of poisoning include dilation of pupils, diarrhea, loss of appetite and loss of muscular coordination.

Black Walnut: The bark, woods, nuts and roots of the black walnut tree contain a toxic compound. Horses are primarily exposed through black walnut shavings mixed in with other shavings as bedding. Clinical signs of exposure include depression, lethargy, laminitis, swelling of the lower limbs, and increased temperature, pulse, respiration rate, abdominal sounds, digital pulse and hoof temperature. Symptoms usually disappear within hours after the horse is removed from the shavings; however, laminitis can present further problems. Since the bark and nut hulls from the black walnut are toxic, these trees should be removed from horse pastures as a precaution.

Maple Trees: Maple leaves are highly toxic, particularly when they are in a stressed state prior to dying (e.g. leaves on a fallen tree limb lying in a pasture or during the fall). Fallen and dead leaves remain toxic for about a month and cause severe kidney damage if ingested in large quantities. It is estimated that an adult horse needs to consume 1.5 pounds of leaves or more to become poisoned. Clinical signs of toxicity include depression, lethargy, increased rate and depth of breathing, increased heart rate, jaundice, dark brown urine, coma and death.

Maple leaves are highly toxic, particularly when they are in a stressed state prior to dying (e.g. leaves on a fallen tree limb lying in a pasture or during the fall).

David Stephens, Bugwood.org

To get a complete list of toxic plants in your area contact your local extension office.

This article was written by Dr. Tania Cubitt. For additional information on Standlee Premium Western Forage, visit www.standleeforage.com.

10 Plants and Chemicals That Are Toxic to Horses

7. Decaying organic matter

Rotting hay, haylage, or other organic matter has the potential to harbor botulism-causing toxins produced by Clostridium botulinum.

Danger to horses Horses are highly susceptible to C. botulinum toxins, which attack the nerves that communicate with muscles, leading to general weakness that progresses to paralysis. Clinical signs of botulism might include inability to eat, drooling, nasal discharge, muscle tremors, difficulty getting up, difficulty breathing, and death.

Risk of exposure Toxin ingestion is one of the most common routes of exposure in horses, notes Gaskill, though it can also be acquired through exposed wounds. Some parts of the country, such as Kentucky, have high levels of the toxin-producing bacterial spores in the soil. “Rotting, decomposing hay or improperly put up haylage or silage are common sources of contaminated material,” says Gaskill. Animal carcasses can also harbor the toxin.

8. Fumonisin (moldy corn)

This mycotoxin (fungal toxin) can infect corn prior to harvest or during storage. Hot, dry conditions followed by high humidity are associated with increased fumonisin concentrations in growing corn, usually in the Midwest and South.

Danger to horses Horses that eat corn containing toxic fumonisin levels develop moldy corn poisoning, or equine leukoencephalomalacia (ELEM), a rapidly progressing, often fatal neurologic disease. “It basically melts the white matter of the horse’s brain,” explains Bischoff. Clinical signs might include those typical of neurologic disease, such as head-pressing, circling, muscle tremors, weakness, or strange, sometimes violent behavior. For those that survive, the prognosis is not good, says Bischoff. Veterinarians typically recommend euthanasia.

Risk of exposure Most commercial feed mills test for fumonisin contamination. But feeding untested corn, such as that which has come directly from the field, can put horses at risk, particularly in regions where fumonisin is more prevalent.

9. Red maple

Researchers have known that wilted red maple leaves can be toxic to horses, but they now suggest that other species, such as sugar and silver maple, might be problematic as well.

Danger to horses While research on the mechanism behind red maple poisoning is ongoing, scientists believe the toxic agent is linked to levels of gallic acid, which increase in leaves throughout the summer. The leaves in combination with certain bacteria produce a strong oxidant that damages horses’ red blood cells, hindering their ability to carry oxygen or destroying them completely. “A lot of horses die from it, but some survive with intensive care,” says Bischoff. Clinical signs include loss of appetite, red urine, increased drinking and urination, and a generally depressed state. To be affected, an average-sized horse would need to consume an estimated 1.5-3.3 pounds of wilted leaves.

Risk of exposure “Red maple poisoning hellip; happens mostly in the late summer and early fall,” says Bischoff. Fallen branches in a paddock following a storm are the most common source of exposure, with wilted leaves remaining toxic for as long as 30 days. Bored or curious, a horse might strip the leaves off and eat them. Scientists believe the bark is also toxic.

10. Tansy ragwort

A nondistinct yellow flowering plant, tansy ragwort grows throughout most of North America.

Danger to horses “If a horse eats enough of the plant over a short period of time, or smaller amounts over a longer period of time, they can develop an irreversible chronic liver disease, though they may not show signs for six months to a year,” says Bischoff. Signs can be neurologic and include head-pressing, circling, and bizarre behavior. Loss of appetite and weight loss over time are also common.

Risk of exposure Horses might consume the plant if it gets baled into hay, but because disease onset is slow and clinical signs often do not appear until well after the initial exposure, poisoning can be difficult to trace, says Bischoff.

Take-Home Message

There are a number of substances that are toxic to horses, with adverse effects ranging from mild to fatal, depending on what is consumed, how much, the horse’s size and health, and other case circumstances. Because horses don’t always know what’s harmful for them, owners and caretakers must be keenly aware of these threats when managing their horses.

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