Updated October 13, 2006

The 10 Most Dangerous Plants for Horses

Of the hundreds of toxic plants in North America, only a handful are likely to bring serious harm to horses.

Take a stroll through any pasture, and there among the grasses you’ll find any number of different plants. Vines, broadleaf weeds and some wildflowers you will recognize and some you won’t!

The chances are good that at least some are toxic to horses. Hundreds of toxic plants grow in North America , and many are extremely common.

The good news is that the vast majority of those plants pose little threat to horses.

First, most of them are unpalatable, and horses filling up on quality forage are unlikely to spend time grazing on a few bitter leaf plants in their pasture. A horse’s size also is a good protection. A thousand- pound animal has to consume a significantly high quantity of most toxins to experience ill effects. For the most part, as long as your horses are healthy and your pasture is in good condition, you have little to worry about.

However, some plants are cause for concern, either because even a curious nibble may spell doom or because repeated browsing over weeks or months can lead to serious illness and/or death. These plants are worth getting to know by sight, not only so you can eliminate them from your pasture, but also so you can recognize them and avoid encounters with them in the woods, on roadsides and along waterways where you ride.

Anthony Knight, BVSc, MRCVS, plant toxicologist from Colorado State University, depending on the area of the country, the following might make the

“Top 10” list of the plants that are most dangerous to horses in the United States .

Hemlock (Conium maculatum) Also known as poison hemlock, spotted hemlock.

• ID: A multi-stemmed perennial weed with toothed, fernlike leaves and cluster of small white flowers. The stems have purple spots, which are most evident at the base of the plant.
• Range: Grows wild along roadsides and other uncultivated open areas.
• Danger: Every part of the plant contains several potent neurotoxins that affect both the central and peripheral nervous system. Four to five pounds of the green plant are a lethal dose for a horse. The mature flowering plant is less toxic than the newly emerging leaves in the spring. Most animals will avoid the plant.
• Signs: Within a few hours of consumption signs appear starting with nervousness, tremors and uncoordination which then progresses to depression and diminished heart and respiratory rates and possibly colic. Death results from respiratory failure.
• What to do: There is no treatment but animal may recover if smaller doses were consumed and supportive care is give for the symptoms. Remove all the plants before they seed to prevent spreading

Senecio, tansy ragwort (Senecio, spp.) Also known as groundsel

• ID: A multi-stemmed annual or perennial with alternating leaves that produce clusters of small daisy like flowers
• Range: About 70 species grow throughout the contiguous United States , in a variety of habitats, many are common along roadsides and pastures.
• Danger: Toxicity varies among different species, but all are thought to contain some concentration of pyrrolizidine alkaloids, which inhibit cell division, especially in the liver. This damage to the liver is cumulative and irreversible. Liver disease is usually found after the animal consumes 50-150 pounds of the plants. Scencio remains toxic after it is dried and may be cut into hay.
• Signs: Often no evidence of a problem until liver failure begins to appear: photosensitization, diminished appetite and weight loss, progressing depression, uncoordination and jaundice.
• What to do: There is no treatment for advanced stages of liver disease for this toxin. It is important to prevent senecio plants from proliferating in pastures or invading hay fields.

Johnsongrass/Sudan grass (sorghum spp.)

• ID: Both are coarse-stemmed grasses with broad, veined leaves that can grow at least six feet tall. They produce large, multi-branched seed heads.
• Range: Johnsongrass is a wild grass introduced to the southern climates, where it grows along roadways and other uncultivated open areas. Certain safe varieties of Sudan grass have been cultivated as a forage crop.
• Danger: Significant levels of cyanogenic glycosides that cause cyanide poising in cattle, are not as likely to affect a horse because his digestive system protects him from plant-induced cyanide poisoning. These grasses also contain lathrogens, which can cause nerve degeneration of the hind legs and urinary bladder in horses. The nerve damage is permanent, and the horses can develop a fatal kidney and bladder infection because of the paralysis of bladder muscles.
• Signs: Weakness in the hindquarters and urinary incontinence are initial indications of this type of poisoning; which may eventually lead to kidney and bladder infections.
• What to do: There is no treatment for nerve damage that occurs. Avoid feeding hay contaminated with these wild grasses. Purchase hay only from reputable dealers who use cyanide free cultivated varieties of Sudan grass.

Locoweed (Astragalus spp. Or oxytropis spp.) Also known as crazyweed.

• ID: Leafy perennials with short stems and compound leaves that grow in tuft like forms from a single tap root. Some species may be covered with silvery hairs. The flowers, often white or purple, are borne on leafless stalks.
• Range: Different species of the plant grow in varied terrains throughout the west and southwest, often in dry sandy soils.
• Danger: Most species of this plant contain the toxin swainsonine, an alkaloid that inhibits the enzymes necessary for complex sugar metabolism in cells; the resulting sugar buildup disrupts the function of cells in many organs, including the brain, digestive and reproductive systems.
• Signs: Strange behavior and weight loss are often the first clinical signs noted; horses may bob their heads, adopt exaggerated, high stepping gaits or stagger and fall. An affected horse may show loss of normal reflexes.
• What to do: There are no treatments for advanced stages and its effects are irreversible. Horses with less severe poisoning may recover when access to the weed is removed. This plant is quite palatable to horses, especially when flowering. It is important to remove the plants from the pasture or remove the horses from the pasture when the plants are flowering if the pasture contains plenty of normal grass.

Oleander (Nerium oleander)

• ID: An evergreen shrub that can reach the size of a small tree, oleander has elongated thick leaves that can grow to three to ten inches long. The flowers, which grow in large clusters at the end of the branches, are one – 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 in a pot in the northern areas.
• Danger: All parts of the plant contain the toxin oleandrin and neriin, which disrupts the beating of the heart. The leaves remain toxic when dried. About 30-40 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, sweating, recumbency and an irregular heart rate. The pulse may either be slowed or accelerated.
• What to do: Call your veterinarian immediately if you know your horses have consumed oleander leaves. Horses can survive if treated early with supportive care, such as administration of activated charcoal to inhibit further toxin absorption and anti-arrhythmic drugs to stabilize the heart. Do not plant within reach of animals.

Red Maple Trees (Acer rubrum)

• ID: A medium sized tree with leaves that are green in spring and summer, with shallow notches, bright red stems and a whitish/silvery underside; in the 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.
• Danger: Ingestion of the fresh, growing leaves seems to do little or no harm, but when the leaves wilt or are dried they become extremely toxic to horses. Access to the wilted leaves is most common after storms when branches or leaves are blown down by the wind into grazing areas. The toxins in the wilted red maple leaves, not all have been identified, cause the red blood cells to break down so 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 the leaves can be fatal.
• What to do: There is no specific antidote for red maple poisoning, and the treatment consists of administration of large amounts of intravenous fluids, Vitamin C and possibly blood transfusions. Recovery depends on the amount of leaves were consumed and how promptly the animal receives veterinarian care.
• Note: Research indicates that the silver and sugar maple varieties may contain the same toxic substances but in lesser amounts. Do not plant red maples or hybrids in and around horse pastures.

Water Hemlock (Cicuta spp.)

• ID: A perennial indigenous plant with erect, hairless stems that can grow to four feet from clusters of fleshy tuberous roots. The stems are hollow and branching and thicker at the base. Leaves are elongated and toothed, and the small white flowers form flat, umbrella-shaped clusters at the end of the branches.
• Range: Water hemlock grows across the contiguous United States and is most likely to be found in marshy areas of meadows and along streams and irrigation ditches.
• Danger: Water hemlock is considered of the most toxic plants in the United States . All parts of the plant contain cicutoxin, an alkaloid that affects the central nervous system. The toxin is most concentrated in the root. Cattle are more likely to pull up and consume the root and are considered most at risk of poisoning, but horses have also been known to browse the plant. Less than a pound of the leaves and stem can be fatal, and an ounce or two of the root would be fatal to an adult horse. The amount of the toxin in the leaves and stem diminishes as the plant ages during the growing season, and additional amounts of the toxin are lost when the plant is dried, but the water hemlock is never considered safe for consumption. Most animals will ignore the plant unless there is little else to graze.
• Signs: The toxins affect neurons primarily within the brain, causing various signs, including excessive salivation, dilated pupils and nervousness, progressing rapidly to difficulty 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, including anesthetizing the horse, must be initiated before the convulsions begin. Horses who survive are likely to have permanent damage to heart and skeletal muscles. Water hemlock plants always need to be removed in their entirety from horse pastures.

White snakeroot (Eupatorium rugosum)

• ID: An erect perennial growing to four feet high, with six to eight opposite leaves with serrated edges. Clusters of small white flowers appear at the end of the branches.
• Range: A common plant of wooded areas or shaded pastures from eastern Canada , south to Georgia and west to Texas and Minnesota .
• Danger: White snakeroot contains the toxin tremetol, which affects the muscles, especially the heart. A fat-soluble alcohol, tremetol accumulates in the body over time and will cause poisoning 1 to 10 percent of the animal’s body weight has been consumed. Tremetol is passed through the milk of lactating animals, and a suckling foal may develop poisoning from the affected mare. The same toxin is found in jimmyweed (Isocoma wrightii), also called rayless goldenrod, which is native to the southwest.
• Signs: Principle signs are muscle tremors, in coordination, difficulty breathing, profuse sweating, difficulty chewing and swallowing food, and collapse; if access to the plant is not removed, the toxin can cause death. Horse that recover may have permanent muscle weakness and poor heart function.
• What to do: There is no specific treatment for white snakeroot poisoning, other than supportive therapy, such as the administration of activated charcoal orally early in the course of poisoning.

Yellow star thistle (Centauria solstitialis) and Russian knapweed (Acroptilon repens)

• 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 one-half to three-quarters of an inch long. Russian knapweed, is a perennial, spreads via creeping root system; its erect, stiff stems grow two to three feet high and are covered with gray hairs, and its thistle like 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 to Canada . They appear as weeds along roadsides, in cultivated fields and pastures. They are listed as noxious weeds by most states.
• Danger: Both plants contain a toxic agent that has a neurological effect on the brain that inhibits the nerves that control the animal’s ability to bite off and chew its food. The poisoning is chronic in nature; to receive a toxic dose, horses must consume 50-200 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. Horses will eventually die from starvation.
• What to do: There is no treatment, and any neural damage is permanent. Euthanasia is recommended once the disease is confirmed and the horse is to 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.
• 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 when dried. As little as eight ounces of yew leaves can be deadly to a horse in minutes.
• Signs: Sudden death is the most typical sign of yew ingestion. Animals found alive may be trembling (or recumbent) and colicky, with difficulty breathing and a slowed heart rate.
• What to do: There is no specific treatment for yew poisoning. Avoidance is critical; most yew poisonings occur when trimmings of the plant are thrown into the pasture after a pruning. Never plant yew in or around animal enclosures.

An ounce of prevention

Eliminating a horse’s exposure to poisonous plants is all but impossible, but there are ways to minimize the risk of serious harm:

• Provide adequate forage – Horses with an adequate supply of quality hay or pasture are much less likely to turn to unpalatable plants.
• Monitor hay quality- Some poisonings occur when toxic plants, such as milkweed, invade a crop and are cut along with the hay. Check your hay regularly as you feed it for the presence of unusual-looking stem, leaves or flowers and remove them
• Alleviate boredom- In some cases, well fed but idle horses may also be more inclined to nibble at unusual plants. Keeping them supplied with free-choice hay can help occupy their mouths, and companionship, toys and regular exercise are also good strategies for keeping horse happy.
• Remove seriously deadly plants- If the threat they pose are minimal, like oaks and horse chestnut trees, can be left where they are especially if your horses tend to leave them alone. Any plants listed here should be removed from fence lines or anywhere else your grazing animals might reach them.
• Inspect new places without delay- Roadsides and trails where people like to ride have many dangerous weeds. Learn to recognize dangerous weeds in your region and avoid letting your horse nibble on unfamiliar greenery when away from home.

Remember

Poisonings among horses are fortunately fairly uncommon. Most horses have a finicky nature when it comes to food, so they tend to avoid plants with questionable odors or flavors, and most of the time, a single, unharmful bite of bitter leaves is enough to send a foraging horse back to sweeter fare. But recognizing the few exceptions can make the difference between life and death.

Information for this article came from the magazine EQUUS and from the book “Horse Owners Guide to Toxic Plants” by Sandra M. Burger and Anthony P. Knight, BVSc, MS, MRCVS