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Poisonous To Horses

The table below gives a guide to the more common poisonous plants, how they act, their source and the likely incidence of poisoning.  This is not an exhaustive list but does include most of the plants and trees that can cause a problem if ingested.  Most horses will not eat plants that are toxic but if hungry or bored they may nibble on these plants and that can be enough to make them sick. Because of the variations of plants across the world you may have to search for more pictures in order to determine if you have any of these plants near your horse.  Please consult your veterinarian if you suspect your horse has come in contact with these or any other toxic vegetation.






Reluctance to move, imbalanced, neural dysfunctions resulting from Vitamin B1 deficiency and can include depression, uncoordinated and blindness.

Coast to coast, except for the Mediterranean and desert climates of Southern California and the Southwest. Found in grazing, especially in farm fields or marshland. This accumulative effect early symptom includes weight loss which can progress to unsteady walking, then staggering with the horse spreading legs to stay balanced, often pressing his head into solid objects. If untreated, death will occur from several days to several weeks after the symptoms appear.

Black Walnut

Edema in legs (stocking up), moderate depression, anorexia, laminitis, increased heart and respiratory rate, increased body temperature, mild colic.

Native to Northeastern U.S. and Canada. Toxicity usually due to exposure to shavings in bedding; as little as 5% black walnut in bedding can induce a toxic reaction. Ingested nuts and nut hulls have also been implicated in digestive disturbances in horses.


Excess salivation, colic, diarrhea, vomiting, wobbly gate, depression, anorexia, and hyper-salivation.

Infested grazing (not poisonous when dried). This is a common plant you can see everywhere next to the roads and on the fields. In the plant is a poisonous substance that has a sharp taste. Two types that cause poisoning are called "Sharp" and "Crawling" Buttercup.


Diarrhea, cardiac arrhythmias, vomiting, weakness, cardiac failure, rapid death.

Hedgerows and primarily in the South Western U.S.A. This plant has also been cultivated in the U.S. and Canada in flower gardens and in herb gardens.

Mare’s or Horse Tail

Reluctance to move, digestive upset, weakness, stumbling.

In grazing or hay made from contaminated grazing. Symptoms are similar fashion as bracken fern. In addition animal may become quiet, unresponsive or comatose prior to death. If the plant is dried into hay the toxin may have a greater effect than in the fresh plant. The toxin in this plant destroys the Vitamin B in the horse's blood.


Green Oak Tree

Hard feces followed by diarrhea, lack of appetite, emaciated appearance, poor or rough hair coat, edema fluid buildup under the skin under the neck, abdomen or on the legs), digestive disturbances.

Oak trees in grazing or overhanging pasture. It is relatively easy for a horse to ingest several pounds of acorns in a relatively short period of time leading to an unfortunate overdose.


There is no evidence of consumption until signs of liver failure begin to appear, diminished appetite and weight loss, photo-sensitization, progressing to depression, uncoordinated and jaundice.

Grazing ragwort infested land, or hay that is made from contaminated grazing. Common ragwort presents the greatest risk for horses because of its widespread distribution in poorly maintained pasture and in grassland used for hay production. Also similar to Stinking Willie, however their alkaloid content seems to be less than senecio jacobaea. About 70 species of senecio grow throughout the contiguous the United States, in many different habitats. Many are common in pastures and along roadsides.

St. John’s Wort

Photo-sensitization, ulcerative and exudative dermatitis.

More than 25 different species known to exist in North America. Found on roadsides, in overgrazed or eroded pastures, abandoned fields, open woods or sandy/infertile soils.

White Hemlock

Salivation, muscle tremors, paralysis, dilated pupils and unaware of surrounding, uncoordinated, progressing to depression, diminished heart and respiratory rates, possibly colic.


Can be found as White Hemlock hedgerows, in pond areas, and grows wild along roadsides or other open uncultivated areas throughout North America.

Yew Tree

Trembling, slow heart rate, cardiac failure, rapid death.

The yew is a small evergreen tree found in the Pacific Northwest. All parts of the yew, along with many of its cousins, are extremely toxic and contain taxine, a cardiac depressant.

Johnson Grass

Signs are consistent with cyanide poisoning, rapid breathing, which progresses to tremors, frequent urination and defecation, gasping and convulsions.

Johnson Grass is a wild grass native to the southern climates, where it grows along roadways and other uncultivated open areas. It is a close relative, Sudan grass. Drug therapy can offset the effects of less severe cyanide poisoning.


Loco Weed (lupine)

Bobbing head, exaggerated, high-stepping gaits or stagger and fall.

Found throughout the West and Southwest, often in dry, sandy soil. Horses that have only ingested small amounts may recover after plant has been removed from contact.

Oleander (rose laurel)

Diarrhea may become bloody, colic, sweating, muscle tremors, uncoordinated, shallow/difficult breathing, recumbency, and possibly death from cardiac failure.

Oleanders are a common ornamental shrub. Native to Mediterranean; cultivated in United States as an ornamental. Thrives in the southern U.S. and California; occasionally raised as a potted plant in other regions. Often growing along roadway sand used as a visual barrier or wind screen. All varieties are extremely toxic.

Red Maple

Signs can appear within a few hours or as long as four or five days after consumption, include lethargy, refusal to eat, dark red-brown or black urine, pale yellowish gum sand mucous membranes at first, advancing to dark muddy brown, increased respiratory rate, rapid heart rate, dehydration.

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.

Yellow Star Thistle

Neurological damage, chewing without being able to swallow, dehydration and malnutrition, possible death due to lack of nutrition.

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.

White Hemlock

Primarily the brain, excessive salivation, dilated pupils and nervousness, difficult breathing, degeneration of the heart and skeletal muscles, seizures and convulsions, respiratory paralysis. Signs of poisoning appear within an hour of ingestion, and death typically follows within two to three hours.

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 throughout Northern North America.

Lily of the Valley

Vomiting, irregular heartbeat, low blood pressure, seizures, disorientation, coma. 

Lily of the Valley is unlikely to be growing in a pasture. It could be accidentally ingested if someone were to throw garden clippings close to a fence line where curious horses might be able to reach. Care should be taken to ensure that well meaning neighbors or gardeners do not put clipping into your pasture.


Disorientation, spasms, loss of muscular control, rapid and weak pulse, diarrhea, respiratory paralysis, dilated pupils, kidney or liver failure.

All parts of the plant are toxic. Living and dried plants (accidentally baled into hay) are toxic. Like most toxic plants horses will avoid milkweed unless they have no other food source.

Red Oak

Depression, coma, dark brown urine, rapid pulse, increased respiration.

Various varieties of oaks live throughout North America. Leaves can remain toxic for several weeks after they've fallen. Don't dispose of red maple leaves in manure piles or compost heaps that might be in reach of your horses. Red maple leaves can cause problems if baled into hay. Water may be contaminated by fallen leaves. Acorns are also toxic if eaten in quantity.

Dogbane (Bitterroot)

Diarrhea (possibly with blood), slow heart rate, weakness, rapid pulse, dilation of pupils, convulsions may occur, coma, death.

Grows on plains and foothills at elevations up to 2,000 meters. It commonly is found in gravelly or sandy fields, in meadows, and along creek beds, irrigation ditches, and fence lines in cultivated pastures. The plant begins growing in late spring or early summer.

Whitesnaker Root

Includes uncoordinated, muscle weakness, tremors, elevated heart rate, cardiac arrhythmias, profuse sweating, and inability to swallow.

Found in the eastern and southern U.S., westward from Minnesota to Texas in open wooded or semi-shaded areas. Plant is toxic year round, although poisonings typically occur in summer and late fall. Horses eating one to ten percent of their body weight in the plant can develop lethal clinical effects. Onset of signs generally occurs after two days to three weeks of ingestion. The toxin is also passed in mare's milk; nursing foals can develop clinical signs as a result.


Weak or rapid pulse, dilated pupils, rapid pulse, diarrhea, uncoordinated gate, muscular twitching, frequent urination, coma and possible death. Symptoms can occur 1 - 4 hours after ingestion.


Jimsonweed is a cosmopolitan weed of worldwide distribution. It is found in most of the continental US from New England to Texas, Florida to the far western states. Jimsonweed is also found in most southern Canadian Provinces as well. It grows in cultivated fields being a major weed in soybeans worldwide and is common on overgrazed pastures, barnyards, and waste land -preferring rich soils.