Plants Toxic to Horses

Horse owners should learn to recognize toxic plants, and be aware of the symptoms they can cause.
Plants Toxic to Horses - Articles


In This Article

Alsike Clover

Two disease syndromes in horses have been associated with grazing Alsike clover: photo-sensitization, and liver disease, which is less common.

Alsike clover is a high quality legume that is frequently planted as forage for livestock. It is not commonly found in high concentrations in most pastures, unless the pasture had been previously seeded with a mix containing this species. Since horses find alsike clover to be palatable, and will not avoid eating it like other toxic plants, even low concentrations may pose a risk. Alsike clover is an annual or biennial clover that is best adapted to cool temperatures and moist soil.

Alsike clover has erect stems like red clover, but the stems are fine and tend to lodge. Flowers are produced along the entire length of the stem rather than at the tip like red clover. The flower is very similar to white clover, but is dark pink on the bottom and white on the top. White clover flowers are white and may be tinged with a pale pink color. Unlike alsike clover that produces upright stems, white clover stems are prostrate and run along the surface of the ground. The leaflets lack the white “V” that is found on white clover.

Toxic Properties
Two disease syndromes in horses have been associated with grazing alsike clover: photo-sensitization, and liver disease, which is less common. The specific toxin in the clover that causes the disease is unknown.

Liver disease is rare and may occur if the horses are feeding on large amounts of alsike clover. Symptoms include weight loss, jaundice, depression, and neurological abnormalities. Symptoms of photosensitization include destruction of skin cells in unpigmented parts of the horse’s body when the skin is exposed to light. Affected skin will blister and eventually slough off.

Liver disease is irreversible. Horses should not be allowed to graze pastures that contain significant concentrations of alsike clover. Alsike clover can be reduced by applying nitrogen fertilizer to pastures to enhance grass forage production. Broad-leaf herbicides can also be used to reduce clover concentrations in pastures.

Rhizoctonia Fungus on White and Red Clover

The toxin, slaframine, is produced by the Rhizoctonia fungus, stimulates the salivary glands and causes horses to drool.

White clover is a very common legume that is frequently found in pastures. White clover can grow if soil fertility is poor, and will survive close grazing. It is a short lived perennial, and it is a prolific seed producer. Although individual plants do not live very long, new plants are constantly being produced from seed. Red clover is also a common pasture legume. Since it does not tolerate continuous grazing, it is not normally found in overgrazed pastures.

White clover is a short lived perennial with a prostrate growth habit. It has no upright stems, and spreads by stolons. Stolons are stems that run along the surface of the ground, and produce new plants. The plant has compound leaves with three leaflets, and a white “V” on each leaflet. Flowers are white in color.

Red clover is a short lived perennial that has an erect growth habit. The plant has reddish purple flowers at the end of each stem. Stems are hairy.

Toxic Properties
The clover plant itself is not toxic. The toxin, slaframine, is produced by the Rhizoctonia fungus which grows on clovers and alfalfa during periods of stress (high humidity, drought, and continuous grazing). Hay made from contaminated forages is also suspect, and the slaframine can remain in hay for several years. It is not uncommon for some horses in a pasture to be more effected than others, since horses vary in their preference for clover and sensitivity to the toxin.

The slaframine stimulates the salivary glands and causes horses to drool. Although this is a nuisance, horses rarely suffer any health effects from grazing infected clover. Other symptoms occasionally include tearing, skin lesions, difficulty breathing, increased urination and feed refusal. It is not uncommon for some horses in a pasture to be more effected than others, since horses vary in their preference for clover and sensitivity to the toxin.

There are several strategies to reduce drooling caused by Rhizoctonia:

  • Remove horses from infected pastures.
  • Mow pastures until the brown fungal spots are no longer present on the leaves.
  • Increase the concentration of grass by overseeding pastures and applying nitrogen fertilizer in spring and fall.
  • Rest and rotate pastures to allow the grass to remain tall and competitive with the clover.
  • Broad leaf herbicides, labeled for pasture use, can be used to remove existing clover plants from pastures.

Tall Fescue

  • Common grass in this region.
  • Large leaf blades with sharp edges and prominent veins, shiny on lower surface.
  • Not a favorite plant until after frost.
  • The toxin is found in all plant tissues and seeds.
  • Plant contains an endophyte that produces a toxin.
  • Affected species: sheep, cattle, goats, horses.
  • Mares have long pregnancies, and may abort foals, or have weak foals if they graze infected fescue in the last three months of pregnancy

Buttercup Species

  • All livestock are affected.
  • Toxicity - low.
  • Common in pastures and marshes.
  • Poisonous part - leaves and flowers.
  • Symptoms: irritated tissues in the mouth and throat. Effects the gastrointestinal system (colic, diarrhea), excessive salivation.


  • Effects all livestock - especially pigs.
  • Toxicity – moderate.
  • Found in rich, disturbed soils such as barnyards, moist woodlands and pastures.
  • Poisonous part - all parts, but mainly the roots.
  • Symptoms: Effects the gastrointestinal system –( colic and diarrhea) and central nervous system – (convulsions).
  • Cooked berries are sometimes used in pies.

Nightshade Species

  • All livestock are affected.
  • Toxicity - moderate.
  • Found in disturbed soils, rich pastures, and woods.
  • Poisonous part - berries and vegetation.
  • Symptoms: Effects: central nervous system- (trembling, paralysis, shock, coma) gastrointestinal system - (colic, diarrhea and impaction).

Poison Hemlock

  • All livestock are affected.
  • Toxicity - extremely toxic, 4-5 lbs. will kill a 1,000 lb. animal.
  • Distribution - disturbed or waste areas, roadsides, ditches.
  • Poisonous part - all parts are extremely toxic.
  • Symptoms: Central nervous system - block spinal cord reflexes, muscle tremors, incoordination, paralysis, frequent urination, sudden death due to respiratory failure.

Water Hemlock

  • All livestock affected.
  • Toxicity - extremely toxic (a piece of root the size of a walnut will kill a cow in 15 minutes).
  • Poisonous part - all parts, especially the root.
  • Distribution - marshes, ditches, wet pastures.
  • Symptoms: Effects central nervous system, causing nervousness, breath-ing difficulties, muscle tremors, collapse, convulsions, death.

Jimson Weed

  • All animals affected, including chickens.
  • Toxicity – extreme, one mouthful can kill a horse.
  • Distribution - crop fields, waste areas, barnyards.
  • Poisonous part - entire plant.
  • Annual plants, mowing helps eliminate plants.
  • Symptoms: Effects central nervous system. Has hallucinogenic properties


  • Affects livestock and poultry.
  • Toxicity – high.
  • Distribution - swamps, bogs, dry fields and pastures.
  • Poisonous part - entire plant.
  • Symptoms: weakness, seizures, respiratory difficulties, coma, death.
  • Latex-like sap makes plant very unpalatable.


  • Affects all livestock.
  • Toxicity - highly toxic.
  • Poisonous part – leaves, twigs, bark and seeds contain cyanide, wilted leaves are more toxic than the rest.
  • Symptoms: anxiety, breathing problems (suffocation), staggering, convulsions, col-lapse, death.