Plants Toxic to Horses

Alsike Clover

Two disease syndromes in horses have been associated with grazing alsike clover: photosensitization and liver disease, which is less common.

• Affected species: Horses
• Low toxicity
• Common in some pastures
• Symptoms: Photosensitization (blistering of unpigmented skin when exposed to sunlight) and liver disease.
• Management: Remove horse from the pasture, manage pastures to promote grass, eliminate clover.

Photo: Photosensitivity injury

White and Red Clover

Horses grazing pastures with red and white clover may become affected by "slobbers." The toxin behind the slobbers, slaframine, is produced by a fungus that afflicts clovers, which stimulates the salivary glands and causes horses to drool.

• The clover plant itself is not toxic.
• Slaframine is produced by "black patch fungus," Rhizoctonia, which grows on clover during periods of stress.
• Symptoms: Salivation and drooling
• Affected species: Only horses
• Management: Remove horses from clover and provide plenty of fresh water

Tall Fescue

Endophyte-Infected Tall Fescue – Kentucky 31

• Common grass in this region
• Large leaf blades with sharp edges and prominent veins, shiny on lower surface
• Not a preferred plant until after frost
• Kentucky 31 tall fescue contains an endophyte that produces a toxin called ergovaline
• The toxin is found in all plant tissues and seeds.
• Affected species: sheep, cattle, goats, horses
• Mares may have long pregnancies, abort foals, or have other reproductive problems if they graze infected fescue in the last three months of pregnancy.
• Endophyte-free tall fescue varieties are available commercially.  Novel-endophyte or "endophyte-friendly" varieties do contain an endophyte for enhanced growth but do not produce ergovaline and are safe for pregnant mares to graze.

Buttercup Species

Tall Buttercup. Photo Credit: Montana Statewide Noxious Weed Awareness and Education Program, Montana State University, Bugwood.org

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

Pokeweed

Common Pokeweed. Photo Credit: Joseph M. DiTomaso, University of California - Davis, Bugwood.org

• Affects 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 - Affects the gastrointestinal system (colic and diarrhea) and central nervous system (convulsions).
• Cooked berries are sometimes used in pies.

Nightshade Species

Bittersweet Nightshade. Photo Credit: Joel Floyd, USDA APHIS PPQ, Bugwood.org

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

Horsenettle

Horsenettle. Photo Credit: Ohio State Weed Lab, The Ohio State University, Bugwood.org

• All livestock are affected.
• Toxicity - moderate
• Distribution - pastures, cultivated fields, hay fields
• Poisonous part - all parts, especially berries. Remains toxic in hay.
• Symptoms - Affects the gastrointestinal (salivation, colic, diarrhea) and central nervous system (muscle tremors, weakness, depression)

Poison Hemlock

Poison Hemlock. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org

• All livestock are affected.
• Toxicity - extremely toxic, 4-5 pounds will kill a 1,000 pound animal
• Distribution - disturbed or waste areas, roadsides, ditches
• Poisonous part - all parts are extremely toxic
• Symptoms - Affects the central nervous system (blocked spinal cord reflexes, muscle tremors, incoordination, paralysis), frequent urination, sudden death due to respiratory failure.

Water Hemlock

Water Hemlock. Photo Credit: Elmer Verhasselt, Bugwood.org

• All livestock are 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 - Affects central nervous system, causing nervousness, breathing difficulties, muscle tremors, collapse, convulsions, death.

Jimson Weed

Jimson Weed. Photo Credit: Howard F. Schwartz, Colorado State University, Bugwood.org

• All animals affected, including chickens.
• Toxicity – extreme
• Distribution - crop fields, waste areas, barnyards
• Poisonous part - entire plant, especially seeds
• Symptoms - Affects central nervous system. Has hallucinogenic properties
• Jimson weed has a strong, foul odor and an unpleasant taste. Animals are often poisoned when feed is contaminated with jimson weed seed.
• Jimson weed is an annual plant; mowing helps eliminate these plants.

White Snake Root

White Snakeroot. Photo Credit: John Triana, Regional Water Authority, Bugwood.org

• Affects horses, cows, sheep, goats, pigs and chickens.
• Toxicity - high
• Distribution - common in moist areas, edge of woods, along roads
• Poisonous parts - leaves and stems
• Symptoms - trembling, stiffness, ataxia, coma, death
• White snake root's toxin, trematol, passes to humans in milk resulting in milk sickness.

Milkweed

Common Milkweed. Photo Credit: Richard Gardner, Bugwood.org

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

Cherry (Black, Pin, Choke)

Black Cherry. Photo Credit: Richard Webb, Bugwood.org

• Affects all livestock, most dangerous to ruminants.
• 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, collapse, death.

Red Maple

Red Maple. Photo Credit: Paul Wray, Iowa State University, Bugwood.org

• Only horses and ponies are reported to be affected.
• Toxicity - extremely toxic (1.5-3 pounds cause toxicity)
• Poisonous part - wilted or dried leaves
• Symptoms - breathing difficulties, jaundice, dark brown urine, death.
• Toxins (gallic acid and others) destroy red blood cells. Red maple hybrids, such as silver and sugar maple, also have toxins.
• Leaves are most dangerous when wilting (i.e. a branch comes down during a storm), but fallen autumn leaves are also toxic for 30 days or so and should be removed from pastures.

Box Elder Maple

Box Elder. Photo Credit: https://www.flickr.com/photos/cricketsblog/26262670762 Melissa McMasters

Seeds from the boxelder maple cause the equine muscle disease Seasonal Pasture Myopathy.

• Affected species - horses
• Toxicity - high. Quantity required for toxicity is unknown, but SPM is 75-90% fatal.
• Poisonous parts - Seeds contain toxin, hypoglycen A, which interferes with fat metabolism and breaks down respiratory and muscle cells.  Only female boxelder trees produce seeds. Male trees are not dangerous.
• Symptoms - Tremors, weakness, stiffness, dark urine, rapid breathing, and death – usually within 48 hours

Black Walnut

Black Walnut. Photo Credit: Jason Sharman, Vitalitree, Bugwood.org

• Species affected - horses
• Toxicity - moderately toxic
• Poisonous parts - bark, root, nuts contain juglone, which may be involved in toxicity. Walnuts and hulls on the ground may become moldy and cause toxicity if consumed.
• Symptoms - Horses bedded on shavings or sawdust containing black walnut develop colic, edema and laminitis.
• No amount of black walnut is acceptable in bedding. 
• If removing trees or branches in pastures, be careful to clean up all sawdust left in the pasture.

Oaks (Black, Chestnut, Red, Pin, White)

Red Oak Leaves. Photo Credit: https://www.publicdomainpictures.net

• Affects cattle, sheep, horses and pigs.
• Toxicity - moderately toxic
• Poisonous part - New young leaves most toxic, acorns more toxic when green than when mature
• Symptoms - Poor appetite, weight loss, diarrhea, increased drinking, increased urination, kidney failure, edema, death.

Buckeye or Horse Chestnut

Horse Chestnut. Photo Credit: John Ruter, University of Georgia, Bugwood.org

• Affects all livestock.
• Toxicity - moderate to high 
• Poisonous part - leaves, seeds, young sprouts; poisoning in spring due to early sprouting
• Symptoms - Affects gastrointestinal and central nervous systems

Black Locust

Black Locust. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org

• Affects all livestock species.
• Toxicity - moderate to high
• Poisonous parts - leaves, seeds, bark, wood (fence posts)
• Symptoms - Causes severe gastritis, colic, depression

Rhododendron, Mountain Laurel, Azalea

Rhododendron. Photo Credit: Richard Webb, Bugwood.org

Common landscape shrub, also found in the forest.

• Affects all livestock.
• Toxicity - high
• Poisonous part - all parts
• Symptoms - Stomach irritation, abdominal pain, abnormal heart rate and rhythm, convulsions, coma, death

Bracken Fern

Bracken Fern. Photo Credit: David Stephens, Bugwood.org

• All livestock are affected.
• Toxicity - low to moderate
• Distribution - moist forests
• Poisonous part - All plant parts contain toxin which destroys vitamin B1.
• Symptoms - Weight loss, weakness, gait abnormalities, abnormal heart rate and/or rhythm, inability to rise, death
• Some animals develop a preference for this plant. 

Yew (English or Japanese)

Yew. Photo Credit: Richard Webb, Bugwood.org

• Affects all livestock and humans
• Toxicity - Extreme
• Poisonous part - all plant parts, especially high in leaves during winter.
• Symptoms - Muscle trembling, incoordination, colic, slow heart rate, death.
• Yews are commonly planted as landscape shrubs on home properties and even show grounds.