Plants Toxic to Horses

Box Elder Maple

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

• 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)

• 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

• 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

• 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

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

• 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)

• 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.