An Equine Guide to Poisonous Plants

Locoweed

Astragalus spp.

Locoweed, also known as crazy weed, is a leafy perennial often with white or purple flowers that grows primarily throughout western states.

Symptoms include

Unusual behavior, Bobbing head, High-stepping movement, Poor coordination, Weight loss, Reproductive problems: Mid- to late-term abortion, birth defects, male infertility

Tansy Ragwort

Senecio jacobaea

Tansy ragwort is a weed that produces small, daisy-like yellow flowers. The entire Senecio species contain pyrrolizidine alkaloids that interfere with cell division and slowly destroy the liver.

Symptoms that may develop over time

Photosensitivity, Poor appetite and weight loss, Depression, Neurological signs, Jaundice, Liver failure

Poison Hemlock

Conium maculatum

Poison hemlock grows on roadsides, fence rows, drainage areas and neglected pastures across most of North America.

Early symptoms include

Dilated pupils, Nervousness, Muscle tremors, Incoordination, Colic, Respiratory failure

Bracken Fern

Pteridium aquilinum

Bracken fern or bracken is a perennial fern that grows throughout most of the U.S. The plant contains thiaminase, a compound that interferes with vitamin B1 (thiamin) metabolism. Chronic consumption over several weeks can result in severe neurological impairment as thiamin is essential for nerve function.

Symptoms include

Muscle tremors, Incoordination, Difficulty standing, Depression, Weight loss, Blindness

St. John’s Wort

Hypericum perforatum

St. John’s Wort is a flowering plant that contains hypericin, a compound that increases skin sensitivity to sunlight — also called photosensitization. Lightskinned horses are particularly vulnerable, as well as all white face, leg and body markings.

Symptoms include

Sunburn-like lesions, Inflammation of the skin, Skin sloughing, Eye irritation and dilated pupils, Photosensitivity

Nightshade Species

Solanaceae

Not all nightshades are poisonous, but several nightshade plants from the genus Solanum contain toxic compounds capable of affecting the nervous and digestive systems.

Symptoms may include

Digestive Upset, Colic, Lethargy or weakness, Excessive salivation, Neurological abnormalities

Water Hemlock

Cicuta

Often considered North America’s most poisonous native plant, water hemlock grows in wet soils, creek banks, irrigation ditches and marshes.

Symptoms include

Excessive salivation, Dilated pupils and nervousness, Violent seizures, Muscle tremors, Respiratory paralysis leading to death

Foxglove

Digitalis

Foxglove's tall flower spikes make it popular in ornamental gardens. Although horses generally avoid the plant, accidental consumption can occur in contaminated hay.

Symptoms include

Irregular heartbeat, Weakness, Colic, Diarrhea, Tremors, Collapse

Buttercups

Ranunculus

Bright yellow buttercups are common in moist spring pastures. Fresh plants are acrid-tasting and contain irritating compounds that usually discourage horses from eating them.

Consuming relatively large amounts may cause

Mouth irritation and severe blistering, Mild colic, Excessive salivation, Bloody diarrhea

Johnson grass

Sorghum spp.

Johnson grass is coarse-stemmed with broad, veined leaves that can grow up to six feet. Native to the southern climates, the leaves and stems contain cyanide compounds.

Symptoms include

Hindquarter weakness and incoordination, Difficulty backing up, Urinary incontinence, Fetal development problems, Abortion

Black Walnut

Black walnut trees themselves are not usually the primary concern in pastures. Black walnut wood shavings used as bedding for stalls, however, can cause serious problems. Even small amounts mixed with other bedding materials can be triggering. Symptoms often appear within hours and include an elevated heart rate, fever, swollen legs, reluctance to move and laminitis.

Red Maple

Red maple trees are beautiful landscape trees throughout the eastern United States. Fresh leaves are generally less dangerous. However, wilted leaves are extremely toxic to horses containing compounds that destroy red blood cells causing anemia and preventing oxygen from being carried in the blood. The greatest risk of consumption occurs after storms or in autumn when wilted leaves drop and blow into reach of grazing horses. Poisoning from red maple trees may result in weak and rapid breathing, dark or chocolate-colored urine and pale or yellow gums. Red maple poisoning requires immediate veterinary treatment that often includes administration of large amounts of intravenous fluids and, potentially, blood transfusions. Early detection and veterinary treatment can, in some cases, save a horse from death.

Cherry

Wild black cherry and chokecherry trees are highly toxic to horses. While the fruit is actually safe, the leaves, bark, twigs and pits contain cyanogenic glycosides. Wilted leaves are particularly dangerous, converting the glycoside compounds into cyanide capable of causing collapse and rapid death if consumed.

Oak

Oak leaves, buds and acorns contain tannins that are highly toxic. In the spring, budding leaves and young blossoms are very palatable and may entice horses, especially if other forage is not available. Most horses will tolerate small amounts of acorns, but during the fall, green, unripe acorns may drop after storms or high winds and pose a significant threat. Consumption can cause colic with constipation followed by diarrhea and kidney damage or failure.

Oleander

Oleander is among the most toxic ornamental plants grown in America. It is common in the southern and western states, and boasts large clusters of beautiful white, pink or red flowers. Every part of the plant contains powerful cardiac glycoside toxins known as oleandrin that disrupt the beating of the heart. Both fresh and dried leaves are dangerous with only a few leaves required to cause sudden death in an adult horse within eight hours of being eaten.

Yew

Popular as an ornamental evergreen shrub with bright red or yellow berries, yews are found throughout much of the country. It is one of the most toxic plants encountered by horses due to potent cardiac toxins. Even small amounts, such as a handful of clippings, can cause sudden death in as little as five minutes after consumption. Every part of the plant is highly poisonous, except for the fleshy part of the berries, containing an alkaloid called taxine that causes respiratory and cardiac collapse.

Azalea (Rhododendron)

Rhododendrons, including the common azalea shrub, are highly toxic to horses. All parts of the plant (leaves, flowers and stem) contain grayanotoxins, which bind to sodium channels in the cell membrane, prevent them from closing and disrupt normal heart rhythm, skeletal muscle function and nerve transmission. Continuous cell activation depletes cellular energy, eventually leading to weakness, incoordination and paralysis. If a large dose is eaten, the continuous cardiac and respiratory distress can progress to coma and fatal heart failure. Horses will usually avoid these plants if good pasture is available, but they may browse on them during the winter when forage is scarce, or if clippings are left within reach.