Poisonous plants are a concern for all horse owners, especially for those with horses who live out on pasture. Virtually every pasture has some degree of toxic weeds and unwanted plants growing in it. Fortunately, many of these toxic plants are bitter and not very palatable, so most horses will choose to eat other plants instead. Most horses are large enough that if they do consume a poisonous plant, they would need to eat an abundant amount to reach toxic levels. Life-threatening poisonings from plants are not common; however, a range of toxicities can occur. If your horse is out on pasture, it’s important to be able to recognize toxic plants and understand the symptoms they can cause if consumed by your horse.
Bracken fern is a plant that’s found throughout North America, but most toxicities occur in the north western states. It contains the enzyme thiaminase, which prevents the absorption of thiamine (vitamin B1) and causes a thiamine deficiency, resulting in blindness, depression, weight loss and uncoordinated gait. For poisoning to occur, a horse must consume large quantities of bracken fern over several weeks. Bracken fern is one of the few toxic plants that horses can develop a taste for, so watch for ferns growing in moist areas of your fields and don’t let your horse munch on any ferns while out on the trail. Although horses may find it in their pasture, most toxicities come from feeding poor-quality hay that contains the plant. Horses eating hay that contains 3-5% bracken fern for a month will be affected. If caught early, bracken fern poisoning can be countered by doses of thiamine to restore normal levels. However, the best prevention methods are to maintain good pastures by eliminating any bracken fern found and inspecting hay to ensure it’s free of the plant.
Buttercups and Pokeweed
Buttercups and pokeweed are two pasture plants that are found in overgrazed pastures and often thrive in drought conditions. Buttercups are characterized by their yellow cup-shaped flowers with long, thin stem; whereas, pokeweed is a large, bushy weed that grows erect like a tree with a reddish or purple stem and brightly colored purple berries. Pokeweed berries are produced in clusters that start off green and turn a blackish-purple color when mature. The roots are the most toxic part of the plant, but horses can be poisoned by consuming the leaves and stems as well. As pokeweed matures, the leaves and stems increase in toxicity. Since both buttercups and pokeweed are not very palatable, horses will normally avoid eating them if they have access to good quality forage. Dried buttercups, which are sometimes found in hay, are not toxic. When these plants are consumed, they can cause oral irritation (blisters and excess salivation for buttercups; burning sensation of the mouth for pokeweed), diarrhea and gastric upset.
Yew is one of the most toxic plants for horses. This ornamental plant is native to the west coast, eastern and central parts of the U.S. and seen as landscape shrubs on home properties or even some show grounds. It’s a woody evergreen shrub with needlelike leaves and bright red berries that have a hole in the end. Yew plants contain taxine, an alkaloid toxin that causes rapid cardiac and respiratory collapse. The highest concentration of taxine is found in the leaves (even dried leaves remain toxic); as little as a mouthful (8-16 oz of yew leaves) can be fatal within 30 minutes of consumption. Symptoms are rarely observed since the toxin works so quickly and equine cases are generally diagnosed after sudden death. There is no antidote or treatment for yew poisoning, so avoidance is critical. Since this is a common ornamental plant, it becomes extremely important to ensure any neighbor trimmings are not thrown into the pasture.
There are over 70 species of nightshade plants and many are toxic to horses, including black nightshade, bittersweet nightshade, horse nettle, jimsonweed and even green tomato and potato vines. Nightshade plants are native to North America and mainly found growing in cultivated fields, overgrazed pastures and gardens. They can be identified by their purple or white 5-lobed flowers with berries that turn yellow or black at maturity. All nightshade plants contain a glycoalkaloid called solanine, which significantly affects the horse’s central nervous system and gastrointestinal tract. Ingestion of the nightshade plants (either fresh or dried) can cause loss of appetite, excess salivation, weakness, lack of muscle coordination, trembling and gastric upset. Toxicity is highest in the green berries, followed by red or black berries, leaves, stems and roots. It is estimated that the consumption of 1 to 10 lbs. of nightshade is fatal to horses. However, horses don’t generally eat these plants and poisoning is more commonly due to nightshade being a contaminant in poor quality hay.
Alsike clover is most frequently found across the northern states of the U.S. and in Canada and can be used for hay, pasture or soil improvement. It has small pink flowers (that turn brown at maturity) on long stems that are 1 to 2 feet in height. Alsike clover is known for causing photosensitization (short-term exposure) and liver damage (long-term exposure), as well as possible nitrate poisoning. Horses consuming hay or pasture with as little as 20% of alsike clover can begin to show signs of liver damage and can be fatal.
Photosensitization is when a horse’s white skin becomes sensitized to sunlight. It causes reddening of the skin, painful lesions, swelling, discharge and the skin will eventually slough-off. Photosensitization most commonly affects the white areas on a horse’s face and legs.
Ragwort is another plant that is highly toxic to horses. There are about 70 different species of ragwort, such as tansy ragwort and St. Johns wort, and are recognized by their 13-petal daisy-like, yellow flowers. All parts of the ragwort plant are toxic, eaten both fresh and dried in hay. Ragwort contains pyrrolizidine alkaloids, which affect the liver by inhibiting cell division. These toxins are cumulative in effect and when consumed over an extended period of time cause irreversible liver damage. Affected horses may be jaundiced and will show neurologic signs from toxin accumulation due to the liver failure. There is no effective treatment for advanced stages of liver disease from this toxin. Unfortunately, since there normally aren’t any early signs of consumption, most horses already have advanced and irreversible liver damage by the time it’s noticed.
Red Maple Trees
Red maple trees grow throughout the eastern portion of the U.S.; they are extremely toxic to horses and one of the most common plant-based poisonings in horses. They are well-known for their leaves that turn bright red in the fall and are frequently found growing in or near pastures where horses are kept. While fresh, green leaves are apparently not toxic if ingested, the tree bark and wilted leaves are highly dangerous and unfortunately have a sweet taste and are more palatable to horses. Wilted leaves are commonly found on tree branches that have fallen into the pasture during a storm, and during autumn months when leaves drop to the ground. The leaves can remain toxic for about a month, so it’s best to immediately remove any that are accessible to horses. The toxin in red maple leaves affects the red blood cells, reducing their ability to carry oxygen and can cause severe kidney damage. It is estimated that poisoning will occur from the consumption of approximately 1.5 lb. of leaves. Depending on the amount consumed, symptoms can appear within a few hours or a few days. Signs of toxicity include lethargy, jaundiced to brown gum color, dark brown urine, breathing difficulties, increased heart rate and death. The only treatment available is to administer IV fluids and possibly a blood transfusion; however, survival depends on the amount of leaves consumed and how quickly supportive care is provided.
Poison Hemlock and Water Hemlock
Poison hemlock and water hemlock are two very toxic plants for horses. These plants look somewhat similar with their clusters of white flowers but can be differentiated by their leaves; poison hemlock has small fern-like leaves and water hemlock has larger leaves with jagged edges. They also grow in different areas; poison hemlock along roadsides or uncultivated areas, while water hemlock prefers the marshy areas of meadows and alongside streams and irrigation ditches. Both plants affect a horse’s nervous system when ingested but do so via different neurotoxins. The leaves, stems and seeds are the most toxic parts of the poison hemlock. Toxicity increases throughout the growing season, especially in their parsnips-like roots. Poison hemlock contains potent neurotoxins that affect the central and peripheral nervous systems. Symptoms of poisoning can occur within 1 to 2 hours of consumption and include increased salivation, nervousness, muscle tremors, incoordination and increased respiratory and heart rates. A horse must consume 4 to 5 lbs. of poison hemlock for it to be fatal, and usually dies due to respiratory failure. Water hemlock, on the other hand, is even more toxic than poison hemlock and as little as 8 oz. can be fatal. All parts of the water hemlock (especially the roots) contain an extremely poisonous neurotoxin that affects the central nervous system. Toxin levels in the leaves and stems decreases as the plant grows, but the roots remain toxic year-round. Signs of poisoning include excess salivation, breathing difficulties, nervousness, violent muscle tremors and convulsions, and death from respiratory paralysis. The onset of symptoms will occur one hour after consumption and death follows typically within 2 to 3 hours.
Black Walnut Tree
The black walnut tree is an interesting poisonous plant. The bark, wood, nuts and roots of the black walnut tree are toxic if ingested; however, most horses rarely eat the leaves or chew on the bark. Horses are primarily poisoned through exposure by shavings that contain parts of the black walnut tree. Bedding that contains 20% or more black walnut shavings will cause toxicity in as little as 10-12 hours after oral or skin contact. Black walnut shavings can be identified by its dark brown color and will stand out from regular light tan shavings. Signs of toxicity include reluctance to move, shifting weight from limb to limb, warm hooves, leg edema and an increased digital pulse. If the affected horse is removed from the black walnut shavings early enough, most symptoms will disappear within a few hours. However, it’s always best to have the horse checked out by a veterinarian to make sure there’s no long-term effects and continuously check your shavings before bedding your horse’s stall.
There’s a long list of other plants that are toxic to horses. You can talk to your veterinarian or contact your state cooperative extension to ask which toxic plants are common in your area and should look out for. When in doubt, take a sample of the plant in question to your veterinarian or use online resources to help identify unknown plants. A comprehensive list of poisonous plants with identification traits, images and potential symptoms can be found at http://www.horsedvm.com/toxicfinder.php. Make sure you walk your fields looking for potential poisonous plants on a regular basis and remove anything suspicious looking immediately. Most horses will avoid toxic plants unless their pastures are overgrazed or suffering from drought conditions. In those situations, try to provide safe forage options such as good quality hay.