Toxic Plants

Do you know what's in your pasture other than grass? Foreign plants can colonize a pasture from remote sources of seed, sometimes conquering large flats in a single year. Fields that are poorly managed, previously flooded, overgrazed, or surrounded by overgrown fields may be prone to weed infestation. Horses that are grazed next to woodlands, roads, ornamental gardens, lawns, or orchards are particularly prone towards toxicity. Take a walk around your pasture and get a sense of what's new? Pasture is the main source of toxic plants, but hay can be another. Hay is viewed as suspect in cases where there is no pasture.

Most horses won't eat toxic plants since they are distasteful and unconventional. Furthermore, it is rare that a mouthful of a toxic plant will be dangerous. Exceptions include Yew (Taxus) or Water Hemlock (Cicuta douglasii) which are quite deadly – fortunately, horses rarely have access to these plants. Goats, on the other hand, seem to get into trouble with them, while satisfying their curious appetites.

Horses often resort to eating toxic plants in times of drought, when pastures are sparse, grazing on recaptured land, with access to woodlands, and in malnourished states, horses. Horses denied forage, roughage, salt, minerals, horses that are fed 'complete feed' pellets instead of hay (old horses with bad teeth), and horses that are bored may start nibbling on toxic plants.

Plant toxicosis should be considered in any horse with unexplained signs of slobbering, laminitis, sudden onset of tremors or weakness, behavioral changes, bloody diarrhea, red urine, cardiac arrhythmias, jaundice or other signs of liver disease, severe anemia, and in cases of sudden death, plant toxicity.

Treatment is rarely specific for individual plants – anecdotes are not the norm. In general, your veterinarian will want to treat all suspected plant intoxications with mineral oil and/or activated charcoal to absorb the toxins, and intravenous fluids to speed the excretion of the toxic principle through the kidneys.

Identification of the plant or testing of hay samples for toxic plants can also be pursued, but more often than not, no plant is found and the intoxication difficult to prove. Extension agents and specialists should be called upon to identify toxic plants if there is any question. Take a photograph and send the plant to a specialist if the whole plant is available.

Plant toxicities are discussed by the symptoms they produce.

Colic and Diarrhea

Several plants fall into this category. One common culprit is Oak (Quercus spp), which contains tannins, the toxic principle. Tannic acid and its toxic metabolites (digallic acid, gallic acid, and pyrogallol) are contained in the leaves, especially when green, as well as the bark, blossoms, buds, stems, or acorns. Oak ingestion causes hard, dark, feces, and colic, later turning to bloody diarrhea, oral ulcers and signs of choke. Liver and kidney damage, and increased blood calcium to phosphorous ratio are additional signs.

Horses that wander into shrubs or woodlands, restricted from good quality pasture or hay, may ingest oak. Horses with bad teeth, given only complete pelleted feed, and young curious horses may taste the leaves. Horses out on the trail may experiment with leaf ingestion if tied near a source. The summertime, when leaves are plentiful, is the most likely time to see oak toxicity. In ruminants, oak causes kidney damage, with signs of urinary dysfunction and edema – these signs are generally not seen in horses.

Other plants that cause colic include the leaves and nuts from Horse Chestnuts and Buckeye (containing aesculin), Morning Glory (containing pseudotopine, an atropine-like toxin), and Solanacae plants (Jimson weed, Potato, and Tomato) which similarly contain atropine-like alkaloids. These alkaloids cause paralysis of the gut and colic, diarrhea, slowing of the heart and breathing, and pupil dilation.

Diarrhea is caused by numerous plants, but fortunately good management and knowledge of these plants can easily keep horses from harms way. Examples include Foxglove and Oleander that contain cardiac glycosides. These are ornamental plants that are readily identified – in addition to diarrhea, they cause signs of choke or regurgitation, and pronounced cardiac arrhythmias, leading to death.

Other plants that cause diarrhea, include Pokeweed (Phytolacca americans), Coffee or Senna weeds (Cassia spp.), the yellow field plant Buttercup (Ranunculus spp), Nightshade and Potato, Tomato, and Avocado (also causes sudden death, but not from flesh of fruit). Mountain Laurels (Kalmia spp), Azaleas (Rhododendron spp), Mountain Pieris (Pieres spp) and Maleberry contain toxins (grayanotoxin and arbutin). These are rarely eaten by horses, but can cause excessive greenish salivation, colic, frequent defecations, diarrhea, weakness and ataxia.

There is a tendency for goats, rather than horses or donkeys, to get into Rhododendron-like plants, where severe hypocalcemia and rapid death ensue from eating a few clippings or leaves. Castor bean (Ricinus communis) is a highly distasteful bean that can get inadvertently mixed into horse feeds, causing colic, diarrhea, depression, severe sweating, leading to severe muscle cramping, convulsions, and seizures or anaphylaxis at higher levels of ingestion. Fortunately, no recent reports (after 1945) could be found. Black Locust (Robinia pseducacia), a very common tree in the USA, contains a similar toxin in the leaves (called 'lectin'), that in addition to diarrhea, cause hypersensitivity reactions.

Slobbering (Hypersalivation)

Slobbering, frothing, or drooling are signs, that must be distinguished from diseases that interfere with swallowing. When caused by toxic plants, the salivation arises from the mouth – when the swallowing function is impaired, there is often saliva and food material coming from the nose as well. Salivation can stem from the irritation of a foreign body in the mouth or tongue, or from direct toxic stimulation of the salivary glands, that drain in the mouth.

Some plants are infested with fungi that cause salivation directly, e.g. alfalfa or clover infested with Rhizoctonia leguminacola; this fungus contains slaframine, a toxin that stimulates the salivary gland through histamine-like actions). A common location for a thorn or foreign body to lodge is the base of the tongue. The tongue can also get infected secondary to the plant material (thorn, awn) migrating into the tissue of the tongue.

Common plants that cause mechanical injury and slobbering include burdock, awn grasses, sand burrs, foxtail barley awns, prickly pear cactus, horse nettle, buffalo burr, porcupine grass, wheat awns, stinging nettle, and cockle burrs. The injury can be very superficial or can result in deep abscessation requiring surgery. The eye can alternatively get injured by these plants, or by fungal agents that enter the cornea after injury from one of these.

Plants that interfere with prehension and swallowing or food, in addition to a whole host of neurologic signs, include Russian Knapweed and Yellow-Star Thistle (these causing severe stupor).

Botulism, Equine protozoal myelitis, West Nile Virus, and Botulism must be distinguished from plant toxicities that interfere with swallowing or induce slobbering.

Liver Disease

The cause of liver disease is often mysterious in horses, but plant-derived toxins must be considered. By the time you see the disease, it may be well after the fact, because the liver has a great reserve, i.e.major damage is required before liver function is compromised. A toxic exposure may be long gone before liver disease appears.

Liver disease may be the result of multiple or long-standing exposure(s) rather than pin-point exposure. However, if a plant was responsible for liver disease, it is probably still in the pasture, and worth identifying. More than one horse effected with liver disease should definitely trigger a suspicion of plant toxicity. The plant toxin also leaves a mark for the pathologist that distinguishes plant-induced from other causes of liver disease. This is another good reason for an autopsy in horses with liver disease.

Photosensitization (sunburn)

Some plants contain "photodynamic" substances that accumulate in the skin. At this location, they are easily excited by the sun, and release "radiant energy" (heat) that simply burns the skin. White hair affords little protection to sunlight, so burns occur in white regions only.

Examples of plants that contain these photodynamic toxins include St John's wort (Hypericum) and Buckwheat. Don't go feeding your horse St John's wort no matter how sad he looks.

Liver disease (jaundice signs) with secondary photosensitization

Other plants cause liver disease directly, and the accumulation of photodynamic chemicals and photosensitization (sunburn) as a secondary effect of liver failure. Examples of plant that contain toxins (ca'lled "pyrrolizidine alkaloids") include Senecio spp (Tansy Ragwort, Lamb'stongue, Groundsels, and Butterweed — ironically, these contain yellow, i.e. jaundice colored flowers). Others include Fiddleneck (Amsinckia spp), Hound's tongue (Cynoglossum spp), Rattlepod or Rattlebox (Crotolaria spp), Heliotrope, Alsike clover (Trifolium hybridum), and Kleingrass pasture (Panicum coloratum). Probably the most important of these are Alsike clover and Kleingrass, both of which cause there liver damage through a fungus (releasing "mycotoxins").

One indication to suspect plant intoxication is neurologic signs – typical signs in cases of plant poisonings include blindness, inability to grasp food with the mouth and chew food, ataxia, convulsions, and depression. Horses that exhibit asymmetric (on one side of the body only) signs or spinal cord disease (as in EPM or West Nile Virus infections) are not likely to have plant poisonings.


The most bizarre behavioral changes are caused by long-term ingestion of locoweeds (Oxytropis spp, Astragalus spp) . These horses are hyperexcitable, with a high stepping spastic gait, head bobbing, coupled with severe weakness. There are many non-toxic forms of locoweeds so it is important to make a correct identification. Curious young animals are most susceptible, since developing neurons are most vulnerable to the effects of the toxin, called "swainsonine" (an indolizidine alkaloid).

Dropped jaw

A dropped jaw and inability to grasp food and swallow is a sign of Yellow Star Thistle and Russian Knapweed (both Centaurea spp) . The mouth is open and the tongue protruds. As previously mentioned, the horse is drooling as well. These horses rarely if ever recover.

Falling forward and weird behavior

Sagebrush (Artemisia) ingestion causes abnormal behavior, and falling toward the front end – you can smell the culprit toxin on your horse's breath. The toxin is a monoterpenoid, so has that volatile smell.


Horsetail is that straight, bamboo-like segmented grass, sometimes found with a rattle-snake tail end that releases spores. You see it along the roads just about everywhere. This plant (Equisetum spp) , which is almost always ignored by your horse despite his wanting to eat everything along the road, makes an enzyme that breaks down Thiamin – your horse becomes short on Thiamin.

Reluctance to move, blindness, ataxia, are signs. Bracken Fern (Pteridium spp) and Sensitive Fern , quite common in woodlands in the USA, also may contain the Thiaminase enzyme. Bracken Fern toxicity is different in horses than in other species – in horses there is progressive depression, hindlimb weakness and ataxia, blindness, recumbency (lying down and unable to rise) and even death. In cattle, a fatal aplastic anemia or bladder tumors are signs of toxicity.


Certain plants contain toxins called "tremetols." These, akin to their name, release tremetols that cause intense whole body tremors, inability to swallow and choking signs. Since the esophagus is made of the same muscle as your horse's legs, it's spastic too. Culprits include White snakeroot, Jimmyweed, Rayless Goldenrod, or Burrow weed.

Sitting or falling when backed, cystitis

Consumption of Sudan or Johnson grasses (both Sorghum spp) affects the back end or your horse, causing weakness and ataxia behind, as well as paralysis of the bladder, anus and tail. Urine dribbles freely from the perineal region. There is loss of sensation around the tail, anus, and vulva, a cardinal sign of this toxicity.

Hoof wall cracks and bobtail ("alkali disease")

Too much selenium can be a bad thing – just ask someone in part of the country where certain selenium accumulating plants reside (Southeast, Central, Southwest). Selenium replaces the usual sulfur that is incorporated into hoof and hair, which disastrous consequences for the formation of keratin. Circular hoof cracks develop, and hair falls out, classically fist in the tail. The short tail looks "bobbed."

Plants that accumulate selenium are numerous, but notables include Golden weeds, Milkvetch, Woody astrers, and Prince's plume . Less likely to do so, but reported nevertheless, include Asters, Saltbrush, Beard tongue, Ironweed, Broomweed, and Gumweed. These plants are not likely to be found in lush green areas, and long-term consumption is necessary.


The most serious lameness is caused by Black walnut (Juglans nigra), which induces serious laminitis. Hence, it is imperative that a horse is not bedded on shavings made from these walnuts. Another plant, Hoary alyssum (Berteroa incana) causes limb edema, fever, and laminitis, confusing the diagnosis of Ehrlichia equi and purpura hemorrhagica (secondary to strangles) in some horses. Hoary alyssum toxicity has been reported from consumption of contaminated hay.

Stiffness, soreness

Other causes of lameness are plants that degenerate muscle ( coffee weed — e.g. Cassia occidentalis), cause calcium deposits in muscle producing stiffness such as jessamine (Cestrum diurnum) . These plants contain a vitamin-D-like substance that stimulates calcium migration into tissues in toxic proportions.


The most important anemia-producing plant is Red Maple (Acer rubrum) The toxin, which is currently unknown, can be found in dried (not green) or wilted leaves, and in the bark. Leaves remain toxic for about 30 days. Only small amounts need to be ingested, and within a couple days, the horse exhibits red-brown urine, severe depression, and signs of shock. A horse with Red Maple toxicity has a poor prognosis, but in some cases can be saved with whole blood transfusions.

Other plants that induce anemia infrequently include onions (Allium spp due to N-propyl sulfide in bulbs and in the plant itself) and moldy Sweet Clover (Melilotus spp – containing molds that accumulate in hay, called dicoumarols). Onions like Red Maple cause red blood cells to break down, and a severe anemia, kidney damage, and shock result. Moldy Sweet Clover causes hemorrhages because the clotting system is impaired. Like sage, onions can be smelled on the breath of the intoxicated horse.

Cyanide poisoning from plants

A special kind of anemia s caused by ingestion of certain berry-laden plants ( Western chokecherry, Serviceberry, Elderberry ) that contain cyanide. Cyanide is found in many plants but is sequestered in a non-digestible form. Free cyanides are liberated in damaged plants. The free cyanide binds to the trivalent iron within cytochrome oxidase, and important enzyme necessary for the function of hemoglobin – the result is that hemoglobin cannot release oxygen to tissues. The blood from an intoxicated horse is bright red because hemoglobin is fully saturated with oxygen. The tissues starve from oxygen, a sort of internal asphyxiation. Rapid death often results, but if treatment is pursued quickly with detoxification chemicals, including sodium thiosulfate and sodium nitrate (intravenously), the life of the horse can be saved.

Hypocalcemia due to oxalates

Some plants contain chemicals called oxalates that bind calcium, reducing the availability to the horse. This is mostly a problem in growing, pregnant, and lactating horses, rarely with healthy mature horses otherwise. Horses have to eat a lot of these oxalate-containing plants to have problems. Plants with oxalates include Pigweed, Sorrel (dock), Sufar beet, Lambsquarter, Rhubarb, Greasewood, Halogeten, Shamrock, Soursob, and Sorrel, with the last 5 being the most common offenders. Horses can get gastrointestinal upsets. In other species, the oxalates settle in the kidneys, blocking their outflow.

Teratogens and abortions

Nothing could be more important than to keep your pregnant mare away from toxic plants. The fetus is most vulnerable to exposure to toxins during the first trimester of pregnancy, when major organ development takes place. The classic example is consumption of Skunk Cabbage / Hellebore (Veratrum eschscholtzii) during the first equine trimester, producing a cyclops foal. A whole host of teratogenic plants exist, but rarely have affects. These include Milkvetch, locoweeds (Astragalus spp) which also cause abortion and congenital skeletal deformities (carpal flexion, hock laxity), and various hemlock species. Suspected teratogenic plants also includes lupine (cause abortions), tobacco, sudan grass, jimson weed, creeping indigo, poppies, groundsel, wild black cherry, periwinkle, mimosa, and wild pea and others. In summary, keep your mares away from ANY unusual pasture or plants.

Heart Damage

Heart damage and even death can result from ingestion of certain plants that make "cardiac glycosides." Digitalis is an example of a cardiac glycoside derived from Foxglove (Digitalis spp) that can stimulate the heart in a beneficial way, but in excess quantities, is quite injurious. The cardiac glycosides produce affects similar to the classic cardiac stimulant, ouabain, by blocking the sodium-potassium pump in the cardiac cell membrance. The most important plants that contain cardiac glycosides include Foxglove, certain Milkweeds, Oleanders, Lily of the Valley, Indian hemp, and Dogbane . Sudden death is a frequent presentation of these toxicities.

Sudden death

Only infrequently will a horse die suddenly from plant toxicity. Death can result from ingestion of cardiac glycosides or cyanide containing plants, as previously discussed. Plants that cause death within a couple hours include Yew (Taxus spp), Poison, spotted, or European hemolock, Death Camas (Zigadenus spp). Avacado peels should never be fed to horses nor should horses be pastured in next to Avacado trees. An unknown toxin derived from this plant causes death following episodes of colic (see above) after a couple days. Avacados are deadly to all large animal species, and goats die within 48 hrs.