(c) 1997, Willis Lamm, TrailBlazer Magazine

"Poisonous Plants"
By Willis Lamm

Reprinted with permission of TrailBlazer Magazine for non-commercial use.

A few years ago my wife, Sharon, and I were visiting some friends when they asked us to walk down the road and look at a neighbor's horse. We found a middle aged horse on a very poor pasture who was hanging its head and drooling. A closer look revealed a swollen tongue.

"Has anyone called the vet?" we asked.

"The owner said she would if the horse didn't get better," was the reply.

Looking around at the number of yellow star thistle plants in the pasture, many of which showed signs of nibbling, our response was "It's probably too late already." Sure enough, a few days later we learned that the vet finally was called and they had to put the horse down.

On the drive back home Sharon and I were struck by the number of horses in other pastures we viewed in which yellow star thistle grew; some literally choked with the toxic weed.

About the same time we learned of a well meaning gardener who would dump grass trimmings over the fence for his neighbor's horses to eat. The horses would eagerly line up for their "snacks" when they heard the lawn mower running. Unfortunately one day the gardener mowed over an area where some oleanders had been pruned, shredding and bagging a number of oleander leaves in the process. A short time after their snacks, all the horses were dead.

Most horses tend to avoid toxic plants, however sometimes they don't, either because they don't recognize them, they are especially hungry, or they accidentally ingest them. Younger and more curious horses are more likely to "sample" their environment, and at certain times of year toxic plants can be more lush and attractive than the native grasses which surround them. Horses under stress can be more susceptible to relatively low levels of toxicity. With many types of plant poisonings, one's first indication of a problem may be the discovery of a dead horse or "terminal" symptoms. In addition, some toxins are stored in the horse for life, the level increasing each time the horse ingests the toxic plants and the effects only becoming visible when the horse has finally accumulated a lethal level of the toxic agent. Thus, it simply makes sense to be aware of the more dangerous toxic plants your horse may encounter and avoid them.

Many toxic plants are not "equally poisonous" when growing under different conditions. Plants which may only be considered a nuisance near your home may be quite toxic in another locale. Your horse's appetite may also play a significant role. Hungry horses tethered or turned out under oak trees have been known to eat huge quantities of acorns, which are generally not a problem when consumed in small quantities, and the sudden loading of such unusual food had fatal consequences.

Other horses may simply have a morbid penchant or acquire a taste for some toxic species. I have one horse who for some reason will nibble on yellow star thistle any chance he gets. Thus I have to be extremely careful where I turn him out and when I let him graze during a trailside rest stop. Mineral deficiencies can also cause horses to sample plants normally outside their traditional menu. Thus a balanced diet and proper nutrition can play an important role in poisoning prevention.

Some plants, such as oleanders, are so toxic that only a few leaves can result in a horse's death. Even the smoke from burning oleander trimmings can be harmful. Locoweed is addictive. Most horses tend to avoid it, but once they have sampled it a few times, they often develop a craving for it and seek it out, eventually suffering neurological damage as a result.

A few of the more common toxic plants are listed below.


The yew is a small evergreen tree found in the Pacific Northwest. All parts of the yew, along with many of its cousins, are extremely toxic and contain taxine, a cardiac depressant. Symptoms include trembling, a slowing of the heart rate and cardiac failure which can occur in as little as five minutes after ingesting the plant. Some deceased horses were found with yew leaves or twigs still in their mouths. There is usually no time to initiate treatment and there is no known antidote.


Oleanders are a common ornamental shrub in the west and south, often growing along roadways and used as a visual barrier or wind screen. All varieties are extremely toxic. The horse's first signs of oleander poisoning will often be profuse diarrhea which may be bloody. Immediate veterinary intervention and a lot of good luck is required to prevent death, which often occurs within 8 to 24 hours after ingestion. Administration of laxatives to purge the remaining oleander from the horse and prompt treatment of symptoms which the horse presents can sometimes save the horse.

Yellow Star Thistle

Eating yellow star thistle and Russian knapweed brings on nigropallidal encephalomalacia, commonly called "chewing disease". Typically horses eat the plant over an extended period of time before symptoms appear. The ingested toxins actually cause a softening of parts of the brain and once symptoms appear, the animal will likely die.

The horse will not be able to eat or drink, although he may attempt to do so. He may chew food only to spit it out when he cannot swallow, often acting as if something is caught in his throat. (Symptoms resemble the early onset of rabies, so you should consider rabies also as a possibility and not go reaching into the horse's mouth to look for obstructions.)

The horse may chew without having anything in his mouth, move his tongue and lips in unusual fashions, yawn frequently and assume unusual postures such as head hanging or pushing against solid objects.

Because this disease damages the brain and prevents the horse from eating, there is no recovery. Clinical experiments where poisoned horses were fed through stomach tubes have shown that even after extended periods of time, affected horses will not regain the ability to swallow.


Locoweed is a common name for plants which fall into two genera; Astraglus and Oxytropis. While there are hundreds of species of these plants, only about 20 are considered poisonous. These plants range from low growing ground covers to two foot tall clumps of flowers. They can be found throughout the west from Canada to Mexico in semi-arid foothills and plains. Some of these species are very similar to each other, so even experienced botanists can have difficulty differentiating between a poisonous and non-poisonous specimen.

Horses typically avoid locoweed, but once they have sampled it a few times, they can become addicted to it. They have to graze on it for a period of time before symptoms appear and the most obvious symptoms may not appear until well after the horse has stopped eating it. These symptoms include altered gaits, aimless wanderings, sometimes in circles, impaired vision (to the extent they bump into things or fall into arroyos or other depressions) and erratic behavioral changes. They may appear listless or complacent, then wildly overreact to some unexpected event.

Locoweeds can have such an alkaloid content that one Nevada species has even poisoned the bees which were pollinating the plants. Alkaloid poisoning has a cumulative effect which can be absorbed over long periods of time until symptoms appear and the effects in many cases are irreversible. Horses found eating locoweeds who have then been confined away from the plants prior to showing advanced symptoms, and who have been fed good quality hay and feed, have experienced a slow but successful recovery.

Timber Milk Vetch

Like the Locoweeds, this group of plants falls into the genus Astraglus although the symptoms of poisoning are different and their onset is much more rapid. When excited, the horse will often produce a roaring sound when exhaling. Other symptoms include salivating and staggering. Death is usually sudden from asphyxiation.


Lupines involve a large genus of plants, many species which are not poisonous. They typically display bunches of white, blue, purple or pinkish pea-shaped flowers on upright stalks. They are found in a wide variety of landscapes from ocean beaches to high mountain pastures. Like locoweeds, it is difficult for even the experts to distinguish the toxic from the non-toxic, so one should assume that all lupines are harmful if eaten.

While poisonous throughout the year, young lupines and those going to seed are the most toxic. Fortunately the effects are not cumulative so a lethal dose must be consumed over a short period, otherwise if removed from access to lupines and if symptoms are properly cared for, a poisoned horse should recover.

Symptoms include gastrointestinal irritation accompanied by diarrhea. The horse's gait may change; the horse being reluctant to move and lifting his feet higher than normal when he does, acting nervous and displaying leg twitching. Loss of muscle control, prostrations, convulsions and coma may follow. While the "one time ingester" of lupines may completely recover, continuous consumption can produce toxic hepatitis. This can result from lupines being baled in hay cut from poorly managed fields and being fed to horses over time. Other than separating the horse from the toxic plants and treating the visible symptoms, there is no published treatment for lupine poisoning.

Poison Hemlock

Poison hemlock, a relatively common range plant, kills a number of horses sheep and cattle each year and is extremely toxic to humans. All parts of the plant, commonly known as deadly hemlock, spotted hemlock, poison parsley, European hemlock, California fern and Nebraska fern, are poisonous. In the spring until the plant flowers, the leaves are especially poisonous.

Poison hemlock is often found along roadsides and creek beds. Humans are poisoned when they confuse poison hemlock roots with wild parsnips, or the seeds as anise seeds. This plant is so deadly that whistles made from the plant's hollow stems have been known to kill children. Fortunately, the toxin is relatively neutral after the plant has cured, so poison hemlock accidentally baled into properly dried hay has not posed a problem.

Symptoms of poisoning will appear within a few hours after ingestion and may range from sudden, unexpected collapse to nervous trembling, salivation, incoordination, especially in the hindquarters, and dilated pupils. The horse's pulse may be weak and he may appear cyanotic (bluish appearance of the mouth and gums), and he may be disoriented or unaware of his surroundings.

In serious cases, death occurs within ten hours of the onset of symptoms due to respiratory paralysis. Less serious poisonings, which still may include periods of coma, can sometimes benefit from early use of stimulants and stomach tubing with mineral oil. If respiratory shutdown can be avoided and the horse can pass any remaining ingested leaves, full recovery is possible without any long term ill effects.

Water Hemlock

All parts of the water hemlock plant, also known as western water hemlock, contain a toxin called cicutoxin. Some consider this to be one of the most poisonous plants in the US. Frequenting moist areas such as wet meadows and pastures or the banks of ponds or streams, it resembles the larger poison hemlock, but only reaches a height of two to three feet.

Tuberous roots and immature shoots and leaves are particularly toxic and only a small amount is needed to poison a horse. The plant seems more attractive to horses after being sprayed with the herbicide 2,4-D. Common names include false parsley, snakeroot and poison parsnip.

Symptoms include muscle tremors which can develop into violent convulsions and respiratory shutdown. Early signs such as excessive salivation and frothing may occur as quickly as 15 minutes after ingestion. Convulsions can be extremely severe, with head and neck thrown back, legs flexed as if running and abdominal pain is generally present accompanied by an associative grinding of teeth.

Coma and death usually follow and there is no known treatment. The toxins act quickly and horses are rarely saved, however horses which make it through the first five to six hours after the onset of symptoms have a good chance of survival.

Ground Ivy

Ground ivy, commonly called Creeping Charlie, is present throughout much of North America. Horses must ingest relatively large amounts for fatal consequences to occur, and such events are commonly traced to the plant being baled into the horse's feed.

When ground ivy is present or suspected, consider unusually severe sweating accompanied by frothing at the mouth and / or difficulty breathing as signs of potential ground ivy poisoning.

Larkspur (Delphinium)

In the Western US, larkspur is the number two cause of livestock losses. Under natural conditions, horses will sample larkspur but will not eat enough to kill themselves if other feed is available. Delphiniums planted in ornamental gardens should be considered as toxic as their wild counterparts and the same precautions should be taken as with oleander.

Bracken Fern

This plant, also known as the brake fern, is common in wooded areas of the Pacific Northwest. Horses generally avoid it, but some can acquire a taste for it. Toxicity is cumulative and generally symptoms appear after the horse nibbles on this plant repeatedly over a long period of time. Early symptoms include weight loss which can progress to unsteady walking, then staggering with the horse spreading with all four feet to stay balanced, often pressing his head into solid objects. If untreated, death will occur from several days to several weeks after the symptoms appear.


Horsetail, also called mare's tail and scouring rush, poisons the horse in a similar fashion as bracken fern. All varieties of Horsetail are poisonous and they are often found near bogs and streams. The only reported difference in symptoms from bracken fern poisoning is that with Horsetail the animal may become quiet, unresponsive or comatose prior to death.

Castor Bean Poisoning

The castor oil plant, or palma christi, is grown as an ornamental plant in California and many southern states. It contains ricin, which causes severe irritation to the intestinal tract. (Castor oil is non-toxic because ricin is not soluble in oil.) The seed is the part of the plant which is poisonous - to all animals. As little as 7 grams of seeds have been reported to kill a horse although it is generally considered that about 50 grams (about 150 beans) are necessary to kill a healthy 1000 lb. specimen.

Symptoms may not appear until two to three days after ingesting the beans. When signs do appear they are generally acute and progress rapidly. The animal may act doped up and lose coordination, followed by profuse sweating. Signs of shock are not uncommon. Neck and shoulder spasms may appear accompanied by an extremely profound but ineffective heartbeat which can be easily felt, but which produces a weak and rapid pulse. Early on a temperature may be present up to 107 degrees F (41.5 C). Eventually a profuse, watery diarrhea appears often accompanied by colic-like pain. Finally the horse may go into convulsions and die.

Red Maple

Red Maples are natives in the eastern US and can be found as ornamental specimen trees in many other areas. The dried leaves and bark of the red maple can produce significant anemia in the horse when eaten. Symptoms include general weakness, and increased respiratory and cardiac rates indicating the animal's attempt to compensate for the anemic condition.

Please note: Some nurseries have crossed silver maples with red maples to produce more color. These hybrids are also toxic to horses. Check with your nurseryman to make sure you are planting true silver maples!


Buckwheat contains a pigment called fagopyrin, which when ingested by the horse, causes photosensitive dermatitis. Symptoms include a weepy, itching dermatitis in those areas exposed to sunlight.

Alsike Clover

Hormones in this plant can cause photosensitization of the skin and hypertrophy of the liver (big liver disease). Visible symptoms include increased sensitivity of the skin (especially the nose and lips) to sunlight.

Rhododendrons and relatives

These plants, along with azaleas, laurels and mountain pieris contain grayanotoxin. Symptoms include an excess of green, frothy salivation which is generally associated with gastrointestinal irritation and colic.

Potato and Tobacco Leaf Poisoning

Nicotine and its related compounds are toxic to horses. The stems and leaves of many types of potato plants contain high concentrations of this alkaloid, as do the wild varieties of tobacco which grow in the western United States and Hawaii. Horses have also been known to be poisoned by domestic tobacco which has been harvested and within their reach, typically when stored in barns where they are stabled.

Nicotine affects the autonomic nervous system. In minor cases the horse may shake, shiver or twitch, particularly around the neck and shoulders. As the symptoms advance, staggering, prostration and paralysis may be evident. The heart may beat violently but produce a weak, rapid pulse. The horse may show an elevated temperature, yet the extremities will feel cold. Sometimes colic and / or labored breathing may be present.

Severe cases will usually produce a rapid onset of symptoms, followed a few minutes later by death, although some horses have been known to struggle with nicotine effects for up to several days. There is no known treatment for nicotine poisoning.

Fiddleneck and related plants

Several members of the amsinckia species have been linked to causing cirrhosis of the liver. These plants include fiddleneck, tarweed, yellow burr weed, fireweed and buckthorn, which are predominantly found in the semi-arid regions of Idaho, Washington, Oregon and California. The plants appear in late winter and early spring. The seeds are the most toxic parts of these plants and fortunately the mature plants are unpalatable to most horses. Most instances of poisoning occur when mature amsinckia species are bailed in early cuttings of hay or when contaminated grain was threshed for horses. Some poisonings have occurred when horses were pastured on wheat stubble in contaminated fields.

Cirrhosis causes hardening and eventual metabolic shutdown of the liver. The liver can no longer filter out toxic wastes which, among other things, produces disturbances of the nervous system. Affected horses have been known to walk aimlessly for miles, running into objects in their path rather than go around them, pressing their heads into solid objects and occasionally becoming frenzied when they cannot continue. Oftentimes death results from the horse getting hopelessly entangled in a fence or literally walking over the edge of a cliff or ravine.

Other behavioral abnormalities include recklessness, charging, lack of coordination, circling, staggering, "dragging" of the hind limbs, which have been described as the "sleepy staggers". The horse may also appear listless, hang its head and acting sluggish or depressed. Signs of colic may be present, which may include straining, diarrhea and rectal prolapse. Small foul smelling ulcers may appear in the mouth.

The chronic patient will become a poor keeper, show yellow membranes around the eyes and mouth, produce a rough coat and eventually become anemic and die. Fortunately healthy horses need to consume amsinckia plants over a period of time before enough liver damage occurs to produce symptoms. Unfortunately, once cirrhosis of the liver develops, it is for all practical purposes irreversible.

Crotalaria Species

Crotalaria, predominantly found in the south and southeast, has been the cause of many horse losses. Known as wild pea, rattle box and rattle weed, crotalaria has been intentionally planted as an agricultural cover crop to enrich the soil from the Atlantic seaboard, west into Texas. Two species, crotalaria sagittalis and crotalaria spectabilis, are particularly toxic and produce the same effects as fiddleneck.

Senecio Species

Senecios comprise one of the largest genre of plants in the midwest and western US. Not all species are poisonous, and of those which are, only a few contain enough alkaloids to cause problems in horses. Those which do can produce the same kind illness as fiddleneck. Senecio jacobaea is particularly toxic. In the Nebraska region, "walking disease" is caused by senecio. In the Pacific Northwest, the disease is commonly called "Hard Liver Disease" or "Walla Walla Walking Disease."

Other species such as ragwort, common groundsel and "Stinking Willie" are generally considered toxic, however their alkaloid content seems to be less than senecio jacobaea.

Chokecherry and Wild Cherry (Prunus)

Chokecherries, growing in bushes up to 12 feet high, are popular for their jelly producing berries. They are common throughout the US, often found along roadsides or creek bottoms. Unfortunately the leaves, which are particularly toxic when stressed or wilted, as well as the bark from chokecherries and wild cherries are cyanide producing.

Death in horses can occur literally in minutes after the horse has ingested the leaves. The horse will appear to have trouble breathing, show flared nostrils and lose bowel and urinary control. Lack of coordination and trembling may also appear, along with agitation. A severely poisoned horse will drop to the ground, kick a few times, then die.

Poisoned horses can be saved, however usually veterinary help cannot arrive in time as the effects of cyanide poisoning progress rapidly.

Sorghum and Sudan Grass

Sorghum and Sudan Grass, both of which can be effective as livestock feed when grown, harvested and cured correctly, can produce cyanide poisoning when improperly managed. After a hard frost or trampling, prussic acid can build up in new growth which grazing horses are likely to seek out. The effects of this poisoning is the same as with choke cherries.

Other problems associated with grazed or improperly baled sorghum and sudan grasses include urinary tract complications, causing thick and viscous urine and bladder infections. Signs of such problems may appear as buildup inside the horse's hind legs. If left untreated, the infection can become fatal. Pregnant mares may abort or give birth to deformed foals.

There are species of sorghum which are developed especially for animal feed, and we have fed them with great success. However, when feeding sorghum to horses, one should be certain what kind of sorghum is being grown and that the grower knows how to properly manage, cure and bale the crop. Unless you really know what you are doing, letting your horse graze on growing sorghum or sudan grass can be a risky proposition.

Bermuda Grass

Brumuda grass can be good feed for horses, however in certain climates a harmful fungus called ergot can be present and which appears as small brown or black nodules on the bermuda grass or dallis grass seed heads. When consumed, a condition known as "Bermuda Grass Shakes" or "Dallis Grass Tremors" can occur, producing such symptoms as lack of coordination, tremors, strange head movements and tongue rolling, and in severe cases, paralysis.

Once the tainted forage is discontinued, horses may recover rapidly, virtually overnight to several days. Pregnant mares, however, may abort.

Black Locust

Horses tied to black locust trees or black locust posts and who have chewed on the bark can become poisoned, becoming very ill in just a few hours. Symptoms include loss of appetite, general weakness and depression. Symptoms of a mild colic may also be present. Horses can ingest enough bark to prove fatal, although most recover after several days or weeks.

Oak Trees

Horses have been known to binge on acorns, particularly if they are hungry and are not used to having them around. Acorns and many oak leaves are high in tannin. It is relatively easy for a horse to ingest several pounds of acorns in a relatively short period of time leading to an unfortunate overdose.

Common Sense

While this listing is by no means complete, it clearly points out that poisonous plants are everywhere. When a horse is introduced to an unfamiliar environment, particularly when there is not plenty of natural grass to graze, one must be observant as to what plants are present and monitor the horse's eating habits. In addition, a hungry horse may sample all sorts of plants which might otherwise not interest him.

Find out what toxic plants are present in your area and in those areas where you may "ride and tie" and allow your horse to feed itself during the trip. A call to your state university's Cooperative Extension should yield a more thorough listing of toxic plants common to your region.

Studies by the Tufts University of Veterinary Medicine suggest that horses kept in stalls and dirt paddocks are the most likely to sample "anything green" and ingest toxic plants when given the opportunity. Dr. Gorden DeWolf suggests making sure that your horse has plenty of roughage and has satisfied his need to chew and then allowing your horse to graze on green grass (for modest periods in an area free from toxic plants) as much as possible during the growing season. The idea here is to let the horse develop natural discriminating habits when grazing whereby he will more likely seek out healthy grasses and avoid harmful forage.

If you do suspect any kind of poisoning, call your veterinarian at once. In many cases time is your enemy and any delays can significantly reduce your horse's chances of recovery or survival.

If the symptoms involve bizarre behavior or some kind of brain disorder, consider the possibility that the problem may be rabies instead of poisoning and avoid compromising your own safety until you know for sure what is going on.

The National Animal Poison Control Center has a toll-free animal poison hot line. It is 1-800-548-2423 and for a $30.00 fee they will provide advice for treating suspected poison cases. (This fee includes follow-up calls and consultations with your veterinarian, if necessary.) They also have available a booklet entitled "Natural Poisons in Horses" which can be purchased by contacting NAPCC, University of Illinois College of Veterinary Medicine, 2001 So. Lincoln Ave., Urbana, IL 61801.

Also, most of these plants can be viewed at the University of Illinois Plants Toxic to Animals data base. Please remember that plants not toxic to horses may be toxic to sheep and cattle, and vice versa.

Our thanks to TrailBlazer Magazine for permission to post this series on our web page.
You can visit the TrailBlazer website at www.horsetrails.com.

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