A look at plants that are toxic to horses …
Horses will avoid toxic plants providing they have sufficient grazing, but if they are very hungry, they will graze on almost anything that appears edible.
Plants Toxic To Horses
Horses can also accidentally ingest toxic plants and they can acquire a taste for some, such as acorns and dried ragwort in hay.
Younger and the more curious horse’s are more likely to sample their environment
And at certain times of the year toxic plants can be more lush and attractive than the native grasses which surround them.
Horses under stress can also be more susceptible to relatively low levels of toxicity.
Plants you may consider a nuisance near your home may be quite toxic near your horse.
Hungry horses 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 can have fatal consequences.
Some plants, such as oleanders, are so toxic that eating only a few leaves can result in a horses’ death.
Even the smoke from burning oleander trimmings can be harmful.
Locoweed is addictive and 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.
The more common plants toxic to horses are…
Table 7i.hh. Plants Toxic To Horses
Yew
The yew is a small evergreen tree and all parts of the yew are highly 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.
Oleander
Oleanders are a common ornamental shrub, often growing along roadways and used as a visual barrier or wind screen. All varieties are extremely toxic.
The first signs of oleander poisoning will usually be profuse diorrhea 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.
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 and from asphyxiation.
Lupine
Lupines involve a large genus of plants, many species of 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 plants from the non-toxic, so you 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.
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 USA. Found in 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.
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
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.
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.
Bermuda Grass
Bermuda 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.
Horsetail – Mares Tail
Horsetail, also called mares tail and scouring rush, poisons the horse in a similar fashion to bracken fern. All varieties of Horsetail are poisonous and they are often found near bogs and streams … More on Horsetail
Your responsibility for your horse …
Make sure you find out what plants that are toxic to horses are present in your area and in the areas where you ride.
Studies by the Tufts University of Veterinary Medicine suggest that horses kept in stables and dirt paddocks are the most likely to sample anything green and ingest toxic plants.
Dr. Gorden DeWolf suggests making sure that your horse has plenty of roughage and has satisfied his need to chew before allowing your horse to graze on green grass as much as possible during the growing season. Let your horse develop natural discriminating habits when grazing and it will be more likely to search out healthy grasses and avoid plants toxic to horses.
If you suspect any kind of poisoning, call your vet immediately. In many cases time is of the essence and any delays can significantly reduce your horses chance of survival.