Poisonous Plants- A primer to keep your horse safe.

 We have tried to research the best sources for information about and photos of poisonous plants. Use it as a guide to help making your pastures safe. Most of the time, a horse's educated nose and palate cause it to shun poisonous plants. (See disclaimer below.) Although most poisonous plants taste bad, hungry horses in a barren pasture might eat them out of desperation. Milkweed, yew, rhododendron, buttercups, azalea, nightshade, even potatoes -  can be deadly to horses.

Some plants respond to environmental stress, such as a hard freeze or a drought, by sharply increasing their toxicity. One of those plants is poison hemlock, which grows in abundance along fence rows, creeks and roadsides.

Some scientists speculate that poison hemlock might have caused spring's foal losses in Kentucky after a hard freeze and 1-inch snowfall one spring made the toxic plant the only lush vegetation available for pregnant mares to eat.**

Recognize Toxic Plants

Hundreds of plants are toxic to horses, some more dangerous than others. Some plants poison horses over time by depositing low levels of toxins in the horse's body. Subtle symptoms of this poisoning could go unnoticed until the horse's liver is nearly destroyed. Plants that are highly toxic can cause a reaction within hours of ingestion; a few of the most deadly plants can cause sudden death.

Experts emphasize that anytime a horse could have eaten a poisonous plant, you should immediately seek veterinary help. Plants that do not kill can cause colic, anemia, diarrhea or neurological problems.

Below are a few of the many toxic plants that affect horses. For more information about poisonous plants in your area, consult your local agricultural extension service or veterinarian.

Azalea or Rhododendron: Perennial shrubs that have tough, glossy, evergreen leaves and produce large, showy flowers with five white, pink or red petals. Toxin causes gastric and cardiac distress. Within six hours of ingestion, a horse can appear colicky and have tremors, along with cardiac arrhythmia. If enough of the plant is ingested, the horse could collapse and die. Treat with supportive therapy.

Black Locust: Tree found throughout the eastern United States and south-eastern Canada bearing sweet-smelling clusters of white flowers. Toxins affect the gastrointestinal tract and nervous system. As soon as one hour after consumption, your horse could exhibit depression, anorexia, weakness, abdominal pain, diarrhea and cardiac arrhythmia. Death can occur from consuming significant amounts of toxin. Veterinary procedures to rid intestines of toxin are indicated.

Bracken Fern: (Pteridium aquilinum) Three-foot-tall plant that has broad, triangular fronds that cause a vitamin B1 deficiency that leads to weight loss, muscle tremors and death when ingested for one to two months. Found throughout North America in barren pastures and acid-soil woodlands.

is one of the more common plants especially on some hilly or moorlands ground.
This is poisonous while green and remains so if cut in the green state, dried and stacked. The roots or (rhizomes) are said to be five times more poisonous than the fronds and this should be remembered if ploughing or digging land on which bracken is growing.

The toxin in brackenfern is thiaminase, an enzyme that destroys thiamine (vitamin B1). The horse then essentially suffers from a vitamin deficiency of thiamine, which causes myelin degeneration of peripheral nerves ( a loss of the fatty insulation layer to nerves that primarily control muscles). Poisoning can occur at any time of year, but is more likely in the late summer between August and October when other forages are scarce and the level of thiaminase is at its peak. Bracken is not considered palatable, but horses will eat it if no other forage is available, or they will consume it in hay or bedding, where it remains toxic. Some horses are believed to acquire a taste for it, and these horses will consume it even if other forages are available.

Horses need to consume bracken for one to two months prior to manifesting clinical signs. After this time horses may then be fed bracken-free forage and yet still develop clinical signs within 2 to 3 weeks. The first signs in horses is weight loss after a few days on bracken. Later, weakness and gait abnormalities are present, which progress to staggering, hence "bracken staggers". Affected horses may stand with their legs widely placed and their back arched. Muscle tremors and weakness is apparent when the horses are forced to move. Early in the course of the syndrome, a slow heart rate and abnormalities of the heart rhythm may be noted. Near the end of the clinical course, the heart rate and temperature rise, and the animals cannot get up and may have spasms and an upward arching of the head and neck. The syndrome runs its course, with death occurring within 2 to 10 days of the onset of signs.
First signs include:

In the advanced stages the horse may experience:

• poor co-ordination
• restlessness
• severe muscle tremors
• gait abnormalities
• difficulty in moving and turning during exercise
• staggering (bracken staggers)
• raised pulse and heart rate
• convulsions
• unable to rise

Horses can be treated if caught in time. If recognized early, symptoms can be reversed with daily injections of vitamin B1 for two weeks.

Buttercups: (Ranunculaceae family) Leafy green plants that support stems bearing five to seven yellow, cup-like flowers. Ingestion of any part of the plant causes protracted, bloody diarrhea and severe blistering of the mucous membranes lining the entire gastrointestinal tract.

Frequently found in horse grazed pasture. Buttercups are potentially poisonous because they contain a compound called protoanemonin, which is a powerful irritant, causing inflammation or ulceration of the mouth, often with sorenes, increased salivation and sometimes colic. However this usually causes the horse to stop eating which makes this condition self-limiting, although it is always advisable to contact your vet if you are concerned.

Cockle-

Weed	Where generally located	Livestock affected	Symptoms
COCKLE *	pastures cultivated fields roadsides waste areas	horses cattle	restlessness grinding of teeth drooling colic diarrhea rapid breathing weak pulse coma death

Cocklebur:

Weed	Where generally located	Livestock affected	Symptoms
COCKLEBUR *	cultivated fields stream banks beaches farm yards	horses cattle sheep	symptoms appear within a few hours weakness unsteady gait twisting of neck muscles depression nausea vomiting labored breathing rapid, weak pulse death

Groundsel, Tansy Ragwort, Fiddleneck and Crotolaria:
Plants containing pyrrolizidine alkaloids that cause liver damage. All bear orange or yellow flowers and different species can be found growing in fields and along roadsides throughout the United States. Ingestion for a week to several months can result in death. Horses become unthrifty and walk in circles or press their heads against fence posts or buildings. If liver damage is not extensive, horses often respond to a low-protein diet supplemented with B vitamins.

Hemlock: -Not to be confused with the Hemlock coniferous evergreen tree- (Conium Maculatum, also called Spotted Parsley, Ciguë Tachetée, Spotted Cowbane, Poison Parsley, St. Bennet's Herb, Bad Man's Oatmeal, Wode Whistle, Cashes, Bunk, Heck-how, Poison Root, Spotted Hemlock, Spotted Conium, Poison Snakeweed, and Beaver Poison) is notorious as the poison used as a capital punishment in ancient Greece. The Greek philosopher Socrates was sentenced to death and died in 399 B.C. after drinking Hemlock juice. Its a very pretty "lacy" plant. Death by Hemlock creeps up from the lower body,  until suffocation occurs. The name Hemlock is derived from the Anglo-Saxon words "Hem" (meaning border or shore) and "Leác" (meaning leek or plant). Another authority derives the British name "Hemlock" from the Anglo-Saxon word "Healm" (meaning straw), from which the word 'Haulm' is derived. A very deadly plant indeed.

Two types of hemlock are toxic - water hemlock and poison hemlock. Plants range from 6 inches to 8 feet tall. Leaves of both plants are delicate, like parsley. White, lacey flowers similar to Queen Anne's Lace emerge in mid-summer. Poison hemlock can be distinguished from water hemlock by small purple spots on the hollow stem. Both plants are prevalent throughout North America:

•Water hemlock grows in wet seepage areas of meadows, pastures and streams. It is considered the most violently toxic plant in North America. Ingestion of a small amount of the plant is often fatal. Livestock usually show signs of poisoning 15 minutes to six hours after they eat the plant; they could die within 15 minutes to two hours after signs appear. Most animals die as a result of the asphyxia and cardiovascular collapse that occur during convulsions.

•Poison hemlock grows in moist areas along fence lines, creek banks and roadsides. The toxin in poison hemlock is different from and less lethal than that in water hemlock. Four to 5 pounds of poison hemlock leaves are required for a lethal dose. However, the plant is well documented to cause abortion and birth defects in lower doses. Symptoms of poisoning include trembling, muscular weakness and incoordination. If a lethal dose is not ingested, the horse may simply lie down until weakness subsides and then fully recover.

Jimsonweed, Jamestown Weed, Devil's Trumpet or Stinkweed: Stout stem holds prickly, saw-toothed leaves and large, trumpet-shaped flowers that range from white to purple. Hard seed pods are round, green and spiny. Jimsonweed is found throughout North America in trampled pastures and barn lots. Plants contain scopolamine, which causes weak, rapid pulse and heartbeat, dilated pupils, in-coordination, coma and possibly death. Rapid onset of symptoms occurs after ingestion. Treatment with cardiac and respiratory stimulates can help.

Weed	Where generally located	Livestock affected	Symptoms
JIMSONWEED*	cultivated fields farm yards	cattle horses sheep goats	dilation of the pupils impaired vision fast, weak pulse nausea loss of muscular coordination violent, aggressive behaviours trembling milk is tainted

Lamb's Quarters and Pigweed***:

Under certain adverse environmental conditions (drought) many weed and crop plants accumulate nitrate to potentially toxic concentrations.  Nitrate-accumulating weeds include pigweed (Amaranthus spp.), lambsquarter (Chenopodium spp.), dock (Rumex spp.) and nightshades (Solanum spp.).  Excessive nitrate may also contaminate sorghum, and grain crops. Nitrate is also found in fertilizers and is a common contaminant of water.  Thus, exposure to these sources can cause intoxication if exposure is of sufficient.

Toxic principle:  Nitrate is reduced in the rumen to nitrite which is the ultimate toxin.NO₃  is turned into- NO₂

Toxicity:  as a salt, nitrate is toxic for ruminants at 0.5 g/kg (single oral dose).  Forages containing > 0.2% nitrate and water containing > 1000 ppm are potentially toxic.  Plants can accumulate 3 to 4% nitrate under appropriate conditions.  Nitrate is not very toxic for monogastrics since it is not efficiently reduced to nitrite.  However, nitrite is toxic for monogastrics.  Unlike cyanide, nitrate does not volatilize and therefore dried forages are toxic.  

MOTA:  the iron in hemoglobin is oxidized from ferrous to ferric iron.  This results in the formation of methemoglobin.  Methemoglobin has significantly reduced oxygen carrying capacity.

Diagnosis: Clinical signs: dyspnea, sudden death, “muddy” mucous membranes, “brownish” appearance to blood.

             Laboratory diagnosis: significant methemoglobin, high serum, ocular fluid or other body fluid nitrate concentration (> 20 ppm in serum or body fluids, > 50 ppm in ocular fluid).  Measurement of high levels of nitrate in plants or water.

            Lesions:  “brownish” discoloration to blood, muscles.

Treatment:  directed at reducing methemoglobin to hemoglobin. Remove from source.  A 1% solution of methylene blue is generally given at a dose of 4 to 15 mg/kg at 4 to 6 hour intervals.  Methylene blue is reduced to leukomethylene blue, which in turn reduces methemoglobin.

Prevention: Test forage prior to feeding.  High nitrate forage turned to silage may lower nitrate concentrations to acceptable levels.

Locoweed: Horses are prone to graze on wooly loco and garboncillo - two variations of locoweed. Wooly loco is found primarily in the western United States - South Dakota, Arizona, New Mexico and Texas. It has low vegetation with blue, purple, yellow or white flowers growing in clumps in the foothills and semi-arid regions of the western United States. Grazing causes chronic intoxication, depression, emaciation, incoordination and abortion. Horses can recover from poisoning, but brain deterioration can make them dangerous to ride.

Milkweed: Tall stems that ooze milky sap produce pods containing silky, hair-encased seeds. Prevalent throughout North America in dry, waste areas and along roadsides, milkweed causes staggering, rapid and weak pulse, coma and death within hours of ingestion. Sedatives, laxatives and intravenous fluids might ease symptoms.

Weed	Where generally located	Livestock affected	Symptoms
MILKWEED *	dry, open areas pastures around woods roadsides waste areas cultivated fields	cattle sheep	loss of appetite constipation drooling excitable difficult breathing rapid, weak pulse convulsions death
		horses	persistent colic

Mountain Laurel***: Kalmia latifolia L- Dense, woody, round-topped shrub or small evergreen tree. Flowers are rose to white with purple markings. Toxin causes cardiac distress. Affected animals exhibit incoordination; watering of the eyes, nose and mouth; irregular breathing; weakness; convulsions; coma; and death.

Nightshades: A relative of the Tomato plant, Nightshades have triangular stems that bear thin, oval leaves and small yellow, red or black fruit that resemble tomatoes. Ingestion of leaves or berries will cause bloody diarrhea, weakness, salivation, trembling, progressive paralysis and death. Gastrointestinal protectants tends to ease symptoms.

Weed	Where generally located	Livestock affected	Symptoms
NIGHTSHADE *	open dry woods cultivated fields pastures fence rows waste areas farm yards	cattle horses   sheep goats	abdominal pain stupidity dilation of pupils loss of appetite diarrhea loss of muscular coordination unconsciousness death

Horse nettle is from the family Solanum spp .This is a large genus of plants with over ~ 1500 species distributed worldwide.  Various species are found throughout the U.S.  The ones of particular concern in the U.S. include Solanum nigrum (black nightshade), S. dulcamara (bittersweet nightshade), S. tuberosum (potato) and S. carolinense (horsenettle) which has courser stems that bear white to purplish flowers and yellow fruits, but it rarely poisons horses.

Toxic Principle:  there are several glycoalkaloids (alkaloids + sugars) that are potentially toxic.  A prototypical glycoalkaloid is called solanine (sugar [solanose] + alkaloid [solanidine] = solanine).  The akaloidal portion of the glycoalkaloid is also generically referred to as an aglycone.  The intact glycoalkaloid is poorly absorbed from the GI tract but causes GI irritation.  The aglycone is absorbed and is believed to be responsible for observed nervous system signs.

toxicity of a given species can vary widely due to environmental conditions, part of the plant or degree of maturity.  Unripe berries are more toxic than ripe berries.  Berries are more toxic than leaves which, in turn, are more toxic than stems or roots.  Overall plant glycoalkaloid content is often higher in the autumn than in the spring. 

Clinical signs: the symptoms observed in a given case will depend on the balance of the irritant effect of the intact glycoalkaloid vs. the nervous system signs caused by the aglycone.  GI signs include anorexia, nausea, salivation, abdominal pain, emesis, constipation or diarrhea (with or without blood).  Nervous system signs include apathy, drowsiness, progressive weakness/paralysis, prostration and unconsciousness.  Nervous signs build to a maximum followed by death or recovery within 1 to 2 days. 

Laboratory diagnosis: although not routinely available, detection of alkaloids in tissues or urine is possible.

Lesions:  gastroenteritis, detection of plant fragments in GI tract.

Treatment:

• GID

• Demulcents

• Fluids 

Oak (Quercus Spp) Poisoning by oak is usually seasonal, being most common in spring when the young buds or leaves are eaten and the autumn when the acorns are eaten. Oak leaves and acorns contain tannic acid which is poisonous to horses and though eating a small number of leaves or acorns is almost certainly harmless, they can also be addictive, and once a horse has acquired a taste for them they can actively search them out. Also some animals seem to be more susceptible to oak poisoning than others with individual animals having different levels of tolerance.
Oak poisoning causes gastroenteritis and kidney damage.
Symptoms include:

• lack of appetite
• staring coat
• constipation followed by diarrhoea which may be bloodstained
• abdominal pain
• depression
• blood in urine

There is no antidote. The horse is treated with drugs to reduce the pain and control the diarrhoea, antibiotics may be prescribed.
Prevention: In general it is best to restrict the access of horses to acorns, particularly if other food is scarce,or else pick up the fallen acorns at least once a day - although this method is time-consuming and less effective as most horses will still find some. The best thing to do is fence off oak trees - either permanently or with electric fencing.

Oleander: Tall ornamental evergreen shrub with showy white to deep-pink flowers.
Ingesting as little as 1 ounce of leaves can cause death. Do not burn the plant to remove
it from your pasture, because the smoke is toxic to humans and animals. The toxin causes cardiac distress, diarrhea, weakness and rapid death. Atropine can be a helpful antidote in some cases.

Pokeweed: Deep red-purple stem grows to 10 feet tall. Plant bears purple fruit that resemble grapes. Horses can be poisoned by grazing on fresh leaves. Symptoms include colic, diarrhea and respiratory failure. Grows throughout North America. Gastrointestinal protectants and stimulants might ease symptoms.

Potatoes: Ordinary potatoes, if grown too close to the soil surface, will develop a green skin due to sun exposure.  The green skin (in addition to young sprouts) can contain harmful glycoalkaloids. Green tissue should be removed from potatoes before eating.

Rayless Goldenrod:  (Haplopappus heterophyllus), rosea, or jimmy weed, is an erect, bushy, unbranched perennial shrub, growing from .75-1.5 meters tall. The leaves are alternate, linear, and sticky, and the flowers are yellow. It is toxic to horses, cattle, sheep, and goats. Ingestion causes trembling and weakness, constipation, coma and death. The presumed toxic substance, trematone, is the same toxin found in white snakeroot (Eupatorium rugosum). White snake root is a common poisonous plant in the Ohio river valley and eastern United States. Trematone is excreted in the milk of lactating animals so that the young may become poisoned by the consumption of contaminated milk (milk sickness). The toxin is present in both green and dry plant material.

    Where and When It Grows
This shrub grows on the dry rangelands from southern Colorado into Texas, New Mexico, and Arizona. It grows especially well in river valleys and along drainage areas. Poisoning is most common in late fall and winter, particularly after snowfall that covers other forage.

    How It Affects Livestock
Daily consumption of 1 to 1.5 percent of an animal's weight of the green plant for 1 to 3 weeks will produce signs of poisoning in horses, cattle, and sheep. Death will result if the affected animals are not removed from access to the plant early. The condition it produces in cattle is known as trembles. As the toxin is secreted in milk, nursing young may become poisoned by consuming milk.  The dam may not show any signs of poisoning when grazing goldenrod.
    Signs and Lesions of Poisoning

• Lassitude and depression
• Stands in a humped-up position; stiff gait when walking
• Trembling of muscles of nose and legs, especially after exercise; as the condition worsens, the whole body may shake
• Weakness
• Constipation and dribbling of urine
• Recumbency
• Acetone odor on the breath
• Coma
• Terminal respiration consists of prolonged inspiration, a short pause, and forced expiration
• Death
• Myocardial necrosis and fibrosis of cardiac and skeletal muscles
• Degenerative changes of the liver

    How to Reduce Losses
Poisoning of livestock by rayless goldenrod can best be prevented by preventing livestock from grazing on ranges infested with this plant for extended periods.   Rayless goldenrod can be controlled by picloram or dicamba (0.5-1.0 kg ai/Ac) in late summer following adequate rainfall.  Follow precautions for handling herbicides.

Rhododendron (Rhododendron ponticum) and its cousin the Azalea (see above) is extremely poisonous. It is generally only eaten when food is scarce. Death may occur a few hours after eating it. Treatment involves removing the stomach contents.

St John's Wort (Hypericum perforatum) Found in grassland, hedges and open woods. This plant causes photosensitation in areas of unpigmented skin, so that when exposed to sunlight they become red and irritated which leads to rubbing and possible infection. When dried this plant loses 80% of its toxicity but can still be dangerous when baled in hay.

White Snakeroot:

Weed	Where generally located	Livestock affected	Symptoms
WHITE SNAKEROOT*	wooded areas persists after woods are thinned out stream banks	horses cattle goats	depression inactivity arched body hind feet place close together excessive salivation nasal discharge nausea rapid, labored breathing
		sheep	above, except sheep stand with legs apart

Sneezeweed:

Weed	Where generally located	Livestock affected	Symptoms
SNEEZEWEED*	wet areas roadside ditches stream banks	cattle horses sheep	symptoms are slow to develop loss of vigor loss of flesh rapid pulse labored breathing loss of muscular control drooling high temperature dizziness spasms convulsions

Spurge:

Weed	Where generally located	Livestock affected	Symptoms
SPURGE *	cultivated fields waste areas roadsides	horses goats cattle sheep (leafy is non-toxic to sheep)	contact with sap causes inflammation of skin eating causes diarrhea vomiting swelling around mouth and eyes abdominal pains muscle tremors sweating tainted milk has reddish colour, bitter taste

Tansy Ragwort:

Weed	Where generally located	Livestock affected	Symptoms
TANSY RAGWORT *	pastures hayfields waste areas roadsides	horses cattle	nervousness chills pale mucous membranes loss of coat lustre strong, rapid pulse high temperature staggering gait weakness death

Yellow Star Thistle: Leaves covered with cottony hair produce clusters of yellow flowers on spikes that sometimes reach 3 feet. Prevalent throughout California and Oregon, this plant has been cited as the cause of a nervous disorder called "chewing disease." Horses often seek out yellow star thistle, even when other forage is available. Poisoning occurs over one to three months. Brain damage caused by the plant's toxin is irreversible.

Yew (Taxus baccata)This ornamental evergreen shrub is found throughout North America. Contains toxic alkaloids and cyanide that cause tremors, collapse and rapid death. Approximately 1/4 pound is sufficient to kill an adult horse in 15 minutes.
All parts of the yew tree are very poisonous. The poison is not reduced by wilting or drying, so that clippings and fallen leaves are as toxic as the fresh plant. The poison is the alkaloid taxine, which affects the heart. In many cases the symptoms of yew poisoning are never seen, as the animal dies a few hours after eating it - one mouthful is enough to kill !
Symptoms include:

• muscular tremors
• staggering
• convulsions
• difficulty in breathing
• collapse
• death from heart failure

Prevention
There is no treatment and so any offending yew should be fenced off or cut down.

In Summary: The best defense is to familiarize yourself with poisonous plants in you area, and to rid your pastures of them.  Also, make sure that your horses have adequate foods at all times, so that they won't eat marginal plants out of hunger or boredom.

Thanks to Kentucky State and University**, USDA PoisonControl, OMAFRA(* Described in Ontario Weeds, Publication 505), University of Pennsylvania***, and HorsewebUK for information used in this article. This article is intended as a guide only. Please talk to your local agricultural representative and veterinarian for accurate information for your region. While every attempt has been used to make sure the above data is correct, Raspberry Ridge Farms cannot be held responsible for any usage of this information.

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