Poisoning from toxic plants is fortunately fairly rare among horses—mainly because those with access to ample hay and/or pasture grass tend to avoid plants that smell or taste funny. Still, it’s a good idea to be familiar with the exceptional cases—plants that are especially deadly, or those that may be more palatable and thus encourage horses to ingest a toxic amount.

Some toxic plants are cultivated widely across the country. Many others grow wild, and their ranges may be limited to specific regions. Other plants are most hazardous mainly when cut into hay. Your local extension agent is a good source for information if you want to know more about which specific toxic plants are a threat in your area.

To get started, try this 10-question quiz to check your knowledge of some of the more hazardous plants your horse might encounter.

1.A 1,000-pound horse needs to consume considerable quantities of most toxic plants to develop any ill effects. But as little as eight ounces of leaves from one shrub, commonly used in landscaping, can be deadly to a horse within minutes. What is it?

a. azalea (Rhododendron spp.)

b. rhododendron (Rhododendron spp.)

c. yew (Taxus spp.)

d. Canadian hemlock (Tsuga canadensis)

c. Yew is a woody evergreen shrub with closely spaced, flat leaves and bright red or yellow juicy berries with a hole at one end, exposing a dark seed. Most parts of the plant—including the leaves, branches and bark—contain taxine, an alkaloid that causes cardiac and respiratory collapse. A horse can get a deadly dose from just one mouthful of yew. To protect your horse, remove all yews from your property, and warn neighbors against tossing landscape trimmings into your turnouts. If you decorate your barn for the holidays, avoid garlands and wreaths that contain yew branches.

Azaleas and rhododendrons—which are commonly planted for their showy flowers—both contain toxins that can cause gastrointestinal and cardiovascular distress, but a horse would need to eat several pounds of the plants to experience any ill effects.

Canadian hemlock, also called eastern hemlock, is an evergreen tree or shrub with short, flattened leaves similar to those of the yew; one difference that distinguishes the two plants is that, instead of brightly colored berries, the hemlock tree produces oval cones that can be up to an inch long. The Canadian hemlock is not toxic.

It is also important to avoid confusing the Canadian hemlock tree with poison hemlock (Conium maculatum), a multistemmed weed with fernlike leaves and clusters of small white flowers. The weed contains a potent neurotoxin; eating four to five pounds would be fatal to a horse.

2.Seasonal pasture myopathy—a serious and often fatal acute muscle disorder—was long thought to be linked to the consumption of a weed called white snakeroot. However, in 2012, researchers announced that the disease is actually caused when horses consume which of these:

a. fescue grasses (Lolium [Festuca] arundinaceum)

b. milkweed leaves (Asclepias spp.)

c. bananas (Musa spp.)

d. seedpods from box elder trees (Acer negundo)

d. Female box elders, also called ash maples or river maples, produce distinctive “helicopter” seedpods, which can be shed into pastures by the hundreds during the autumn months. They contain a toxin that blocks the metabolism of fat, and when horses pick up the seedpods while grazing, they can develop the fatal myopathy. Box elders are most common in the central United States, from the Appalachian to the Rocky mountains, and although the trees prefer colder climates, they can be found as far south as Texas. (To read more, see “Mystery Solved,” EQUUS 426.)

Fescue grasses that have been infected with a fungus can cause problems—such as diminished milk production and difficult births—when grazed by pregnant mares. Milkweed contains toxins that affect the digestive, cardiac and nervous systems, but horses will rarely graze on the live plant. However, the plant remains toxic when dried, and it may sometimes be cut and baled with hay. Bananas are not toxic, and many horses enjoy them when they are fed as snacks.

3.Some plants contain chemicals, called photodynamic agents, that react when struck by the photons in sunlight. When horses consume these plants, the chemicals circulate in the bloodstream; as the blood passes through capillaries near the surface of unpigmented (pink) skin, the chemicals are exposed to sunlight and release energy, which damages the surrounding tissues. The result is painful blistering and scabbing. Which of these plants can cause photosensitization?

a. alsike clover (Trifolium hybridum)

b. St. Johnswort (Hypericum perforatum)

c. buckwheat (Fagopyrum esculentum)

d. all of the above

d. Alsike clover has three-lobed leaves and pink or white flowers that grow on upright stems. St. Johnswort is a weed with oblong, oval-shaped leaves and clusters of yellow, five-petaled flowers. Buckwheat is cultivated for its seeds, but has become naturalized as a weed throughout much of the United States. It has reddish stems with rounded leaves up to four inches long and small clusters of tiny white flowers.

Although they contain different photodynamic agents, each of these plants can cause photosensitization. The condition will heal when the horse is shielded from sunshine and he no longer has access to the plants that caused the reaction. If healing goes slowly or your horse seems ill, call your veterinarian, who may prescribe medications for the pain. She may also want to run tests to rule out liver damage, which can also cause photosensitization. (To read more, see “Scorched,” Case Report, EQUUS 404.)

4.Most toxic plants taste bitter, and a browsing horse with ample forage isn’t likely to take more than a harmless bite or two of a bad-tasting weed before moving on. However, sometimes horses develop a taste for a particular toxic plant and seek it out even when other forage is available. Which of these plants is toxic, yet may still be attractive to horses?

a. bracken fern (Pteridium aquilinum)

b. sagebrush (Artemisia spp.)

c. red clover (Trifolium pratense)

d. tomato plants (Solanum lycopersicum)

a. Bracken fern, also called brake fern or eagle fern, causes a thiamine (vitamin B1) deficiency that can lead to neurological impairment and blindness. Horses need to eat hundreds of pounds of the ferns to develop any ill effects. Most bracken fern poisoning occurs in late summer or autumn when other forage dies back; however, some horses may seek out the younger shoots and leaves. Bracken poisoning can also occur when horses are fed hay that contains dried ferns. Horses can be treated effectively with thiamine supplements if the poisoning is caught early.

Sagebrush is not harmful when consumed in small quantities, but if horses are forced to rely on it as their sole sustenance, it can cause behavioral and neurological signs. The signs will diminish after two to four weeks when a horse resumes a nutritious diet. Red clover itself is not toxic, but when the plant is stressed—such as by overgrazing or drought—it can be infected by a fungus that stimulates excessive drooling in the horse who consumes it. Most horses suffer no serious harm and recover as soon as they stop grazing the infected clover. Tomato plants, like other members of the nightshade family, contain toxins that can cause colic, but horses aren’t likely to encounter or consume enough to do serious harm.

5.Grazing on fallen, wilted or dried leaves in the autumn or after a windy summer storm can be fatal to a horse—if the leaves came from one of these species of a common tree. Which one is it?

a. red maple (Acer rubrum)

b. Japanese red maple (Acer palmatum)

c. red oak (Quercus rubra)

d. red hawthorn (Crataegus mollis)

a. Ingesting a substance called gallic acid found in red maple leaves leads to damage to a horse’s hemoglobin, the molecules within the red blood cells that transport oxygen. Starved of oxygen, organs and tissues throughout the body will begin to fail.

Nibbling the occasional mouthful of green leaves from a tree will likely do little harm to a horse, but when leaves drop in autumn or wilt on a fallen branch after a storm, they dry out and the toxin becomes concentrated, and eating even a pound or two may become dangerous. The same toxin is also present in the branches and bark of the tree, but horses are not likely to eat enough to be affected.

The threat from red maples is well established, but gallic acid has also been found in the leaves of sugar and silver maple trees. A number of hybrids and cultivars have also been bred from these and other native and imported maples, most of which have never been tested for toxicity. For now, it is safest to assume that the wilted or dried leaves of any maple species are potentially toxic. (To read more, see “Maple Tree Menace,” EQUUS 457.)

Japanese red maple, often called simply “red maple,” is an Asian native widely cultivated in U.S. landscapes. Many varieties have leaves that are red or burgundy throughout the summer, with five to seven or more deep, pointed lobes. Japanese red maples are not known to be toxic to horses.

Red oaks, like other oak species, contain toxins that damage the gastrointestinal tract and kidneys. All parts of the tree are toxic, but cows and sheep are more likely to consume fatal amounts than are horses or goats. Horses who graze on large quantities of ripe acorns in the fall or on new leaves and buds in the spring may develop colic.

Red hawthorn, also called downy hawthorn, is native to the eastern United States. The tree produces clusters of white flowers each spring and edible red fruits in late summer and fall. No part of the plant is toxic to horses, but it’s not a good idea to let horses have access to these trees in turnouts because they produce potentially hazardous thorns up to two inches long along their branches.

6.Many toxic plants grow as weeds along the trails and back roads where people like to ride. Which of the following common roadside weeds is not toxic to horses?

a. senecio (Senecio spp.)

b. common dandelion (Taraxacum officinale)

c. water hemlock (Cicuta spp.)

d. yellow star thistle (Centaurea solstitialis)

b. Common dandelions are not toxic, and many horses seem to seek them out while grazing.

It is important, however, to be able to distinguish common dandelions from false dandelions (Hypochaeris radicata), also called flatweed or cat’s ears. One of the most noticeable differences is that each flower on a common dandelion grows on a single, hollow stem, while the false dandelions may have multiple flowers growing on forked, solid stems. Horses who graze false dandelion for extended periods may develop Australian stringhalt, an exaggerated hyperflexion of the hind legs that results from damage to the long nerves.

The Senecio genus, also called tansy ragwort or groundsel, includes about 70 species found throughout the United States. Most are multi-stemmed weeds that produce clusters of yellow flowers. The toxicity may vary among species, but all potentially contain dangerous amounts of an alkaloid that causes permanent damage to the liver. Signs may include photosensitization, weight loss, depression and jaundice. Horses need to eat 50 to 150 pounds of the plants to develop liver disease.

Water hemlock, which can reach four feet high on hollow stems with umbrella-shaped clusters of small white flowers, is considered one of the most toxic plants in the United States. The toxin is concentrated in the root, but all parts of the plant contain dangerous levels of an alkaloid that damages neurons primarily in the brain, causing signs including seizures, degeneration of the heart and skeletal muscles, and respiratory paralysis. Less than a pound of the leaves and stems can be fatal to a horse. The plant can be found in marshy areas and along streams and irrigation ditches throughout the United States.

Yellow star thistle, a spherical plant with round yellow flowers surrounded by stiff spines, contains a neurotoxin that inhibits a horse’s ability to bite and chew food. The effects are cumulative—to consume a toxic dose, a horse must eat 50 to 200 percent of his body weight over 30 to 90 days. Horses do find the plant palatable. The plant is found throughout the Western United States.

It’s a good idea to learn to identify common toxic weeds in your region, and avoid letting your horse graze on unfamiliar plants when you’re out on the trails.

7. Shrubs and small trees that produce showy flowers are popular landscaping choices. Which of these species, found primarily in hotter climates across the southern United States, is highly toxic to horses?

a. oleander (Nerium oleander)

b. Texas mountain laurel (Sophora secundiflora)

c. crape myrtle (Lagerstroemia indica)

d. Southern magnolia (Magnolia grandiflora)

a. Oleander, also called rose laurel, is an evergreen shrub that can reach the size of a small tree, with thick, leathery, elongated leaves up to 10 inches long; flower clusters in white, pink or red bloom in the spring and summer. All parts of the plant contain toxins that disrupt the beating of the heart. The plant remains toxic when dried, and eating as few as 30 to 40 leaves can be fatal for a horse.

Texas mountain laurel is an evergreen shrub or small tree that produces clusters of fragrant, violet/blue flowers and bright red seeds in woody pods. This plant is not toxic to horses, but browsing mature foliage or chewing the seeds can cause muscle disorders including trembling, stiff gaits and difficulty rising in cattle, sheep and goats.

Crape myrtles are multi-stemmed small trees with distinctive smooth, mottled bark that in summer and autumn produce clusters of flowers ranging in color from white to pinks, reds or purples. Southern magnolias are large evergreen trees with broad, smooth leaves up to eight inches long that produce white flowers up to 12 inches across. Neither of these species is toxic to horses.

8. Most toxic plants cause problems when horses consume them—with one notable exception. Which of these can cause deadly harm to a horse simply through physical contact?

a. poison ivy (Toxicodendron radicans), if horses brush their nostrils against it while grazing

b. black walnut (Juglans nigra) shavings incorporated in stall bedding

c. Douglas fir (Pseudotsuga menziesii) sap, rubbed on a horse’s skin

d. buttercups (Ranunculus spp.), when horses lie or roll on them in pastures

b. A wood shavings mixture with as little as 20 percent black walnut can cause acute laminitis0 within eight hours of exposure. Black walnut shavings are a distinctive chocolate brown that will stand out if they get mixed in with other woods used in beddings, which are typically much lighter. To avoid potentially toxic shavings, purchase bedding materials only from reputable suppliers who are familiar with the needs of horses.

Poison ivy may give you an itchy rash, but it rarely affects horses, who can graze on it without problems. Pine saps, such as from the Douglas fir, may make a sticky mess if an itchy horse rubs against a tree, but they won’t do him any serious harm. Buttercups release an oil that causes profuse salivation when horses eat the plant, but rolling on it will do him no harm.

9.

Moving to a property that comes with fruit trees might seem like a welcome bonus source of horse treats, but one common supermarket staple can be toxic. Which of these should never be fed to horses?

a. figs (Ficus carica)

b. lemons (Citrus limon (L.) Osbeck)

c. avocados (Persea americana)

d. pomegranates (Punica granatum)

c. The fruits, seeds, bark and leaves of the avocado tree contain toxic compounds that can cause mastitis0, colic, diarrhea, damage to the heart muscle, and edema of the head and neck that may inhibit breathing. Most cases of toxicosis are associated with the Guatemalan avocado, but other varieties are not considered safe.

Figs, lemons and pomegranates are all safe to feed to horses, rinds and all (cut into small pieces, of course, to reduce the risk of choke0). The bark and leaves of these trees are not toxic, but it’s still not a good idea to allow horses to have free access to any fruit trees in pastures. The high sugar content in these fruits may cause weight gain and could trigger laminitis in horses with metabolic disorders. (To read more, see “Fruit Trees in Pastures?” Consultants, EQUUS 445.)

10. A plant doesn’t have to contain toxins to cause health problems for horses. Which of these can cause mouth ulcers painful enough to stop a horse from eating?

a. milk thistle (Silybum marianum)

b. foxtails (Setaria spp.) and foxtail barley (Hordeum jubatum)

c. stinging nettle (Urtica dioica)

d. locoweed (Astragalus spp. or Oxytropis spp.)

b. Foxtails and foxtail barley are weed grasses that form bushy seedheads that resemble foxtails or bottle brushes. As the seedheads mature throughout the summer, they harden, dry out, and form sharp barbs (awns) to catch in the coats of passing mammals for dispersal. If horses eat dried foxtails, the awns can become embedded in the tissues of the mouth, causing bleeding ulcers. Horses generally avoid grazing on mature foxtails growing in pastures, but the wild grasses can cause problems if they are cut and baled into hay. Foxtails in the field pose a serious hazard to dogs, who can get the sharp awns embedded in their ears and noses. Eradicating wild foxtails is difficult, but they can be controlled by mowing them before the seedheads mature. (To read more, see “Something They Ate,” Case Report, EQUUS 419.)

Milk thistle, also called bull thistle or variegated thistle, is toxic to cattle and sheep but not to horses. Stinging nettle, also called common nettle, has hollow hairs on the leaves and stem that inject noxious chemi-cals into the skin of people and animals who brush against them, causing a stinging sensation. Stinging nettles are not toxic to horses who graze on them, but rolling on the plants may cause a painful skin rash.

Note that stinging nettle is a different plant than horse nettle (Solanum carolinense), a member of the nightshade family that can cause excessive salivation, diarrhea or constipation, depression and colic when eaten; serious cases may be fatal. Horses tend to avoid horse nettle unless they have no other forage. (To read more on horse nettles, see “How Toxic Is This Weed?” Consultants, EQUUS 414.)

Locoweed is highly toxic. Various species of this plant, also called crazyweed, grow throughout the West and Southwest, often in dry, sandy soils. Most contain an alkaloid that inhibits sugar metabolism, causing a buildup of sugars that disrupts the function of the brain and other organs. Weight loss, sudden changes in behavior or temperament, ataxia and unusual gaits are often among the first signs. There is no effective treatment for locoweed poisoning, and the effects may be permanent.

Horses tend to be naturally finicky eaters, which is good news when they’re picking through all of the plants in the pasture in search of the sweetest grass. Your horse will avoid many serious threats all on his own. And by learning to recognize and avoid the plants that might still do him harm, you’ll be able to keep him even safer. 

This article first appeared in the June 2017 issue (#477) of EQUUS magazine