Plants Poisonous or Injurious to Horses

Poisonous plants rarely cause issues for horses unless there is not enough proper feed/forage. However, some horses may ingest things they should not, particularly young or curious horses (Figure 1). Boredom and deficiency are other reasons horses may ingest poisonous plants. In some cases, such as bracken fern poisoning, horses acquire a taste for a harmful plant and will continue to eat it after sufficient forage is provided.

Unwanted plants are present in many pastures due to the natural weed seed bank and seed moving in from other areas. Many poisonous plants are opportunistic and often fill in any time we have a gap caused by overgrazing, improper forage harvest, or forage stands becoming old and dying out. Sacrifice areas, exercise paddocks, and dry lots should be scouted often due to not having good forage cover to outcompete potentially harmful plants. Wilted leaves on branches falling into pastures may taste sweet, so tree lines should be checked, especially after storms. Unfortunately, poisonous plants are not limited to pastures. They may also be present in harvested hay.

Why Are Poisonous Plants a Concern?

Some plants can cause serious harm and even death with just a few mouthfuls. Others may need to be consumed in great quantities before becoming problematic. Symptoms of plant poisoning vary depending on the plant species, toxins, etc. Poisonous plants can cause incoordination or colic. Unlike most mammals, horses cannot regurgitate easily. This means they cannot expel offending material once it is ingested. Seasonality and weather conditions (e.g., drought stress) play a role in poisonous plant risk.

Problematic Forage Species

Some forage species can become issues under certain conditions. An example is tall fescue, one of the most common plants in Ohio pastures. Tall fescue can be a problem when it is infected with endophyte, a microscopic symbiotic fungus. Most wild tall fescue is endophyte-infected and can cause problems for horses during the last three months of pregnancy and early lactation. However, under most conditions and for most horses, tall fescue is a nutritious forage option that is more resistant to trampling than other grass species.

Clovers are another example. Red clover, when infected with a fungus, can cause slobbers. Additionally, alsike clover under certain conditions and grazed at high levels can cause photosensitization. Horses with pink skin are most affected (Figure 2). However, under most conditions, clovers can be a useful component of a pasture mix by fixing nitrogen and contributing dietary protein for livestock, including horses.

Injurious Plants

Injurious plants are not poisonous but can cause other issues. For example, foxtail awns in hay can become embedded into the gums causing ulcerations. Burdock, cocklebur, and thistles have sharp spines and hooks that can get caught in the haircoat, manes, forelocks, and tails. They can also injure eyes or mucous membranes.

Types of plants that can be a problem (See “additional resources” for images and identification guides)

• landscaping shrubs (e.g., ornamental yew, rhododendron, and oleander)
• trees (e.g., red maple, black locust, black walnut, buckeyes, oaks, and cherry trees)
• forage crops that can become poisonous or antinutritional (e.g., sweet clover, tall fescue, red clover, and alsike clover)
• pollinator and butterfly habitats may include poisonous plants (e.g., milkweed)
• other plants that may end up in your horse’s forage (e.g., poison hemlock, poke weed, cressleaf groundsel, buttercup, and nightshade)

Prevention

• Identify possibly harmful plants in your area. Scouting is the best method. Once located, remove or eradicate potentially harmful plants. Walk your pastures regularly and incorporate scouting into fence walks.
• Provide plenty of good quality forage that is green in color, palatable in texture, pleasant in smell, and free of weeds, mold, dust, and foreign matter. Ensure that it is free of insects, particularly blister beetles.
• Meet the horse’s nutrition and water needs with quality forage, grain if needed, and a fresh, clean water supply.
• Discourage boredom by providing horses with plenty of exercise, salt, feed, and water.
• Provide preventative healthcare and know what is normal for your horse.
• Proper fertilization of forage and pasture fields will help reduce weeds and unwanted plants.
• Educate landscapers and others who make decisions about what plants are grown on the property and along fence lines about plants harmful to horses.
• Especially in residential areas, consider posting “Do not feed the horses” signage and discourage the dumping of yard waste, including grass clippings and trimmings from ornamental shrubs, in pastures.

Remove Poisonous Plants When Possible

Eradicating poisonous plants can be done manually, using herbicides or in combination. This will be determined by the type of plants. Trees and shrubs can be removed manually and most will not regrow. When using herbicides, read the label and follow the directions for grazing restrictions and residual effects. Some of these products can have residual restrictions for up to 18-24 months. Broadleaf herbicides will not affect grass plants. Controlling grass plants is much more difficult and a non-selective herbicide will need to be used. Non-selective herbicides will destroy most, if not all, treated plants. Therefore, reseeding will need to be done in these areas.

What to Do if You Think Your Horse Has Ingested Poisonous Plants

Immediately contact your veterinarian and remove access to the potentially problematic forage. Depending on the plant, time of year, toxins involved, and symptoms presented, veterinarian recommendations will vary. Providing your veterinarian with accurate vital signs and a description of any symptoms will help them determine the urgency of the situation. Next, take steps to identify the offending plant.

Additional Resources

Download a PDF of this fact sheet.

Local County extension office directory

• Locate an Office
  (extension.osu.edu/lao)

Other resources

• A Guide to Plant Poisoning of Animals in North America
  (ivis.org/library/guide-to-plant-poisoning-of-animals-north-america)
• Colorado State University:

Guide to Poisonous Plants
(southcampus.colostate.edu/poisonous_plants)

• Cornell:

Weeds of the Northeast
 (cornellpress.cornell.edu/book/9781501755729/weeds-of-the-northeast/#bookTabs=1)

Department of Animal Sciences - Plants Poisonous to Livestock
 (poisonousplants.ansci.cornell.edu/comlist.html)

Horse Owner's Guide to Toxic Plants: Identifications, Symptoms, and Treatments
(skyhorsepublishing.com/9781510741676/horse-owners-guide-to-toxic-plants)

• Penn State:

Plants Toxic to Horses
(extension.psu.edu/plants-toxic-to-horses)

How to Tell Foxtail from Timothy and Control It
(extension.psu.edu/how-to-tell-foxtail-from-timothy-and-control-it)

Trees in Horse Pastures
(extension.psu.edu/trees-in-horse-pastures)

Landscaping with Native Plants on Horse Farms
(extension.psu.edu/landscaping-with-native-plants-on-horse-farms)

• Purdue University:

Guide to Toxic Plants in Forages
(extension.purdue.edu/extmedia/WS/WS_37_ToxicPlants08.pdf)

• The Ohio State University:

Forages
(forages.osu.edu)

Ohio Perennial and Biennial Weed Guide
(weedguide.cfaes.osu.edu)

OSU Weed Management: Weed Management Information for Agronomic Cropping Systems
(u.osu.edu/osuweeds)

Weed Control Guide
(extensionpubs.osu.edu/category-2c-agricultural-weed-control)

• University of Georgia:

Blister Beetles in Georgia Alfalfa Hay
(secure.caes.uga.edu/extension/publications/files/pdf/C%20917_7.PDF)

• University of Michigan:

Ornamental Plants Toxic to Michigan Horses
(canr.msu.edu/resources/ornamental_plants_toxic_to_michigan_horses_e3059)

Toxic Plants of Concern In Pastures and Hay for Michigan Horses
(canr.msu.edu/uploads/resources/pdfs/bulletin-e3060.pdf)

Michigan Trees Toxic to Horses and Donkeys
(canr.msu.edu/uploads/resources/pdfs/e3062.pdf)

• University of Minnesota:

Plants Deadly to Horses
(extension.umn.edu/horse-pastures-and-facilities/plants-deadly-horses)

• University of Pennsylvania:

Drs. Lisa Murphy and Elizabeth Woodward, “Poisonous Plants in Pastures and Ponds” - First Tuesday Lecture - Nov 2023
(vimeo.com/899836895)

• USDA Plants Database
  (plants.sc.egov.usda.gov/home)
• Wild Parsnip (Pastinaca sativa)-induced photosensitization
  (sciencedirect.com/science/article/abs/pii/S0041010119301801)