Poisonous Plants for Horses: Common Toxic Plants and Prevention

Toxic plants can grow as weeds in a variety of places, and horses might eat them when they’re hiding in plain sight. For example, if a toxic plant was growing in a hay field, it might accidentally end up in the hay bale you feed to your horse.  

Horses may also eat poisonous plants when pastures are overgrazed and grass is no longer available. Hungry horses might eat toxic plants out of desperation, and poisonous plants may also become more appealing during droughts, frost, or snow. For this reason, it’s important to know how to safely transition your horse off pasture.

Lastly, curious or young horses are more likely to eat toxic plants by investigating new or unfamiliar things. Horses taste and mouth strange objects because they are naturally curious prey animals who use their lips, teeth, and tongue to gather sensory data about their environment.

The most common plants horses may accidentally ingest, as well as side effects they may experience with each:

• Yew
  • Trembling, sudden heart and respiratory problems, death
• Red maple
  • Lethargy, lack of appetite, dark red-black urine, pale gums, increased effort to breathe, dehydration, and rapid heart rate
  • Can lead to organ damage
• Oleander
  • Colic, tremors, irregular heart rate and rhythm, and difficulty breathing
  • Deadly in small amounts
• Foxglove
  • Weakness, digestive upset, heart problems, seizures, and death
• Rhododendron and azalea
  • Drooling, weakness, heart or digestive problems, and sudden death
  • Similar toxin to oleander
• Tansy ragwort
  • Liver damage leading to peeling of white markings, decreased appetite, weight loss, jaundice, and lack of coordination
  • Common in pastures and along roadsides
• Poison hemlock and water hemlock
  • Tremors, excessive salivation, lack of coordination, nervousness, depression
  • Can progress to severe neurologic signs, depressed heart and lung function, and death
  • Often grow near wet areas
• Hoary alyssum
  • Swelling, fever, stiff joints, diarrhea, and laminitis
  • Common through the upper Midwest and western states

• Bracken fern and horsetail
  • Lethargy, depression, blindness, lack of coordination, and severe neurologic problems when eaten in significant amounts or over time

Symptoms associated with plant poisoning can appear suddenly or gradually. There’s a wide range of symptoms depending on each plant species, as noted above.

General signs of plant toxicity in horses include:

• Drooling or difficulty eating
• Colic, diarrhea, or loss of appetite
• Weakness, depression, or unusual behavior
• Muscle tremors, lack of coordination, or seizures
• Trouble breathing
• Irregular heartbeat or collapse
• Swelling
• Increased sensitivity to sunlight
• Laminitis or unexplained lameness

Any unexplained illness or behavior should be treated as an emergency, particularly after exposure to pasture, new plants, or new hay.

If you know that your horse has eaten a toxic plant, follow these steps to ensure they receive appropriate care:

1. Remove the horse from the area of the suspected plant or contaminated hay.
2. Call a veterinarian immediately.
3. Identify the plant and take clear photos and samples if it is safe to do so.
4. Note when the exposure occurred and how much may have been eaten.
5. Do not wait for symptoms or give home treatments unless directed by a veterinarian.

The Pet Poison Helpline (855-764-7661) is an excellent resource if you suspect toxic plant ingestion in your horse. It can help you identify symptoms and provide information on what to do. It’s also best to contact your veterinarian right away for any suspected poisoning.

To help prevent poisoning, eliminate access to toxic plants. Use a reliable plant identification source and know the most common toxic plants in your area so you can remove them when you see them.

• Remove all potentially toxic and unfamiliar plants from barns, pastures, and fence lines.
• Walk all pastures, fence lines, and perimeters regularly to look for dangerous plants.
• Apply appropriate herbicides to remove unwanted plants, and do not allow your horse in the treated area for the recommended time frame.

• Horses tend to investigate other plants when hungry, so ensure pastures are adequately stocked with safe forage.
• Pay careful attention to your horse’s diet during and after droughts, frost, storms, and herbicide application.
• Never feed hay that contains any unfamiliar weeds, plants, molds, or spots.
• If hay quality is uncertain, a forage analysis can be done to confirm safety.

• Check all fence lines and gates before turnout so horses cannot get loose and go where toxic plants may be.
• Keep all ornamental plants out of reach and away from the barn area.
• Be extra cautious with toxic landscaping plants, such as yew, oleander, azalea, and rhododendron.
• Never throw clippings or trimmings of any plants into the pasture.

Pasture safety can be regulated largely through soil, management, and forage choices. Forage refers to the plants specifically meant for consumption, such as grass, legumes, grains, and hay.

The safest pastures are those that are maintained for the growth of good-quality grass and other plants that horses can eat, and they are monitored closely for any plants that do not belong.

Different plants are common to different regions, and many grow as weeds in the pasture. Local agricultural contacts, extension offices, agronomists, or equine professionals are excellent resources to help with planning a safe and healthy pasture.

Prevention is the absolute best protection when it comes to keeping toxic plants away from your horse. To avoid ingestion of poisonous plants and prevent emergencies, focus on careful inspection of pastures and preparation of horse feed.

If you ever find yourself with a horse showing signs of plant intoxication, call your vet immediately and, if possible, collect the plant itself for analysis.

What is the most poisonous plant to horses?

There is no single “most poisonous plant” for horses. However, this list includes some of the most common plants that are toxic in very small amounts:

• Bracken fern
• Hemlock
• Water hemlock
• Tansy ragwort
• Yellow star thistle
• Oleander
• Red maple trees
• Yew
• Locoweed
• Johnson grass

Do horses sometimes eat poisonous plants in hay?

Absolutely. This is one of the most common ways horses accidentally consume toxic plants and molds.

Will horses avoid poisonous plants?

Some poisonous plants are not particularly appetizing, so horses tend to avoid them unless conditions like bad weather or severe hunger make the plants more desirable. Others, like oleander, look like a tasty treat, so horses will not avoid them.

What should I do if my horse eats an unknown plant?

If you witness your horse eating an unknown plant, it’s best to take a picture of that plant and note its specific characteristics—like “small twiggy plant with yellow flowers.” Use these details to identify the plant with a reliable resource (like an AI photo ID app or the USDA plants database) and determine if it’s toxic.

If toxic, call your veterinarian immediately. Even if you are unable to identify the plant, vets are typically well-versed in the toxic plants of your region and may be able to help you identify the plant and determine next steps.