Plants Toxic To Horses – Weed & Grasses To Be Wary Of




One essential aspect of horse care is ensuring access to safe and nutritious food. While most horse owners provide equine companions with high-quality hay and feed, many horses also graze as part of their diet. As such, it’s important to know that certain plants can be toxic to horses. In this article, we explore some common plants that can be highly toxic.


Key Takeaways:


  • A range of weeds, grasses, and other plants are toxic to horses
  • Common symptoms of toxic exposure include problems in the gastrointestinal tract, neurological issues, and increased pulse and respiration rate
  • If a horse eats something harmful, contact your vet immediately for diagnosis and treatment
  • Prevent your horse from ingesting poisonous plants by fencing safe areas off for grazing and using a grazing muzzle to limit foraging


Bad Grass and Weeds for Horses


Bad plants for horses may feature attractive flowers or bright red berries, or they may have a more boring appearance. We discuss some extremely toxic greenery below.


Poison Hemlock


poison hemlock in the field



Poison Hemlock is a toxic plant that can be deadly to horses. The plant contains coniine, a toxic alkaloid that affects the central nervous system and causes muscle paralysis, respiratory failure, and death.

Poison Hemlock is often mistaken for wild carrots, and its leaves, stem, and seeds can be toxic. Since Poison Hemlock impacts the central nervous system, symptoms of hemlock poisoning often include trembling, salivation, colic, dilated pupils, difficulty breathing, and convulsions.


Hoary Allysum


Hoary Allysum, also known as hoary mustard or white top, is a common weed that can be toxic if ingested. The plant contains nitrate, which can accumulate in the horse’s body and cause nitrate poisoning. Symptoms include weakness, rapid heartbeat, difficulty breathing, diarrhea, and collapse.

Hoary Allysum is especially dangerous to stressed horses or those with a compromised digestive system, as they are more susceptible to poisoning.


Bracken Fern



bracken fern in the woods



Bracken Fern is a common plant found in many pastures that can be toxic if ingested. The plant contains a compound called thiaminase, which destroys vitamin B1 and can cause a deficiency in horses.

Symptoms of Bracken Fern poisoning in horses can include weight loss, weakness, lethargy, stumbling, and neurological problems, such as seizures. In severe cases, the horse may become comatose or die.

Bracken Fern is also a known carcinogen and can increase the risk of developing bladder cancer in horses that graze on it regularly.


John’s Wort


St. John’s Wort is a medicinal herb commonly used to treat depression and anxiety in humans. However, it can be toxic if ingested by horses. The plant contains hypericin, which can cause photosensitization. Photosensitization is a painful skin condition that can induce blistering, peeling, and scarring when the horse is exposed to sunlight.

Other symptoms of St. John’s Wort poisoning in horses include fever, depression, and anorexia. While St. John’s Wort is not considered extremely toxic, it’s still crucial for horse owners to be aware of its potential dangers and prevent horses from accessing the plant.




Nightshade is a highly toxic plant that can be deadly to horses. The poisonous plant contains a toxic alkaloid called solanine, which affects the horse’s central nervous system and can cause symptoms like dilated pupils, difficulty breathing, tremors, colic, and convulsions.

Nightshade’s leaves, flowers, and unripe berries are particularly toxic to horses, and even small amounts can be fatal. Nightshade poisoning can also cause long-term damage to a horse’s liver, kidneys, or other organs.




Buttercup is a toxic flower that grows wild and is commonly planted in yards. The plant contains a poisonous compound called protoanemonin, which can cause blistering and irritation to the horse’s mouth and digestive system.

Symptoms of buttercup poisoning in horses can include colic, diarrhea, excessive salivation, and reluctance to eat or drink.





ragwort in a field



Ragwort is very toxic when consumed over time. All parts of the plant are poisonous, even when dried, because the plant contains pyrrolizidine alkaloids. Pyrrolizidine alkaloids accumulate in the body as a horse consumes ragwort and eventually cause permanent damage to the liver.

Sadly, there are no outward signs of early consumption in most cases, so many caretakers aren’t aware their equine companion is eating ragwort until liver damage occurs and the horse exhibits neurological symptoms or jaundice. 


Red Maple Trees


Red maple trees are beautiful trees known for their showy fall leaves, especially in the northeastern United States. This type of tree is also incredibly toxic and is one of the most common plants to cause poisoning among our equine friends. 

When healthy and green, the leaves of a red maple typically aren’t dangerous, but when wilted, they release a kidney-damaging toxin. Even worse, they taste sweet to horses when wilted, encouraging consumption.




Commonly found in pastures, milkweed features clusters of flowers that are often pink or purple and long, elliptical leaves. True to its name, milkweed contains a white sap. Every part of the milkweed plant is toxic, whether fresh or dried, and ingestion can lead to milkweed poisoning, which is potentially deadly. 

A horse that ate milkweed may display signs of weakness or respiratory difficulties. It may also have seizures or fall into a coma. Fortunately, most horses will only eat milkweed if no other food options are available.


Lily of the Valley


Lily of the Valley is an ornamental plant that produces gorgeous flowers in many gardens, but it’s also toxic to humans and their beloved animals. It’s not likely that Lily of the Valley will grow wild in a pasture, but caretakers should still know its effects and avoid planting it near a fence line, around barns, or in other areas horses where horses may ingest it. 

Lily of the Valley poisoning may lead to low blood pressure, irregular heart rhythm, weakness, colic, or seizures. Ingestion can ultimately be fatal. 




Pigweed is a toxic plant that grows in a range of environments, so it pops up in a lot of places, including horse pastures. It’s toxic both when eaten fresh and when dried. When consumed in large quantities, pigweed may cause kidney failure, so keep your horse away from this weed. 

If a horse does consume it, it may experience muscle tremors, weakness, or a lack of coordination. Most horses will only eat pigweed if there’s nothing else in the pasture.  


How to Recognize Toxic Plants and Prevent Poisoning


Use a book like The Horse Owner’s Field Guide to Toxic Plants by Sandra Burger to spot toxic elements in your horse’s pasture and barn area. Once you’ve spotted a problem plant, remove it promptly or fence off the areas where it grows.

Also, use a grazing muzzle, such as the Flexible Filly Slow Feed Grazing Muzzle, to limit grazing. Grazing muzzles are designed to fit over the horse’s nose and mouth, allowing them to feed but limiting the amount of vegetation they can consume. They are an excellent tool for horses prone to overgrazing or at risk of ingesting toxic food in the pasture.

Finally, make sure the horse has good quality forage readily available. Hungry horses are more likely to eat toxic flowers, leaves, and plants. And avoid feeding poor-quality hay, which could include dried plants that remain toxic even after being bundled into a haybale.



ThinLine Grazing Muzzle

Click to view the Flexbile Filly Grazing Muzzle


Diagnosing Toxic Plant Exposure


Diagnosing toxic plant exposure in horses can be challenging, as the signs and symptoms can vary depending on the specific plant and the amount ingested.

Diagnostic tests, like bloodwork, may be necessary to confirm the horse has eaten toxic weeds and choose the appropriate treatment.

If you suspect a horse has eaten a toxic plant, immediately remove them from the exposure area and contact your vet for further evaluation and treatment.

Also, watch for the following signs of poisoning:




Poisonous plants affect many horses by inducing lethargy when the toxins disrupt the normal metabolic processes within the body. Ingested toxins also often damage the kidneys or liver, heightening lethargy.




The ingestion of poisonous plants may disturb the function of the liver, leading to a buildup of bilirubin in the body. Bilirubin is a yellowish substance, and the buildup causes the horse’s skin to appear yellow, a clear signal that the horse has jaundice and may have liver disease or damage.


Increased Heart Rate


Certain plant toxins stimulate the sympathetic nervous system, which is responsible for the “fight or flight” response, leading to a rapid pulse, high blood pressure, and an increased respiratory rate. This can cause the horse to become restless and potentially develop cardiovascular complications.


Neurological Symptoms


Certain plant toxins interfere with neurotransmitters, which are responsible for transmitting signals between nerve cells. When plants poisonous to horses affect the central nervous system, the horse typically shows neurological symptoms, like difficulty with coordination, seizures, and tremors.



a vet leading a horse with neurological symptoms

Image source:



Brown Urine


Most toxic plants damage the liver or kidneys, which filter waste as part of their job. When waste isn’t filtered out, it builds up, resulting in dark-colored urine. When a horse has dark urine, contact a vet immediately, as kidney problems can lead to severe kidney damage and even kidney failure.


Gastric Distress


Toxins can irritate the digestive system. A typical signal of plant toxicity in horses is gastrointestinal upset, such as colic, diarrhea, or constipation.


Oral Irritation


When a horse’s skin comes into direct contact with toxins, it may show signs of skin irritation, especially around the muzzle. The skin irritation may take the form of blisters, bumps, or inflammation. The horse may also drool or foam from the mouth.


What to Ask Your Vet During the Examination


Immediate vet attention is critical after a horse eats something toxic. During the exam, give the vet as much information as possible about the plant the horse consumed.

Also, ask your vet questions, like:

  1. What toxins have my horse ingested?
  2. What treatments are available to counteract the poison or reduce symptoms?
  3. Are diagnostic tests necessary? If so, what tests need to be performed?
  4. What is the prognosis for the horse’s recovery, and what is the expected timeline?
  5. Are there any long-term effects or complications I should know about?
  6. How can I prevent my horse from toxin exposure from now on?


FAQs About Plants Toxic to Equines


We answer a few FAQs about plant toxicity below.


Can Horses Develop Immunity to Toxic Plants Over Time?

No, horses cannot develop immunity to toxic plants. In fact, some plants cause long-term damage to a horse’s liver, kidneys, or other organs, even after only one ingestion.

Therefore, it’s crucial to prevent your horse from coming into contact with any potentially harmful plants. Restrict pasture access, provide plenty of good quality hay and feed, and use a grazing muzzle to limit the intake of unwanted plants.

How Often Should I Check for Extremely Toxic Plants in a Grazing Area?

The frequency with which you should check for potential toxicity in a grazing area depends on several factors, including the location, climate, and types of vegetation present.

In general, it is recommended that you inspect your grazing area for new signs of toxicity at least once per season. However, you may want to check more frequently if there has been a recent change in the environment or if you have reason to suspect that toxic plants may be present.

Are All Parts of a Toxic Plant Dangerous to Horses?

No, not all parts of every toxic plant are dangerous to horses. For example, some plants may have poisonous leaves or flowers but have non-toxic stems or roots or vice versa. However, to prevent plant poisonings, always assume that the entire plant is toxic until you’ve confirmed which parts are harmful.

Additionally, some plants remain toxic even after wilting and drying out, so be sure to remove any dried plants lying on the ground.