Poisonous Plants: The First Five of the Top 10

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The pastures are in bloom, it’s that time of year we all have been anxiously awaiting. Winter is over and the green is taking over!

When you take a walk through your pasture, I’m sure you will find plants that are toxic to your horse. Lots of these plants in general will pose little threat to your horse because the horse will have no desire to eat the poisonous plants. One reason horses will eat non-palatable plants is hunger. 

Since our equine friends are often on the order of 1,000 or more pounds, it takes quite a bit of plant intake to clinically affect them, for the most part. However, some plants are a cause for concern with just a little nibble or repeated browsing over weeks to months. Results are not always good after ingestion and sometimes lead to serious illness or even death. Be on the look out! Poisonous plants are worth recognizing so they can be promptly removed to protect the health of your horse. If you have additional questions, seek the help of your local Equine Extension office as well as your veterinarian.

The top 10 poisonous plants to horses in the United States as compiled by the American Association of Equine Practitioners (AAEP) are:

  • Brackenfern
  • Hemlock
  • Water Hemlock
  • Tansy Ragwort
  • Johnsongrass and Sudangrass
  • Locoweed
  • OleanderRed Maple Trees
  • Yellow Star Thistle/Russian Knapweed
  • Yew

Bracken Fern

This grows in woodlands and moist open areas. The entire plant is toxic. It contains thiaminase (an enzyme), which inhibits absorption of thiamin (vitamin B1). Thiamin is necessary for nerve function and if there is a thiamin deficit, this can lead to neurological problems. Clinical signs are related to neural dysfunction, including depression, incoordination/ataxia and blindness. It is said a horse needs to consume 3-5% of body weight over a month to induce clinical deficits. If you catch this before neurological signs are severe and start treatment with your veterinarian, you will have a better outcome.

Courtesy Steve Dewey, Utah State University, Bugwood.org

Hemlock (aka Poison Hemlock)

This is a weed with clusters of small white flowers. All parts of the plant--leaves, stems and seeds--contain neurotoxins that affect both central and peripheral nervous systems. Four to five pounds is a lethal dose for a horse. Most horses will opt not to graze on it, unless hunger has driven them to no other choice. Clinical signs appear within an hour or two after eating the plant. These signs begin with nervousness, tremors and incoordination. You can even see signs of depression, possibly colic, and decreased heart and respiratory rates. Ultimately, death results from respiratory failure. If suspected, supportive treatment may help in the recovery of your horse if non-lethal amounts are ingested.

Elmer Verhasselt, Bugwood.org

Water Hemlock

This is likely found in marshy areas of meadows, along streams, or irrigation ditches. Water hemlock is considered one of the most toxic plants in the United States. All parts of the plant contain a cicutoxin alkaloid that affects the central nervous system. Horses will browse this plant. Less than a pound of leaves and stems can be fatal; however, most of the toxin is concentrated in the root. Toxins primarily affect the brain. Clinical signs may include excess salivation, dilated pupils, nervous attitude, difficulty breathing, seizures, convulsions and/or possibly leading to death by respiratory paralysis. Clinical signs can appear within one hour of ingestion. Death typically follows within 2-3 hours. Supportive care is absolutely necessary before convulsions begin. Horses that survive may have permanent damage to the heart and skeletal muscles. Most likely, euthanasia is recommended due to rapid onset and severity.

Eric Coombs, Oregon Dept. of Ag, Bugwood.org

Tansy Ragwort

This is a weed that produces small daisy-like yellow flowers. There are about 70 species that span the United States and many different habitats. These plants contain pyrrolizidine alkaloids and the concentration can vary among species. These plants are especially harmful after being cut, as they are less bitter upon eating; however, just as toxic. These alkaloids are cumulative in nature and inhibit cell division, especially in the liver where cells are slowly killed and prevented from regenerating. This liver damage is irreversible. Horses can be the target of chronic exposure over time. Consuming 50-150 pounds of the plant can result in chronic exposure and disease. Clinical signs of poisoning do not appear until the liver damage is severe and include photosensitization, jaundice, incoordination, depression, decreased appetite and weight loss. Euthanasia is recommended when liver failure occurs. The best prevention is to eliminate exposure to the plant.

Howard F. Schwarts, CSU, Bugwood.org

Johnsongrass and Sudangrass

Both plants can grow up to six feet and are a wild grass native to southern climates. Leaves and stems contain cyanide compounds. These compounds are metabolized and inhibit the body’s ability to absorb oxygen resulting in suffocation. Young plants of Johnsongrass contain the highest concentrations of the toxin. Clinical signs with acute poisoning are rapid breathing, gasping, frequent urination and defecation, bright red/brick red mucous membranes and progression to convulsions and death. One of the problems in feeding sorghum hays to horses is the permanent damage to nerves of the urinary bladder, which can cause urinary incontinence and hind end weakness. It is not recommended to feed sorghum hay to horses for prolonged periods of time unless it is from a “cyanide free” sorghum hybrid plant. Once your horse is affected, there is no effective treatment for nerve damage; however, if caught early enough, supportive therapy can diminish the effects if cyanide poisoning is less severe.

Credit: Gerald Holmes, CA Polytechnic State, Bugwood.org

Credit: Gerald Holmes, CA Polytechnic State, Bugwood.org

For an article covering the remaining five poisonous plants visit the Hagyard website.

Information in this document has been provided with the help of the American Association of Equine Practitioners. For more horse health information also visit the AAEP website for horse owners www.aaep.org/horseowner.