Toxic Plants

Plants can become a problem for horses during drought.

Poisonous plants are commonly found in pastures in most areas of the United States. They tend to be a problem in the fall of the year when the grass is short, but are especially a problem this year in those parts of the country experiencing drought.

Horses at greatest risk for plant poisoning are those that are on overgrazed pastures, are not receiving adequate nutrition or are in dry lots.

Usually, horses find poisonous plants distasteful and will not eat them if there is adequate grass or hay available. However, in overgrazed pastures or during drought periods, noxious weeds proliferate and may be the only forage available. Some plants become poisonous only after a frost, which can increase their toxin levels and improve their palatability. Other toxic plants become palatable after they have been treated with herbicides or after rainfall.

What might not be a problem plant during one time of year can become a threat during a different season. Some plants, such as sudan grass, are relatively safe to graze part of the year but then become lethal as the season changes and their toxin levels rise. Horses are at greater risk of plant poisoning if they have the opportunity to consume large quantities of toxic plants during a period of several weeks or months. It is rare for a horse to be poisoned by a single mouthful.

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Because the symptoms of plant poisoning are varied and may develop over time, it can be difficult to recognize that your horse is suffering from plant toxins. Symptoms may vary from frothing of the mouth and convulsions (hemlock) to weakness and liver failure (ragwort) to colic and bloody urine (buttercups) to anemia (red maple) and sunburn.

Good pasture management and prevention is key. Here are a few recommendations to help you protect your horses against plant poisoning.

    • Avoid overgrazing, and maintain appropriate stocking rates.
    • Ensure that horses on pastures have adequate hay if the grass is short, so they don’t resort to eating poisonous weeds.
    • Practice good pasture management, including pasture rotation to prevent overgrazing.
    • Mow pastures two or three times during the growing season and regularly drag pastures to break up manure piles.
    • Don’t allow horses to graze pastures immediately after herbicide application.
    • Never give garden, grass or ornamental plant clippings to horses.
    • Remove broken tree limbs and plant debris from pastures after storms.
    • Save samples of suspected toxic plants.
    • Learn to identify poisonous and toxic plants in your geographic area.

Without a doubt, the most difficult aspect of preventing plant poisoning is learning to identify toxic plants. That is because there are so many plants that can become toxic under the right circumstances and horses experience a wide variety of symptoms following their ingestion. Here are a few of the most dangerous toxic plants: nightshade, bracken, boxglove, hemlock, horsetail, iris, larkspur, lily of the valley, linseed, monk’s hood, potato, cherry trees, oleander, privet, ragwort, red maple, rhododendron and yew.

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Some of the lower-risk plants are acorn, buckthorn, buckwheat, buttercup, cherry laurel, chickweed, columbine, ground ivy, hemp nettle, horseradish, marsh marigold, poppy, rhubarb, rush, St. John’s wort and spurge. There are virtually hundreds of other plants that can become toxic.

If you have questions on plants that may be toxic to your horses, contact your local equine extension specialist or equine veterinarian. Sensible horse pasture management will prevent almost all plant poisonings. Be vigilant during periods of drought, and call your veterinarian immediately if your horse becomes ill.