Poisonous Plants in the Pasture


Some plants are good for us; others are not. It’s a simple rule, and it also applies to our horses. Some plants benefit an equine’s body nutritionally, some can be used medicinally, while still others cause irrevocable harm.

Many wild plants are potentially toxic, but most are avoided by horses in pasture because they are unpalatable compared to available forage. But some taste too good for horses to resist, or are so toxic that even small amounts pose serious danger. Also, horses in poor pastures and lacking adequate fodder will often turn to poisonous plants they would normally avoid.

Here are the 13 most poisonous plants in horse pastures. In all cases, avoidance and attentive pasture management are the best strategies to avoid accidental poisoning. In most cases, removing plants by the root before they go to seed will eliminate them. If you see something in your pasture you are not familiar with, always identify it as soon as you can: field guides and the internet are invaluable resources.

1. Yew (Taxus species) is one of the most poisonous plants for horses in North America. All parts except the flesh of the berries contain taxine, a respiratory and cardiac depressant – just one handful can cause death within minutes. Yew is native to North America, and planted widely as a landscaping ornamental; yew clippings must be handled with extreme care. Do not put pruned twigs anywhere a horse or other ruminant animal will have access to them.

2.  Although the roots of Water hemlock or cowbane (Cicuta species) are the most toxic part of the plant, even its seeds and leaves contain cicutoxin, an unsaturated alcohol deadly to horses. Eating even small amounts of the leaves can produce poisoning symptoms within an hour. Death may result soon after, although immediate treatment may save a horse. Signs to watch for are nervousness, rapid pulse, pupil dilation, excessive salivation, respiratory distress and seizures.

3. Oleanders (Nerium oleander) are beautiful evergreen bushes with pink, white or purple flowers. It takes only a few handfuls of the leaves to kill a horse, and all parts of the plant are deadly, containing toxins that cause arrhythmia of the heart. Within a few hours of oleander consumption, a horse will present with colic, a fast or slowed heartbeat, respiratory distress or shaking. Immediate veterinary treatment can save a horse.

4.  Two varieties of Centaurea , Yellow star thistle and Russian knapweed,
are poisonous plants that affect the nerve pathways in the brain that control chewing. Repeated ingestion is necessary to cause poisoning, generally over the course of a month or more, but once symptoms present they are permanent, often resulting in weight loss and possible starvation.

5.  Tansy (Senecio jac obeae), despite its pretty yellow flowers, should be culled from all horse pastures. It contains alkaloids that accumulate and harm the liver over time, resulting in serious, sometimes irreversible damage. Signs of liver failure are jaundice, decreased appetite, photosensitivity and weight loss.

6.  Poison hemlock (Conium maculatum ) is common throughout most of North America, and is toxic in any amount. Unrelated to water hemlock, it will cause tremors, nervousness and decreased heart rates within hours of ingestion, and consumption of just a few pounds will lead to respiratory failure in horses.

7.  Oak trees (Quercus species )
themselves are not poisonous plants, but the acorns, which contain high amounts of tannins and protein, can become an acquired taste for horses and cause severe dehydration within hours. Oak trees may be eliminated from the pasture, or you can keep your horses out of the area during the fall until there have been several frosts (the frost lowers an acorn’s toxicity) and keep the area well supplied with choice hay.

8. Common to the western United States, locoweed refers to species of both Oxytropis and Astragalus (also known as milk vetch). These are known for their ability to disrupt neurological and motor function. Horses that eat locoweed may move their heads up and down, walk strangely, act aggressively or stumble. Horses can recover from eating small amounts, but larger doses will cause permanent nerve damage.

9. Red maples (Acer rubrum), now planted and growing throughout North America, have leaves whose toxins present as they die and wilt, whether in autumn or after storm damage or pruning. Just one pound can poison a horse, damaging his red blood cells and starving of oxygen vital organs such as the liver and kidneys. Symptoms can appear anywhere from hours to days after consumption: the more leaves are ingested, the sooner signs will present and the more serious the damage will be.

10. Sudan grass and Johnsongrass, both species of Sorghum, contain cyanide compounds that will cause oxygen starvation in a horse’s red blood cells. Young or frost-damaged grasses contain the highest levels and are considered the most dangerous. Sorghum grasses properly dried and prepared for hays contain the lowest levels and are generally believed safe for consumption.

11. Trees in the Prunus genus, including all plums, cherries, peaches and apricots, produce leaves, bark and fruit pits that contain cyanide. Wild cherry trees are the most toxic of all, but any of these trees should be eliminated from pasture areas. The leaves behave similarly to those of the red maple, producing more cyanide compounds when they wilt, and can cause dangerous symptoms in horses within minutes of ingestion. Signs to watch for are trembling, loss of bowel and urinary control, labored breathing, flared nostrils and collapse.

12. Wild pea or Crotalaria is found throughout the western United States and is quite toxic to horses, containing liver and lung damaging alkaloids. Depending on how much is ingested, and how often, it can can produce both acute or chronic liver damage, resulting in fatigue, jaundice and weight loss.

13. Tobacco and potato plants
are the most dangerous members of the nightshade family (Solanum) for horses, containing toxins such as nicotine, pyridine and solanine. Poisoning by this family of toxins can produce the following symptoms: a weak, rapid pulse, cold extremities, shaking, elevated temperature, difficulty breathing and walking, and paralysis.

Although some of these poisonous plants and trees are very common, you can protect your horse with vigilant pasture management. Weed out the bad, make sure he has access to plenty of nutritious forage, and he’ll stay safe and healthy.


Maya Cointreau is an herbalist with 15 years of experience and the author of Equine Herbs & Healing: An Earth Lodge Guide to Horse Wellness. She can be contacted at info@earthlodgeherbals.com.

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