A Horse Owners Guide to Toxic Plants

A horse lovers worst nightmare is to see a horse in pain, for any reason. Sometimes it can't be helped - but there are a lot of things a horse owner can do to minimize the chances of a hurt horse. One of the simplest is knowing which plants need to kept out of reach.

Several plants have been noted as having systemic effects on horses. Some plants cause intense gastrointestinal difficulties, which can lead to colic and even death. While not all plants are found in every pasture and climate, being familiar with those that grow wild or cultivated in your area is an important step to keeping your horse safe.

If a plant is poisonous to humans, it's a safe bet that your horse should not eat it either. However, several plants that have no side effects in humans are poisonous to a horse - including such plants as avocado or mustard greens, both healthy delicacies on our plates.

Most of the time horses avoid toxic plants naturally. There are a number of reasons why they might accidentally ingest them. A young or curious horse might decide to nibble on an unfamiliar plant, or the horse might be exceptionally hungry or thirsty. Depending on the time of year, a toxic plant may simply look irresistibly green and lush compared to surrounding plants.

Some plants are so poisonous that a horse can eat just a handful of leaves and die. A brief list of some common and extremely toxic plants include:
Oleanders - Death can follow within 8-24 hours after ingestion.

Yew trees - A small evergreen native to the Pacific Northwest so toxic, a horse will die almost instantly with no time for treatment. There is no known antidote.

Poison hemlock - Symptoms will show within a few hours, but a full recovery is possible if treated immediately.

Water hemlock - Common plant that grows in ditches and other wet areas; symptoms show up about 15 minutes after digestion. Looks similar to poison hemlock.

Yellow Star Thistle - Symptoms typically occur after a horse has digested the plant over a period of time, and by the time they do show up, damage to the brain may be irreversible and death is likely.

Castor beans - As few as 7 grams have been reported as killing a horse, although about 50 grams or 150 beans is considered the typical amount needed to kill a 1-ton animal. Symptoms can take up to 2 days to show, but will then progress rapidly.

Timber Milk Vetch - Fast onset of symptoms that usually lead to death by asphyxiation.

Plants that will show symptoms after repeated ingestion includes the addictive Locoweed and Lupines, both of which lead to death - Lupines in particular have no known antidote, although recoveries from Locoweed digestion have been reported. Locoweed, Lupines, and other plants contain many varieties that are not poisonous. For the sake of safety, and due to the difficulty of distinguishing nontoxic varieties from poisonous ones, horses should be kept away from all potential dangerous plants.

Less toxic plants can induce dermatitis, milder gastrointestinal difficulties, and other symptoms that may not be immediately fatal but still require treatment. Different toxic plants produce varying reactions, many of which are also associated with other diseases that affect horses.

Symptoms of poisoning depend on the source and can include frothing at the mouth, a weak or rapid pulse, heavy or rapid breating, colic, a staggering walk, disorientation, weakness, behavioral changes, and many others. If your horse is acting unusual in any way, it is always best to call a veterinarian immediately.

The best thing to do is to familiarize yourself with a full list of toxic plants, what they look like and if they grow in your area. The pasture should be checked regularly, as well as other outdoor areas your horse frequents. For a compilation of the most common toxic plants visit:
www.aspca.org/site/PageServer?pagename=pro_apcc_horsetoxicplants

Sources:
Willis Lamm, "Poisonous Plants." Trailblazer Magazine, 1997.