Take a stroll through any pasture, and there among the grasses you'll find any number of different plants. Small vines, broad-leafed weeds, some wildflowers you recognize--some you don't. And, as disquieting as it may be to contemplate, the chances are pretty good that at least some are toxic to horses. Hundreds of poisonous plants grow in North America, and many are extremely common. "I defy anyone to tell me they have a pasture with zero poisonous plants," says Jeffery Hall, DVM, PhD, a toxicologist at Utah State University.
The good news, of course, is that the vast majority of those plants pose little threat to horses. For one thing, most of them are unpalatable, and horses who are filling up on quality forage aren't likely to spend a lot of time grazing on the few bitter leaves populating their pasture. Another factor that protects horses is their size--a 1,000-pound animal has to consume significantly higher quantities of most toxins than a smaller animal does to feel any effects. So, for the most part, as long as your horses are healthy and your pasture is in good shape, you have little to worry about.
However, some plants are cause for concern either because even a curious nibble can spell doom or because repeated browsing over weeks or months can lead to serious illness and death. All are worth getting to know by sight--not only so you can eliminate them from your horsekeeping areas, but also so that you can avoid encounters with them in the woods, on the roadsides and along the waterways where you ride. According to Anthony Knight, BVSc, MRCVS, plant toxicologist from Colorado State University, these 10 plants are those most dangerous to horses in the United States:
Bracken fern (Pteridum aquilinum)
Also known as: brake fern, eagle fern
ID:A perennial fern with triangular leaves that can reach two to three feet high. Grows in clumps in woodlands and moist open areas.
Range: Coast to coast, except for the Mediterranean and desert climates of Southern California and the Southwest.
The danger: Bracken fern contains thiaminase, which inhibits absorption of thiamin, which is vitamin B1. Thiamin is necessary to nerve function, and deficiencies can lead to neurological impairment. The relative toxicity of individual leaves is low--horses must consume hundreds of pounds to experience ill effects. However, bracken fern is unique among the toxic plants in that some horses seem to develop a taste for it and will seek it out even when other forages are available.
Signs: Signs are related to neural dysfunctions resulting from vitamin B1 deficiency and can include depression, incoordination and blindness.
What to do: Large doses of thiamin over the course of a week or two can aid in the recovery of horses whose bracken consumption is discovered before the neurological signs are severe.
Hemlock (Conium maculatum)
Also known as: poison hemlock, spotted hemlock
ID: A multistemmed perennial weed with toothed, fernlike leaves and clusters of small white flowers. The stems have purple spots, which are most evident near the base of the plant.
Range: Grows wild along roadsides and other open uncultivated areas throughout North America.
The danger: Hemlock leaves, stems and seeds contain several potent neurotoxins that affect both the central and peripheral nervous systems. Four to five pounds is a lethal dose for a horse. Most animals will avoid the plant.
Signs: Signs appear within an hour or two of consumption, starting with nervousness, tremors and incoordination, progressing to depression and diminished heart and respiratory rates and possibly colic. Death results from respiratory failure.
What to do: There is no treatment, but if smaller doses were consumed, animals may recover with supportive care.
Tansy ragwort (Senecio spp.)
Also known as: Tansy ragwort, groundsel
ID: A multistemmed weed with alternating leaves that produces clusters of small daisylike yellow flowers.
Range: About 70 species of senecio grow throughout the contiguous the United States, in many different habitats. Many are common in pastures and along roadsides.
The danger: Levels of toxicity vary among different members of the species, but all are thought to contain at least some concentration of pyrrolizidine alkaloids, which inhibit cell division, especially in the liver. Damage to the liver is cumulative and irreversible, and most horses succumb to chronic exposure over time, after consuming between 50 and 150 pounds, in total.
Signs: Often, there is no evidence of consumption until signs of liver failure begin to appear: photosensitization, diminished appetite and weight loss, progressing to depression, incoordination and jaundice.
What to do: There is no treatment for advanced stages of liver disease due to this toxin.