COLUMBUS, Ohio – Plants and other items dangerous for horses to eat could be lurking in barns and pastures, according to a University of Kentucky associate professor.

Cythia Gaskill, DMV, also a veterinary clinical toxicologist at UK’s Veterinary Diagnostic Laboratory, has come up with the top plants, substances, organisms and chemicals most dangerous to horses.

“There are a number of substances that are toxic to horses,” Gaskill said, “with adverse effects ranging from mild to fatal, depending on what is consumed, how much, the horse’s size and health, and other case circumstances. Because horses don’t always know what’s harmful for them, owners and caretakers must be keenly aware of these threats when managing their horses.

“People often assume that horses know what to eat and what not to eat, but that’s just not true in a lot of cases. There are many things horses should never eat. From a curious nibble of a tree branch to accidental consumption of a contaminated grain meal, there are a number of ways horses can ingest toxic substances that put their health, and lives, at risk.”

Gaskill’s top 10 list has yew, oleander, ionophores, blister beetles, rodenticides/pesticides, herbicides, decaying organic matter, moldy corn, red maple leaves and tansy ragwort.

The American, English, Japanese and Western yew can be devastating to a hungry horse. As little as a mouthful or two of yew, Gaskill said, can be lethal.

“The plant’s alkaloid toxin taxine causes cardiac and respiratory failure, often within minutes,” she said. “It’s not uncommon for horses to be found dead with the plant still in their mouth.”

Oleander is a common decorative perennial evergreen shrub. It is common in the southern United States. Relatively small quantities of the plant are considered lethal. Ingestion might also cause colic. Horses are often exposed to oleander when people toss clippings into pastures and horses ridden on trails might also take a quick bite out of curiosity.

Ionophores are antibiotic feed additives used as growth promotors in cattle and poultry diets. Farmers also use these as antiprotozoal agents to control Coccidia infections. Ionophores are cardiotoxic to horses as they damage the heart muscle. Clinical signs of consumption include a loss of appetite, rapid heart rate, sweating or colic.

“Exposure to cattle feed with the approved amount of ionophore is rarely a cause of intoxication in horses,” Gaskill said. “It’s when horses get into a concentrated pre-mix or an improperly formulated cattle feed with a higher dosage that we typically see the problem.”

Abundant in the Midwest, blister beetles swarm alfalfa fields and can be baled into alfalfa hay during harvest. The beetles feed on the tops of alfalfa plants. They contain cantharidin, a toxic chemical and blistering agent.

Alfalfa hay can become contaminated with beetles that are crushed during the crimping process. Even dried and crushed, the beetles remain toxic for a long time.

“When the horse eats it, (he) can get blisters in the mouth and esophagus and ulcerations in the stomach and intestines,” Gaskill said, “and on the way out it can damage the kidneys and cause ulceration in the urinary bladder.”

Rodenticides and pesticides are products formulated to kill rats and mice, gophers, birds, snails, slugs, ants and other offending pests. They often consist of pelleted, granular or powdered bait. In sufficient doses, the toxic agents used in rodenticides and pesticides can be lethal to horses.

These bait-type products often contain sweet flavorings or grain bases designed to attract pests.

“Unfortunately, these qualities often make them attractive to horses as well,” Gaskill said. “Exposure typically occurs due to inappropriate storage.”

For herbicides, the most common means of exposure is when an owner has used them in a horse’s field and forgotten about it. Horses can have diarrhea or colic post-exposure. Gaskill said when herbicides are used, they can bring certain chemical changes in the plant, which for some reason can make it more attractive to the horse.

Gaskill said “rotting hay, haylage or other organic matter has the potential to harbor C. botulinum toxins.

“Toxin ingestion is one of the most common routes of exposure in horses, though it can also be acquired through exposed wounds,” she said. “Some parts of the country, such as Kentucky, have high levels of the toxin producing bacterial spores in the soil. Rotting, decomposing hay or improperly put up haylage or silage are common sources of contaminated material. Animal carcasses can also harbor the toxin.”

Fumonisin (or moldy corn) is a fungal toxin that can infect corn prior to harvest or during storage. Hot, dry conditions followed by high humidity are associated with increased fumonisin concentrations in growing corn, usually in the Midwest and South.

Most commercial feed mills test for fumonisin contamination. But feeding untested corn, such as that which has come directly from the field, can put horses at risk particularly in regions where fumonisin is more prevalent.

Researchers have known that wilted red maple leaves can be toxic to horses, but they now suggest that other species, such as sugar and silver maple, might be problematic as well.

The leaves in combination with certain bacteria produce a strong oxidant that damages horses’ red blood cells, hindering their ability to carry oxygen or destroying them completely. To be affected, an average-sized horse would need to consume an estimated 1.5 to 3.3 pounds of wilted leaves.

Tansy ragwort is a non-distinct yellow flowering plant that grows throughout most of North America.

“If a horse eats enough of the plant over a short period of time, or smaller amounts over a longer period of time, they can develop an irreversible chronic liver disease, though they may not show signs for six months to a year,” Gaskill said.

Horses might consume the plant if it gets baled into hay.