Horses have a surprisingly sensitive digestive system, and many common foods, plants, and yard waste can cause serious illness or death. Some dangers are well known, like moldy feed. Others catch owners off guard, like lawn clippings or a black walnut tree near the fence line. Here’s a practical breakdown of what to keep away from your horse.
Fruits and Vegetables That Are Toxic
Avocado is one of the more dangerous foods you might not suspect. It contains a compound called persin that can cause swelling of the head and neck in horses, along with cardiac problems. Every part of the avocado plant is a concern, including the fruit, pit, leaves, and bark.
Potatoes and tomatoes belong to the nightshade family, and certain parts of these plants contain glycoalkaloids, compounds that irritate the gut and can affect the nervous system. Green, sprouted, or damaged potatoes are especially concentrated with these toxins. Symptoms range from nausea, vomiting, and cramping to more serious neurological effects like confusion, shaking, weakness, and disturbed vision. The green stems and leaves of tomato plants carry similar risks. As a rule, keep all nightshade plants and their scraps out of your horse’s reach.
Stone fruits like cherries, peaches, apricots, and plums pose a double threat. The pits are a choking hazard and can cause esophageal obstruction. More critically, the seeds and leaves of these plants contain cyanide. When a horse chews the seeds or when the foliage wilts after a frost, cyanide is released. Apricots and peaches tend to have the highest concentrations of toxin in their seeds.
Chocolate, Coffee, and Caffeine
Chocolate and caffeinated products contain methylxanthines, a group of stimulant compounds that horses cannot process safely. Ingestion can cause vomiting, diarrhea, excessive thirst, hyperactivity, abnormal heart rhythm, tremors, seizures, and in severe cases, death. Darker chocolate and baking cocoa carry the highest concentrations. Even white chocolate, which has the least, is not worth the risk. Keep all chocolate, coffee grounds, and caffeinated drinks well away from the barn.
Onions and Garlic
All members of the allium family, including onions, garlic, leeks, and chives, contain sulfur-based compounds that damage red blood cells. When a horse chews or digests these plants, the compounds are absorbed and begin oxidizing red blood cells within 24 hours. This damage peaks around 72 hours after exposure, and by days three to five, red blood cells start breaking apart in a process called hemolysis. The result is a form of anemia that leaves the horse weak, lethargic, and oxygen-deprived. Even cooked onions and garlic are dangerous because the harmful compounds survive heat.
Too Much Grain or Starch
Cereal grains like corn, wheat, and barley are common in horse feed, but overfeeding them is one of the most frequent dietary mistakes. Horses are designed to digest fiber, not large loads of starch. When too much grain is consumed at once, a significant portion escapes digestion in the stomach and small intestine and arrives in the hindgut, where it ferments rapidly. This produces a surge of lactic acid that drops the gut’s pH, killing off the beneficial fiber-digesting bacteria and allowing acid-producing bacteria to take over. The cascade causes mucosal irritation, inflammation, water flooding into the intestine, and diarrhea. In more severe cases, this process, sometimes called hindgut acidosis, triggers colic or laminitis.
The practical takeaway: grain should be fed in small, measured meals rather than large single portions. If your horse’s workload changes, adjust grain quantities gradually over a week or more.
Wheat Bran and Mineral Imbalances
Wheat bran is sometimes offered as a treat or mash, but feeding it regularly throws off the calcium-to-phosphorus ratio in your horse’s diet. The ideal ratio is about two parts calcium to one part phosphorus, with anything between 1:1 and 6:1 considered acceptable. Wheat bran is very high in phosphorus and low in calcium. When a horse takes in too much phosphorus over time, the body pulls calcium out of the bones to compensate. This leads to a condition called nutritional secondary hyperparathyroidism, historically known as “big head disease.” It starts as shifting lameness and progresses to brittle, weakened bones. An occasional bran mash is unlikely to cause problems, but making it a dietary staple without balancing the minerals is a real risk.
Toxic Trees and Plants
Red maple is one of the deadliest trees a horse can encounter. The leaves contain multiple toxins, including gallic acid, tannic acid, and a compound called pyrogallol. Some of these destroy red blood cells outright. Others prevent the remaining red blood cells from carrying oxygen by converting them into a dysfunctional form. The combined effect leaves the horse severely oxygen-deprived, causing weakness, depression, colic, and laminitis. A telltale sign is muddy brown mucus membranes. Wilted or fallen leaves in autumn are particularly dangerous, and even a small amount can be lethal.
Yew is another deadly tree. All parts of it are toxic to horses, and ingestion can cause sudden cardiac failure. Black walnut is harmful in a different way: the innermost wood contains a toxin that causes acute laminitis (founder) within just a few hours of contact. This is especially relevant for bedding. If black walnut shavings are mixed into stall bedding, even skin contact with the horse’s legs can trigger a severe laminitic episode. Never use shavings from an unknown source without confirming they are free of black walnut.
Chokecherry is lethal. Its leaves and seeds contain cyanide, which is released when horses chew the plant or when the foliage wilts. Walk your pasture and fence lines regularly and remove any chokecherry, wild cherry, or related plants growing within reach.
Lawn Clippings and Yard Waste
Dumping grass clippings over a pasture fence is one of the most common and underestimated dangers. Fresh lawn clippings begin fermenting almost immediately once piled together. The warm, moist, low-oxygen environment inside a heap of cut grass is ideal for the growth of Clostridium botulinum, the bacterium that produces the botulism toxin. This neurotoxin blocks communication between nerves and muscles, causing progressive paralysis that can be fatal.
Horses also tend to gorge on clippings rather than graze them slowly, which increases the risk of colic. Garden trimmings are equally risky because they may contain ornamental plants, nightshades, or other toxic species mixed in. The simplest rule: never feed your horse anything that came out of a lawn mower or garden cleanup.
Moldy or Spoiled Feed
Mold in hay or grain produces mycotoxins, invisible chemical byproducts that can cause a range of serious health problems. These include colic, neurological disorders, paralysis, hypersensitivity, and brain lesions. One particularly devastating condition linked to mycotoxins in moldy corn is a brain disease that destroys white matter in the horse’s brain.
Inspect hay before feeding by breaking open flakes and checking for dust, discoloration, a musty smell, or visible mold. Store hay in a dry, well-ventilated area off the ground. Grain should be kept in sealed containers and used before expiration dates. If hay or feed gets wet or looks or smells off, discard it. The cost of replacing a bale is nothing compared to treating mycotoxin poisoning.
Quick Reference: Common Items to Avoid
- Avocado: all parts, including fruit, pit, and leaves
- Chocolate and caffeine: especially dark chocolate and baking cocoa
- Onions, garlic, and related plants: raw or cooked
- Potatoes: especially green, sprouted, or damaged
- Tomato plant leaves and stems
- Stone fruit pits and leaves: cherries, peaches, apricots, plums
- Red maple leaves: especially wilted or fallen
- Yew: all parts
- Black walnut: wood, shavings, and nuts
- Chokecherry: leaves and seeds
- Lawn clippings and garden waste
- Moldy or spoiled hay and grain
- Excessive cereal grain fed in large single meals