Horses have a surprisingly sensitive digestive system, and many common foods, plants, and even tree shavings can cause serious illness or death. Some of the most dangerous items are ones you might not suspect: lawn clippings, wilted leaves from a storm-downed branch, or a well-meaning treat from your kitchen. Here’s what you need to keep away from your horse and why.
Chocolate and Caffeine
Chocolate contains theobromine and caffeine, both of which are toxic to horses. These compounds overstimulate the heart and nervous system by blocking the body’s natural braking signals at the cellular level. They also increase calcium inside muscle cells, forcing the heart to contract harder than it should.
Mild signs like restlessness and elevated heart rate begin at around 20 mg of theobromine per kilogram of body weight. Severe symptoms, including dangerous heart rhythms, appear at 40 to 50 mg/kg, and seizures can start at 60 mg/kg. Dark and baking chocolate contain far more theobromine than milk chocolate, making even small amounts more dangerous. A horse that raids a bag of baking chocolate is in significantly more trouble than one that steals a milk chocolate bar, though neither situation is safe.
Avocado
Every part of the avocado plant is dangerous to horses. The leaves, fruit pulp, peel, and seed all contain toxic compounds called acetogenins, the most notable being persin. These substances damage heart muscle cells directly. In a two-year study of six horses that regularly ate fallen avocado fruit and leaves, two died. Necropsy revealed severe scarring and fibrosis of the heart tissue. This isn’t a mild irritant; avocado causes permanent cardiac damage, and there’s no safe amount for horses.
Onions and Garlic
Onions and garlic belong to the Allium family, and both destroy red blood cells in horses. When chewed or digested, these plants release sulfur-containing compounds that oxidize the surface of red blood cells, forming clumps called Heinz bodies. The damaged cells are then destroyed by the body faster than they can be replaced. This process begins within 24 hours of ingestion, peaks around 72 hours, and leads to visible anemia 3 to 5 days later.
Garlic is 3 to 5 times more potent than onion. Despite garlic supplements being marketed for horses (often for fly control), the risk of hemolytic anemia is real with repeated or large doses. If you use a garlic-based supplement, the margin between “possibly tolerated” and “toxic” is narrow enough to warrant caution.
Nightshade Plants: Potatoes and Tomatoes
White, red, and yellow potatoes all belong to the nightshade family and contain solanine, a compound concentrated in the skin, eyes, and any green-colored flesh. Tomato plants, especially the leaves and stems, carry similar toxins. Symptoms of solanine poisoning include diarrhea, slow pulse and breathing, abdominal pain, and abnormally low body temperature. Never feed raw potatoes to a horse, and keep them away from potato peels in compost or kitchen scraps. Sweet potatoes are a different botanical family and are generally considered safe in small amounts.
Wilted Cherry Leaves and Other Cyanide Sources
Fresh cherry leaves on a living tree pose minimal risk because horses rarely eat enough to cause problems. The danger spikes dramatically when branches break during storms and the leaves wilt. As cherry leaves dry, they release hydrogen cyanide, one of the fastest-acting poisons in nature. Signs of cyanide toxicity can appear within 15 to 20 minutes: drooling, rapid breathing, a weak pulse, convulsions, and collapse. The mucous membranes turn bright red because the blood is carrying oxygen the cells can no longer use.
Research on ruminants shows that as little as 1.2 to 4.8 pounds of wilted black cherry leaves can kill a 1,200-pound animal. Horses are similarly vulnerable. After any storm, walk your pastures and fence lines to remove downed cherry branches before turning horses out.
Red Maple Leaves
Fresh red maple leaves on the tree are not toxic, but wilted or dried leaves and bark are extremely dangerous. According to Tufts University’s veterinary school, fallen red maple leaves remain toxic for about four weeks. Most cases occur after late summer and autumn storms knock down branches.
Red maple contains multiple toxins that attack red blood cells in two distinct ways. Gallic acid and tannic acid directly destroy red blood cells, a process called hemolysis. A separate toxin, pyrogallol, prevents the remaining red blood cells from carrying oxygen. The combination is devastating: the horse loses red blood cells while the surviving ones can’t do their job. Affected horses develop dark or reddish urine, lethargy, and rapid breathing. Without aggressive treatment, the condition is often fatal.
Black Walnut
Black walnut is unusual because a horse doesn’t need to eat it to be poisoned. Shavings or sawdust that contain as little as 5 to 20 percent black walnut wood can trigger laminitis, an extremely painful and potentially crippling inflammation of the tissue connecting the hoof wall to the bone inside the hoof. Research has shown that compounds in black walnut heartwood cause selective dysfunction of the tiny veins inside the hoof’s laminar tissue, disrupting blood flow during the early stages of laminitis. If your horse is bedded on wood shavings, verify the source does not include black walnut.
Rhubarb
Rhubarb leaves contain high levels of oxalic acid, which binds to calcium in the bloodstream and forms insoluble crystals. These crystals can deposit in the kidneys, leading to kidney stones and progressive kidney damage. The concentration of oxalic acid in rhubarb varies with the season, growth stage, and weather conditions like drought or heavy rain, so there’s no reliably “safe” amount. Keep horses away from rhubarb plants entirely, including any trimmings thrown into garden waste piles.
Lawn Clippings and Garden Waste
Dumping lawn clippings over a fence into a horse pasture is one of the most common and least understood dangers. Freshly cut grass begins fermenting almost immediately when piled together, and horses tend to gorge on clippings rather than graze them slowly. This rapid intake of fermenting material can cause severe colic and dangerous gas buildup in the gut.
Even worse, grass clippings can harbor Clostridium botulinum, the bacterium that produces botulism toxin. In 2010, four horses in northern California developed progressive paralysis after eating grass clippings from a nearby park. The clippings tested positive for preformed botulism toxin type A. Botulism in horses causes a creeping flaccid paralysis that can be fatal even with treatment. The simple rule: never feed lawn clippings to horses, and make sure neighbors know not to toss theirs over the fence.
Other Foods to Avoid
Several more items belong on the no-feed list:
- Brassicas (cabbage, broccoli, cauliflower, kale) can cause gas and digestive upset and may interfere with thyroid function in large quantities.
- Persimmons can form a sticky mass in the gut that causes intestinal blockages.
- Buttercups, commonly found growing in pastures, cause blisters in the mouth and facial swelling with mild exposure. Larger amounts can trigger colic, diarrhea, tremors, seizures, or paralysis.
- Acorns and oak leaves contain the same tannic acid found in red maple and can cause kidney damage and red blood cell destruction when consumed in quantity.
Stone Fruits and Safer Treats
The flesh of peaches, apricots, plums, cherries, nectarines, and mangoes is safe for horses in small amounts. The pits are the problem. They pose a choking hazard and, like cherry leaves, can release cyanide compounds. Always remove the pit completely before offering stone fruit as a treat.
Plain crackers and cookies without chocolate are generally safe in small quantities as occasional treats. Carrots, apples (cored and sliced), and watermelon remain the go-to safe options. When in doubt about a food, skip it. Horses don’t need variety in their treats the way people do, and the consequences of guessing wrong can be severe.