A horse has one stomach. Unlike cows and sheep, which have four stomach compartments, horses are single-stomached animals (monogastric). That single stomach is surprisingly small for an animal that can weigh over 1,000 pounds, holding only about 4 gallons, the smallest stomach relative to body size of any livestock species. But what makes the horse’s digestive system interesting is how that one stomach is divided internally and how it works with the rest of the gut to break down tough plant material.
Why Horses Aren’t Ruminants
Cows, sheep, and goats are ruminants. They have a massive four-compartment stomach where microbes break down grass and hay before the nutrients ever reach the intestines. This is called foregut fermentation: the plant material gets fermented first, digested second. The microbes themselves become a protein source once they’re killed further along in the digestive tract.
Horses take the opposite approach. They digest food with enzymes in the stomach and small intestine first, then send whatever is left, mainly fiber, to a large, specialized section of the hindgut called the cecum and large colon. There, billions of microbes ferment the plant fiber and produce fatty acids that the horse absorbs through the colon wall for energy. The horse’s large colon is dramatically elongated compared to other animals, giving microbes maximum contact time with the fiber passing through. This makes the horse a hindgut fermenter, sometimes called a pseudo-ruminant because it accomplishes a similar goal through a completely different route.
One trade-off of this design: horses can’t extract protein from their gut microbes the way cows can. In ruminants, the microbes multiply in the stomach, then get digested downstream, supplying the animal with microbial protein. In horses, the microbial action happens after the main protein-absorbing sections. So horses rely entirely on the protein in the food they eat.
Two Regions Inside One Stomach
Though horses have a single stomach, it’s divided into two distinct regions that look and function differently. The upper portion, closest to where the esophagus enters, is lined with pale, tough tissue similar to the lining of the esophagus itself. This nonglandular (squamous) region doesn’t secrete acid or digestive enzymes. It serves more as a holding and mixing area, and the pH here stays relatively neutral, especially near the top.
The lower portion is the glandular region. It’s lined with tissue that actively produces hydrochloric acid and digestive enzymes, similar to a human stomach. The pH drops significantly here, reaching 1.5 to 4.0 near the exit valve that leads to the small intestine. A visible ridge called the margo plicatus marks the boundary between these two zones. This boundary matters clinically because it’s one of the most common sites for stomach ulcers in horses.
Between these two zones, along the margo plicatus itself, the pH sits in an intermediate range of roughly 3.0 to 6.0. This gradient from near-neutral at the top to highly acidic at the bottom means different parts of the stomach face very different chemical environments, which influences where problems tend to develop.
Why Horses Can’t Vomit
One of the most important quirks of the horse stomach is that food can only go in one direction. Horses have an exceptionally strong muscular valve where the esophagus meets the stomach. This lower esophageal sphincter clamps shut so tightly that horses are physically unable to vomit. If gas or fluid builds up in the stomach, the organ will rupture before the contents can travel back up the esophagus. This is one reason colic, a broad term for abdominal pain in horses, can become life-threatening so quickly. There’s no pressure relief valve.
How Food Moves Through
Because the stomach is small relative to the horse’s size, food doesn’t stay there long. The majority of a meal passes into the small intestine within about 12 hours. Horses are designed to eat small amounts almost continuously. In the wild, they graze 16 to 18 hours a day, keeping a slow but steady flow of forage moving through a stomach that was never meant to handle large, infrequent meals. When domestic horses are fed two big meals a day with long gaps in between, the stomach sits empty for hours while still producing acid, which is a setup for problems.
Stomach Ulcers Are Extremely Common
The combination of constant acid production, a small stomach, and modern feeding practices makes horses highly susceptible to gastric ulcers. Ulcers in the upper squamous region are the most frequently reported type, with prevalence ranging from about 11% in leisure horses to over 80% in competing racehorses and high-level endurance horses. The squamous lining lacks the protective mucus layer that the glandular region has, so when acid splashes upward during exercise or when the stomach is empty, that tissue takes damage quickly.
Ulcers in the lower glandular region are reported less often but still affect a substantial number of horses. Studies have found glandular ulcers in 17 to 65% of horses depending on the population, with Thoroughbred racehorses and sport horses at the higher end. One UK study of 51 domesticated horses found squamous ulcers in about 61% and glandular ulcers in roughly 71%, suggesting the problem is widespread even outside elite competition.
Feeding smaller, more frequent meals, ensuring constant access to forage, and reducing the time a horse’s stomach sits empty are the most straightforward ways to lower ulcer risk. The presence of forage in the stomach helps buffer acid and keeps it from splashing onto the vulnerable upper lining.