After a horse has ingested a mouthful of food, the circular chewing motion is used to grind up the food into smaller particles which will then be mixed with saliva, forming a bolus. The bolus is then pushed to the back of the pharynx, where it is swallowed and passed into the esophagus. The esophagus is a muscular structure approximately 4 to 5 feet in length which carries the food to the stomach by way of the cardiac sphincter. No digestion actually takes place in the esophagus.
Horses are herbivores, meaning that their natural diet consists of plant materials. Unlike ruminant species such as cows which possess multi-compartmented stomachs, horses have a simple (monogastric) stomach similar to humans. Only approximately 10 % of the capacity of the gastrointestinal tract is made up of the stomach. This small size evolved in order to manage the natural eating habits of horses—smaller portions of roughage consumed frequently while grazing. With domestication and the human desire for convenience, the more common feeding trend nowadays is to feed larger amounts of hay and grain less frequently. This husbandry practice is at odds with the digestive health of the horse.
The horse’s stomach lies on the left side of the abdomen and contains both glandular and non glandular portions which are separated from one another by a region called the margo plicatus. The stomach’s capacity is approximately 8 to 16 liters and it is rarely empty in its natural state. The retention time of food and fluids differs—while fluids pass from the stomach into the duodenum rapidly, feed particles may remain in the stomach for more than 48 hours while the process of digestion begins. Horses are unable to vomit due to the presence of the strong cardiac sphincter. This is a muscular ring which connects the esophagus at an oblique angle with the stomach. Any condition which prevents the stomach from emptying properly into the small intestines at a normal rate will lead to the accumulation of gastric fluid and can ultimately lead to the rupture of the stomach.
The bottom portion of the stomach is glandular and secretes acid. It has a protective coating to help prevent it from being damaged by this acid. The top (non-glandular) portion of the stomach is designed to mix the liquid and solid contents and lacks the protective coating that the bottom portion of the stomach contains. As a result, the non glandular region is much more susceptible to the development of gastric ulcers.