Pfizer gut video
The main function of the small intestine is to move nutrients from the non-cellulose parts of the diet into the bloodstream. This part of the GIT works similarly to that of the dog with the restriction to a vegetarian diet.
Once foodstuffs such as grasses, corn, oats, barley and fats enter the small intestine, they are mixed with enzymes and buffers from profuse pancreatic secretions to start breaking down the food products into glucose, amino acids and lipids. The intestinal wall contracts rhythmically to mix the ingesta with the enzymes. The simpler components are absorbed through capillaries and lymphatics in the intestinal wall where they can be transported to other organs for use or storage. At the same time, the ingesta is moved from the duodenum, through the jejunum and into the ileum. Gradually more and more of the ingesta is exposed to the gut lining and can be absorbed if it hits the right receptors. The downstream (aboral) movement occurs through coordinated movements of the intestine, aka peristalsis. Most of the simple sugars and proteins are absorbed in the jejunum while fats are absorbed in the ileum. The ingesta will often be held and mixed in the ileum until the fat content is minimal.
During the process, the starches (including those from oats and corn as well as from hay) are not broken down but are being hydrated by active fluid secretion. The water exchange that occurs in the SI equals about 1.5x the total extracellular fluid volume of the animal over a 24 hour period. In all species, the SI is the main site of water absorption; in the horse, the large colon also does a decent job. Eventually the remaining components are moved into the cecum where microbial digestion will start.
One major difference from dogs is the type of motility patterns. Dogs eat set meals while horses graze continuously. This means there are different electrical and contractile patterns that result in continual activity with fewer periods of mass movement.
The cecum is sometimes called the fermentation vat. Ingesta empties into the cecum via control by the ileocecal valve. Receptors in the ileum permit emptying after the fat content is properly lowered by nutrient absorption. Microbes in the cecum start the process of digesting cellulose. The cecal contents should always be liquefied, despite the fact that a large amount of water is absorbed here. Even though the cecum is a relatively small organ, the transit time averages 5 hours to allow proper mixing of contents with microbes and fluid. Afterwards, intestinal contents are moved into the right ventral colon.
The equine large colon has been highly modified to allow cellulose digestion. It is very elongated to allow maximal exposure of the cellulose products to the microbes. Motility is largely mixing with propulsion slowed to keep the ingesta in the colon for long periods of time (transit time is 30-50h). This maximizes the nutrition that the horse can obtain from the hay diet. The microbes use the hay (and any leftover carbohydrates) for energy and produce volatile fatty acids that are absorbed by the colon wall and used by the horse. These fatty acids supply the major portion of the horse’s daily energy requirements (and is why they do well on a strictly hay diet).
The colon also buffers ingesta coming from cecum and absorbs fluid from the ingesta. A pony resorbs approximately 30 liters of fluid/day from the colon. As with the small intestine, the colon (with its microbes) is designed to handle frequent eating and continuous flow of ingesta. If horses are fed only twice daily, they will have much more extensive fluid shifts into the ingesta and out of the colon, particularly if they are fed highly fermentable carbohydrates. Increasing carbohydrates entering the cecum and colon also means happy microbes – and more gas production.
The small colon continues to remove water from the ingesta, forming fecal balls. The fecal balls can get drier and smaller with dehydration and impactions (more water is removed). It also continues some microbial digestion and buffering. Some retention also occurs (waiting on the defecation reflex).