(I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be added to the sidebar. Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments. Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)
Although it may take as little as 7 to 8 hours for the contents of the digestive system to reach the large intestine, or colon - with most of this time spent in the cecum - those contents reside in the colon for as long as 48-65 hours. The large colon has a length of 10-12 feet, with a capacity of 80 quarts, and the small colon also is about 10-12 feet long and has a capacity of about 14 quarts. The large and small colons together constitute about 45% of the digestive system, and together with the cecum, 60%.
As the food exits the cecum, it carries the cecal microorganisms with it, and during its stay in the large colon, the food continues to be digested and fermented. The colons are primary sites for absorption of water, electrolytes and volatile fatty acids produced by fermentation. Very little protein is absorbed, although proteins are produced by fermentation (ruminants, with the rumen in front of the small intestine, do benefit from the proteins produced by fermentation). Up to 95% of the fluids absorbed by the digestive system are absorbed in the cecum, large colon and small colon.
The food within the upper parts of the colon is moved both by peristalsis (contraction of muscles that move from front to back of system) and by anti-peristalsis (food is moved back in the direction it came from). This is a main reason why the food takes so long to transit the colon. It is theorized that the nervous system of the colon can sense the size and consistency of food particles, and since food particles can move both in the normal direction and also retrograde, digestion is thereby maximized.
The structure of the colon is complex - there are several distinct segments: right ventral colon, left ventral colon, left dorsal colon, right dorsal colon, short transverse colon (all these parts constitute the large colon) and descending (or small) colon. Due to the many turns the large colon makes and abrupt narrowing of its diameter midway through its course - at a point where a turn in direction also occurs - the large colon can be a site for impaction colic. It is also not attached to the abdominal wall during part of its course, which can result in displacements or torsions.
The descending, or small, colon, is where fecal balls are formed. Both the small colon and the rectum are delicate in their structure and easily damaged, and rectal exams for pregnancy or for impactions should only be done by qualified veterinary professionals, and even then injuries can occur.
Finally, the fecal balls exit the digestive system through the anus.
To summarize, the equine digestive system is perfectly suited to a diet composed of small, continuous meals of forage that is not too high in non-structural (easily digested) carbohydrates. Healthy functioning of the digestive system is facilitated by free access to clean water and salt, and by the movements the horse makes while foraging. Any significant divergence from this - infrequent, large meals or meals that are too high in easily-digested carbohydrates, too little forage in the diet, too little time spent moving around, or inability to access clean water and salt, can seriously disrupt the equine digestive system, leading to such conditions as improper tooth wear and TMJ problems, choke, ulcers, colic and laminitis. In many respects, many of the ills our horses are subject to are really problems created by our feed and stabling practices. Although keeping horses invariably involves some compromises, the closer we can come to keeping our horses in a way that maximizes their healthy digestive functioning, the better.