What should I feed my horse?
Horses are nonruminant animals, meaning they have a simple, one-chamber, stomach similar to humans, dogs and pigs. However, their diets are very different. Horses are able to obtain nutrients from ingredients that most other nonruminants can't because they have a very well developed cecum, and they have more capacity for nutrient absorption in the large intestine than many other animals. The cecum is a blind pouch that sits between the small intestine and large intestine, and serves as a large fermentation vat. This means that the cecum contains millions of bacteria and other microorganisms that are able to breakdown feed ingredients that the animal can't on its own. For example, cellulose, which is a structural component in grass and hay, can't be broken down by enzymes produced by the horse, but can be utilized by microorganisms in the cecum, which in turn produced volatile fatty acids (VFAs) that can be used as an energy source by the horse. Ruminant animals, like sheep and cattle, also have large fermentation vats, but in the case of ruminant animals this occurs on the from end of the digestive tract, shortly after absorption.
In the case of the horse, fermentation occurs much further down the digestive tract. As a result, the process is somewhat less effective than in ruminants, but the horse has partially compensated by having increased absorptive capacity for nutrients in the large intestine. With most species, we assume that almost all absorption occurs in the small intestine. This is not true for the horse. Deciding what to feed your horse can be complex because horses come in all sorts of breeds, sizes, live in different climates, have widely different expectations in terms of work, and may or may not have access to pasture. Therefore, prior to making a decision on what to feed your horse, it is important to have an understanding of how horses consume and digest feed.
Below are a series of videos which attempt to provide detail on these issues. The first is a very nice 3D animation of the horse gastrointestinal tract put together by PurinaMills and available through their YouTube channel. The second is a video put together by a group of undergraduates at Purdue University and walk through the anatomy and physiology of an actually equine GI tract.