Deep within your horse’s gastrointestinal tract, there’s a whole civilization in miniature. Microorganisms, billions of them, have taken up residence in his cecum. But there’s no cause for alarm; these thousands of different species of bacteria, protozoa, yeasts, and fungi not only belong there, the majority of them actually are beneficial to the horse.

The horse’s gastrointestinal tract is not unique in this regard. Almost all animal species, including humans, have their share of intestinal "microflora" (a catchall term describing the normal microscopic inhabitants of an environment). The types of microbes can vary from species to species, but their presence and function are universal. They exist in a symbiotic relationship, helping their host animal extract and absorb the full nutrient value from his food. Some organisms are specialists in helping to digest cellulose (tightly linked sugar molecules contained in tough, fibrous foods such as hay, which are broken down into simpler glucose units). Others have a talent for handling the lactose sugars of milk, or for taking the byproducts of cellulose digestion and constructing from those bits and pieces the vitamins for the horse to use. Still others are responsible for maintaining the airless environment of the gut, removing oxygen so that their anaerobic microbial kin can continue to function normally