Horses carry billions of passengers with whom they have a mutually beneficial relationship. No, they’re not his human riders—they’re the plethora of bacteria that make up his gut microbiome.

These bacteria don’t exist in a vacuum, but rather develop cooperative and competitive interactions within the horse’s digestive tract. The bacteria utilize food sources, are sensitive to pH and salinity, and have populations largely determined by the horse’s diet. In turn, bacteria produce B vitamins and volatile fatty acids (VFAs), on which the horse depends for energy production.

Does this sound complex? That’s because it is! To help simplify the topic at the 2017 Mid-Atlantic Nutrition Conference, held April 6 in Hunt Valley, Maryland, Amy Biddle, MEd, PhD, of the University of Delaware Department of Animal and Food Sciences, presented an overview of the equine gastrointestinal microbiome.

“The bacteria are master biochemists,” Biddle said. “They are extremely metabolically flexible, can adjust to the environment, and there’s nothing wasted in that system. We know there are differences in the bacterial populations found in horses with and without equine metabolic syndrome, young and old horses, horses with colitis versus without, and semi-feral and domestic horses.  How those microbiome differences translate into health outcomes is not well understood