Grazing is a full-time job for horses. Given their druthers, they’d graze for 12 hours or more every day, their broad, flat teeth and sideways chewing motions making short work of the tough, stemmy grasses and weeds they favor. Like all true herbivores, horses get most of their daily energy requirements from eating plant fibers. Yet, ironically, horses can’t digest fiber.

In fact, no animal can digest fiber on its own. Animals don’t produce the enzymes needed to break the beta bonds of polysaccharide fibers and make the nutrients within available for use. Fortunately, horses, like most other animals, have an almost invisible ally–a population of intestinal bacteria, resident in the cecum and colon, that are specially adapted to digest the fiber that horses cannot. Through a fermentation process, these gut flora produce the necessary enzymes to convert fiber to volatile fatty acids (VFAs), which the horse can absorb. Not only do the bacteria benefit (making this a truly symbiotic relationship), but the VFAs they create provide 30-70% of the horse’s total digestible energy needs.

While we often provide grain and supplemental fats to our domestic horses to give them the energy to do hard work, it’s important to remember that it’s fiber that horses were meant to use as fuel–and fiber remains the first and most important ingredient in every equine diet. It provides all the energy horses need for everyday maintenance metabolism–ordinary stuff like breathing, walking, grazing, and sleeping. Without adequate fiber, the horse’s digestive system doesn’t function properly–it loses the ability to move food particles efficiently through the gut, and its ability to conserve water and electrolytes also is compromised. Without fiber in the system, high-carbohydrate feeds tend to "pack" in the gut as well. The result is a horse at risk for dehydration, colic, and laminitis (not to mention stable vices like cribbing and wood-chewing, which often develop when a horse’s fundamental urge to chew is not satisfied).

Except in the most strenuous circumstances (such as 2-year-olds in heavy race training), fiber should make up at least 50% (by weight) of your horse’s daily diet. For the vast majority of adult horses, that percentage can be pushed up considerably higher–even to 100%, if the horse is an easy keeper and/or not being asked to do work. The basic principle is this: grain is an optional part of a horse’s diet; roughage (fiber) is not