Updated: Sep 15
A horse is a hind gut fermenter, meaning he is designed to eat fibrous material, being largely fermented in a holding chamber called the cecum, home to trillions of bacteria, archaea, protozoa and fungi working in synergy to break down poor quality and largely indigestible food material (woody- stalky material, seed heads) into energy. He is also designed to process rapidly degradable starch (grains, the leaves of grass and hay, alfalfa) in the stomach and small intestine, a horse needs this rapidly available energy when speed or work is required from him.
The fermenting process is a slower process compared to the rapid release of energy from easily degradable starch, fermented energy takes time to produce, but this method allows him to survive the winters in the wild, when food (especially starch) may be hard to come by. The horse is different from many animals because he is designed to consume large amounts of highly degradable starch in the spring and summer, which he needs for breeding, galloping, feeding young and storing up energy in fat deposits to use in the winter. Then when food is scarce he relies upon the fermentation process in the hind gut to produce energy much more slowly. In other words, he has two survival systems in place, perfectly matched to provide rapid energy (for moving at speed and breeding) and to produce slower energy during times when food is scarce.
Getting the balance right for a modern stabled horse can be challenging and metabolic imbalances such as laminitis, obesity, ems, ulcers, colic and acidosis of the hind gut are very common, the microbes of the gastro intestinal tract are very sensitive to changes in pH. If the degradable starch levels are higher than the slower-release fibrous material, then the lactic acid-loving bacteria can outnumber the acid-eating members of the gut biota and the horse is then susceptible to low-grade continuous acidosis, leading to the previously mentioned modern metabolic dysfunctions.
The long- term answer is to feed according to work and lifestyle, for most horses this is likely to mean a switch to a diet higher in natural fibre intake, just look at the fibre available in the photo of a Tuscany meadow taken in October.