We humans are just starting to recognize dietary fiber benefits for ourselves in terms of preventing colon cancer, modulating blood glucose levels and supporting good bowel function. In horses, there is even more to it. Fiber is also a major and important source of energy/calories.
Humans and animals, including the horse, cannot digest fiber because they do not produce the enzymes needed to break it down to more simple compounds that can be absorbed and used by their bodies. However, microorganisms in their intestinal tracts can break it down. Because the horse has a large hind gut where food spends several days they can efficiently use fiber as an energy source.
Not all fiber is created equal. For the purpose of this discussion, I will focus on fiber types found in hay. Lignin is a very rigid and strong fiber, found in high amounts in wood and increasing with time as grasses age. Straws are also high in lignin. It is the most resistant to the fermentation efforts of intestinal organisms.
Next in line are cellulose and hemicellulose, also strong structural fibers but easier to ferment. All three are classified as insoluble fibers because they do not dissolve in water. Residual unfermented insoluble fiber gives manure much of its bulk.
On the other hand, soluble fiber which does dissolve in water is very easily and quickly fermented. This groups includes fructooligosaccharides of various sizes including fructans, pectin and beta-glucan. A large proportion of the calories present in hays come from this class of fiber.
Fiber is an important prebiotic because it feeds the beneficial organisms in the intestinal tract and stimulates gut-associated immune tissue. Insoluble fiber also works to keep organisms in the intestine, not flushed out, by providing microscopic rafts where bacteria can establish colonies. In humans, adding fiber to a meal can reduce the glucose and insulin spikes but research has not consistently shown this effect in horses.
As natural and beneficial as fiber is in the diet, some horses may react to high levels with diarrhea, particularly as they age. If a hay analysis is available, this usually correlates with an acid detergent fiber (ADF) level in high 30s or higher, neutral detergent fiber (NDF) high 50s or higher. The hays will be stemmy and stiff.
The solution is to find a younger, softer cut of hay. You could also try a supplement with cellulase and hemicellulase activity, fed in a small amount of beet pulp as a prebiotic. Beet pulp is rich in soluble fiber.
The fiber in your horse’s diet is much more than just roughage.
Eleanor Kellon, VMD