Although many people think of hay as nothing but ”gut fill,” the truth of the matter is that fermentation of fiber produces 50% or more of the calories in most equine diets — or at least it should.
The fiber components of your horse’s hay, pasture and other feeds do vary in how easily they are fermented. Hemicellulose is more fermentable than cellulose, and lignin is poorly fermented, if at all. Although technically not fiber, there are other complex carbohydrates that escape digestion in the small intestine but are highly fermentable. These include pectin, beta-glucans, simple plant sugars like xylose and more complex plant sugars such as fructans. These substances are also ”digested” by the micro-organisms in the large bowel. They are typically referred to as soluble fiber, because they will dissolve in water to form a slime or gel. These are the substances that will make the milk in a bowl of cereal thicken if you let it sit.
Grass and hay is the major source of cellulose, hemicellulose and lignan, but depending on the species also contains variable levels of fructans (cold-tolerant hays), beta-glucans (primarily grass hays) and pectin (primarily alfalfa and clover). Other sources of easily fermented soluble fiber include hulls (e.g., soy, oat), beet pulp, flax, apples and other pulpy fruits, oat bran, rice bran and psyllium. Wheat bran is higher in insoluble fiber and more difficult to ferment.
The higher the level of fermentable materials in the diet, the greater the contribution. Because fiber fermentation occurs at a slower and more constant rate than the digestion of a grain meal, it provides a constant flow of calories without swings in blood sugar.
Lignin and poorly fermentable cellulose add bulk to the manure and are believed to help regulate intestinal motility. Far more important, however, is the fermentable material, like hemicellulose and soluble fiber. These materials are fermented by intestinal organisms to produce what are called VFAs, volatile fatty acids, which include acetate, butyrate and propionate.
Lactate may also be produced from plant sugars or undigested concentrate that reaches the large bowel, but this is normally only a small amount. Lactate and propionate are taken up by the liver to be converted to glucose.
Glucose can then either be released into the blood stream, stored as glycogen in the liver, or used to manufacture fats or amino acids. Acetate is the most abundant VFA. It can be used directly as an energy source by the liver or other body tissues. The butyrate produced is primarily used as an energy source by the intestinal lining cells.
The VFAs also stimulate water and sodium uptake by the colon and increase the absorption of other minerals in the colon. In the horse, the fiber is absorbed, the minerals released and the VFAs enhance their absorption, making the colon a much more important location for mineral absorption in the horse.
Finally, healthy populations of fiber-fermenting organisms in the colon protect from intestinal disease by several mechanisms. They provide gentle, constant stimulation to the rich layer of immune-system cells lining the intestine. They maintain an environment in the colon that discourages the growth of disease-causing organisms and compete with them for nutrients.
