Everyone is familiar with fiber as roughage, the stuff laxatives are made of. With equine nutrition, there is much more to it than that.
Thanks to the specialized population of organisms living in their large intestine, and the prolonged amount of time their food spends there (about 2 days), the horse is able to break down fiber into nutrients called volatile fatty acids, VFAs. These can either be directly utilized by the body or converted into glucose and fat.
Forage as hay or grass is the major source of fiber but since the horse eats a vegetarian diet virtually all foods contain fiber – grains, seeds, brans, nuts, fruits.
Not all fiber is created equal in terms of its potential calorie yield. Fiber is broadly classified as soluble or insoluble, depending on whether or not it will dissolve in water. Insoluble fiber includes lignin, cellulose and hemicellulose. These are the forms that give structure to cell walls, form the skin on fruits, the outer coverings to seeds and nuts and the more rigid components of vegetables and fruits, like the “strings” in celery. There are several soluble fibers in the equine diet with the major ones being pectin, glucans and fructans.
Insoluble fiber, which is measured in the ADF (acid detergent fiber) of a laboratory analysis, has lesser nutritional value. Lignin is virtually resistant to breakdown by intestinal organisms. Cellulose has a limited fermentability, with hemicellulose a bit easier for the organisms to break down. The nutritonal factor isn’t everything though. Fragments of insoluble fiber forms rafts in the intestine which organisms colonize. They are housing units for the beneficial bacteria.
On the other hand, soluble fiber, found in the NDF fraction (neutral detergent fiber), is very easily reduced to VFAs so the energy/calorie yield can be considerable. For example, beet pulp is very high in pectin with virtually no starch that is the major calorie source in grains but can yield as many calories as plain oats.
Soluble fiber is also an excellent prebiotic for the large intestine because it supports the growth of fiber fermenting organisms. Fructans, aka oligofructose or inulin, can be found in some prebiotic supplements but high soluble fiber foods like beet pulp or flaxseed are also prebiotic.
Psyllium husk is particularly high in soluble fiber. Large doses form a gel in the intestines. This is used to help horses move sand out of their digestive tract but it only works when used intermittently for no more than a few days because the intestinal organisms will adapt to its presence and begin to efficiently break it down.
Psyllium is also a useful feed ingredient for horses that tend to choke. Adding psyllium and water to a meal forms a slick coating that helps food travel down the esophagus. The prebiotic effect is an extra bonus.
In short, fiber is much more than filler in your horse’s diet. It’s an essential component of their diet both for calories and intestinal health.
Eleanor M Kellon, VMD