Figure 1. The equine digestive tract. Courtesy Dr. Reynolds
Why should it matter if we are in the 21st century or any other century as far as equine nutrition is concerned? Actually, the science of equine nutrition is very different today than it was only a short time ago. As we learn more about how horses digest and utilize nutrients from feeds, and as more feeds become available, our feed choices broaden and change. We have recently learned that digestibility of a feed is almost as important as its nutrient content. We also now have many newly available feeds, like beet pulp, soybean hulls and rice bran, which come from the human food manufacturing industry. Following is a discussion of current equine nutrition topics, starting with the equine digestive tract.
Figure 2 - Simplified digestion. Courtesy Dr. Reynolds
The Equine Digestive Tract
No matter what your horse looks like on the outside, and no matter what job you ask him to do, he has one thing in common with all horses--his digestive tract. Everything he eats must be processed through that tract in order to be used by his body for energy, growth, reproduction and maintenance of health (See Figure 1).
Horses are grazing animals with digestive tracts best suited for eating forages for 15-20 hours per day. They have relatively small stomachs, which hold only about as much as a 5-quart ice-cream pail. The actual physical capacity of the stomach is larger, but the equine stomach begins to empty when it is only two-thirds full. This is a safety mechanism to prevent lethal stomach rupture, since horses cannot vomit. Feed proceeds from the stomach through the small intestine, a 70-foot-long, tubular-shaped organ, where most of the starch, sugar, fat, vitamins and minerals and about half of the protein from the feed should be digested and absorbed into the bloodstream. The remaining nutrients and the plant fibers continue on to the hindgut (See Figure 2 -- Simplified Digestion in Horses).
Horses have huge fermentation vats in the hindgut that hold 23-30 gallons of fibrous material while billions of bacteria and other organisms work to digest it. Fiber-digesting bacteria produce volatile fatty acids (vfa), or very short-chain fats, that are used by bacteria and horses for energy. In fact, horses on total forage diets receive 70% of their energy from these vfas. Some of the remaining protein and some minerals, especially phosphorus, are absorbed from the large intestine, along with water, which is recycled within the body. Bacteria in a healthy horse's hindgut also produce B-vitamins that can be used by the horse.
The equine digestive system is very efficient if horses are fed mainly grass or hay. But, if we feed more than a few pounds of grain in meals, the system doesn't work very well. Grains are very high in starch (50-75%) compared to grasses and hays (<10%), which the digestive tract is designed to process. Often, the excess grain starch can't be digested in the beginning of the tract by the horse's enzymes for several reasons:
He just doesn't have enough enzymes,
The starch is too compact to be broken down,
There isn't enough time, because most feed goes from the mouth through the foregut and into the hindgut in less than 6 hours. Starch entering the hindgut is used, by starch-digesting bacteria, to produce lactic acid. The addition of lactic acid, to the hindgut, results in a lowering of hindgut pH.Many of the beneficial, fiber-digesting bacteria cannot tolerate the more acidic conditions, so they die and release toxins into the hindgut. The presence of these toxins often results in colic and founder. This cascade of events begins with high-grain (starch) rations and ends with colic and founder. Research results show that feeding 5 pounds or less of
Figure 3 - The digestive tract at a glance. Courtesy Dr. Reynolds
grain-based feeds daily greatly reduces the risk of colic and founder in horses.We know that horses doing only light work do very well on high-forage rations, and don't need much grain. But, many horses need more energy than forage alone can provide. Since grain starch in the hindgut is such a problem, how do we choose an energy source? Do all grains contribute equally to this problem, or are some better than others?
The Digestive Tract at a Glance Foregut - Stomach and Small Intestine - 1-6 hours - Horse Enzymes digest sugar, starch, fat and protein. Hindgut - Cecum and Colon - 18-36 hours - Bacterial Enzymes digest plant fibers.
Part Two: More on making the right feeding choices.