Carbohydrate Digestibility in Horses By Dr. Joe Pagan · March 21, 2012
Because monosaccharides are the only form of carbohydrates that can be absorbed from the equine intestine, more complex carbohydrates must be broken down into simple sugars before they can be used by the horse. Starches are broken down into the disaccharide maltose (two glucoses) by the enzyme amylase. Maltose, sucrose, and lactose are split into their two monosaccharide units by the disaccharide enzymes maltose, sucrase, and lactase, which are produced in the intestine. These disaccharides are completely digested in the small intestine of the healthy horse. This is not the case, however, for starch. The horse’s ability to produce amylase is limited. Horses produce only about 8 to 10% as much amylase as the pig. Therefore, a great deal of the starch in a horse’s diet escapes digestion in the small intestine.
If starch is not digested in the small intestine, it travels to the large intestine, where it is broken down by bacteria that are quite efficient at their job. In fact, regardless of source, total tract digestibility of starch by the horse is better than 95%. However, flooding the hindgut with too much starch causes a buildup of lactic acid, the product of starch fermentation. This acid changes the pH in the intestine, killing some types of gut bacteria and leading to a release of endotoxins that are absorbed into the bloodstream. This cascade of events is a known risk factor for laminitis.
Prececal starch digestibility is affected by the source and processing of the starch, the amount of intake, the source and timing of forage feedings, and differences between individual horses.
All starch is made up of chains of glucoses, but the way the starch molecule is constructed is quite different for each type of grain. These differences in architecture impact digestibility in the horse’s small intestine. Oat starch is most easily digested by horses followed by sorghum, corn, and barley. Because oat starch is so digestible, processing has little effect on its digestion in the small intestine. On the other hand, corn starch digestibility is greatly enhanced by grinding (29% for whole corn, 45% after grinding). Cooking the corn by steam rolling, extruding, or micronizing will improve digestibility even more, and popping corn increased digestibility to 90% in the small intestine. Grinding of some other grains before feeding does not seem to increase digestibility, perhaps because the horse with good teeth will grind feed itself by chewing before swallowing. If a horse has dental problems that prevent effective chewing, grinding any grain will probably increase digestibility.
Smaller grain meals are generally digested more thoroughly in the small intestine than larger grain meals. In a study conducted in Germany, horses were fed small, medium, or large meals of oats or corn. The results showed that, while oat starch was processed well at all levels, horses fed larger meals of coarsely ground corn showed increasingly larger drops in cecal pH, indicating that significant amounts of starch had escaped digestion in the small intestine. This finding supports the practice of feeding grain in frequent small meals rather than in one or two large meals.
Time of forage feeding can also influence starch digestibility. At Kentucky Equine Research, a study was designed to compare three different feeding plans. Six Thoroughbreds were fed 2.26 kg (5 lb) of sweet feed (a mixture of oats, corn, molasses, and a supplement pellet) as a morning meal. The grain was fed either alone, with an equal weight of hay, or two hours after hay. There was a large difference in glycemic response between the “grain only” treatment and the two treatments where hay was fed either before or with grain. The horses were offered water each hour during the study. They consumed the highest amount of water two to three hours after being fed hay. This increased water intake coincided with an increased total plasma protein level. Total plasma protein is an indirect measure of plasma volume where a higher level correlates with a reduced plasma volume. Eating hay stimulated saliva production and an increase in the secretion of digestive juices into the intestines. Much of the fluid in these secretions came from blood plasma, resulting in a drop in plasma volume. Decreased plasma volume stimulated a thirst response and the horses drank more water. All of these factors, combined, increased the dilution of the intestinal contents and increased rate of passage. Starch digestibility in the small intestine was reduced and glycemic response diminished.
Finally, starch digestion is influenced by individual differences in particular horses. These variations may be due to different rates of feed intake and passage through the digestive tract or the amount of digestive enzymes produced by a horse.