The Types of Substrate and Fermentation Patterns Are Essentially Identical for Forestomach and Hindgut Fermentation
Structural and nonstructural carbohydrates as well as proteins form the major substrates for hindgut fermentation. However, the passage of material through the stomach and small intestine before its arrival at the cecum and colon may have some important effects on fermentative digestion.
First, hindgut fermentation may be aided by prior gastric action. The effects of soaking and acid exposure on plant particles in the stomach may increase their susceptibility to microbial attack and thus raise their rate of digestion in the hindgut. Second, some of the readily available carbohydrate, particularly sugars and starches, may be digested and absorbed before the other material arrives in the cecum. Most evidence indicates, however, that glandular digestion of carbohydrate in the horse is not extremely efficient and that substantial amounts of starch and sugars reach the cecum. Further, cell-wall carbohydrate appears to interfere with the digestion or absorption of nonstructural carbohydrate, so that diets high in cell-wall content result in relatively little starch digestion and absorption in the equine small intestine. Even with a high-grain diet, up to 29% of dietary starch may reach the cecum and colon.Protein as well as carbohydrate is absorbed in the small intestine, potentially leading to a deficiency of nitrogen for colonic microbes. However, there is extensive urea recycling into the colon and cecum, similar to that occurring in the rumen (see Figure 31-6). Thus, urea plus protein escaping small-intestinal digestion supplies the nitrogen needs of the microbes. In contrast to ruminants, horses do not have an efficient means of recovering the microbial protein synthesized in the hindgut, and most of it passes out in the feces. Some experiments have shown a small amount of amino acid absorption from the equine cecum or colon, but the amount does not compare with microbial protein availability in the ruminant.