Introduction

How do animals digest their food? Considering this question, it is possible to differentiate two groups, namely, those species that are able to digest the ingesta with their own enzymes (autoenzymatic digestion) and, secondly, species eating a food that can only be digested with the help of microbial fermentation (alloenzymatic digestion). The latter type is necessary when plant material is eaten, as plants contain structural carbohydrates in their cell walls: mainly cellulose with pi-4 linkages between D-glucose monomers. The cell-wall carbohydrates can be encrusted to different extents with lignin and can thus be made highly resistant to microbial degradation. Bacteria and, in many cases, protozoans and sometimes even fungi, are the organisms that have to colonize the gastro-intestinal tract of mammals with alloenzymatic digestion (Savage, 1972, 1983; Church, 1988). Both the large intestine and, in some cases, the forestomach are regions of the tract that have been widened to ‘fermentation vats’. Although the main products of microbial activity, namely short-chained fatty acids, can be absorbed in the forestomach and in the large intestine (Stevens, 1988), the efficiency of digestion in both regions of the tract is quite different (Demeyer and de Graeve, 1991). Protein and amino acids are microbially synthesized in the forestomach of ruminants and are digested in the hindstomach and small intestine with subsequent absorption of their end-products (Stevens, 1988).