Whilst herbivory is widespread in the animal kingdom, no vertebrates and few invertebrates are capable of synthesizing cellulose- or hemicellulose-digesting enzymes. Instead, herbivores have evolved symbiotic associations with microorganisms | 8 Rumen Microorganisms and their Interactions . Theodorou1 and J. France2 1BBSRC Institute for Grassland and Environmental Research Aberystwyth Dyfed SY23 3EB UK 2Centre for Nutrition Modelling Department of Animal Poultry Science University of Guelph Guelph Ontario N1G 2W1 Canada Introduction Whilst herbivory is widespread in the animal kingdom no vertebrates and few invertebrates are capable of synthesizing cellulose- or hemicellulose-digesting enzymes. Instead herbivores have evolved symbiotic associations with microorganisms. Two main types of herbivory exist among mammals. The ruminants cloven-hoofed mammals of the Artiodactyla are best equipped for maximal digestion of plant biomass which is achieved by prolonged retention within the gastrointestinal GI tract. The second type of herbivory is exemplified by members of the Equidae horses and Elephantidae elephants where plant material is passed through the GI tract more rapidly at the expense of maximal plant cell wall digestion. With this form of herbivory a greater proportion of the nutrient supply to the animal is obtained from plant-cell contents than from cell-wall polymers. Both types of herbivory are dependent upon microorganisms for the degradation and fermentation of plant-cell contents cellulose hemicellulose and pectin. Ruminants rely on a predominantly pre-gastric fermentation in the rumen whereas in horses and elephants the fermentation occurs in the hindgut predominantly in the caecum. Although this chapter is concerned with quantitative aspects of rumen microbiology it may have wider relevance since many similarities exist between microbial populations in the rumen and those found within the GI tract of post-gastric herbivores. Due to microbial activity conditions in the rumen are highly anaerobic with a redox potential of between 300 and 350 mV. Temperature remains relatively static at 38-42 C due in part to the heat generated during fermentation but mainly to the homoeothermic metabolism of
