Microbial Rumen Fermentation

With the ruminant animal, we have essentially two ecosystems, namely, the microbial ecosystem within the rumen and the animal's external environment. In dealing with the microbial ecosystem, we have made significant strides in the last few decades toward accomplishing the overall goal of ecology, which is to understand the relationships of organisms to their environments. As pointed out by Hungate (47), a complete ecological analysis of any natural habitat requires an elaboration of: 1 ) kinds and numbers of organisms, 2) activities of the organisms, and 3) extent to which their activities are expressed. Within the last 25 yr, much information has been gained in the first two aspects of the ecological analysis. Currently we are just beginning to address the third aspect. The rumen is an ideal fermentation site. In most ruminant species, the rumen is approximately one-seventh of the mass of the animal, is maintained at a relatively constant temperature (39°C), is buffered well by salivary secretions, and compared to many other microbial ecosystems is well supplied with nutrients. End-products of the fermentation (e.g., volatile fatty acids), which can be toxic to microbial metabolism, are removed across the rumen wail. The microflora inhabiting the rumen is dense and contains approximately 10 l° to 1011 bacterial and 106 protozoal cells per milliliter. Diversity within this population is extensive, and approximately 200 species of bacteria and 20 species of protozoa have been isolated (13). Although a few of these bacteria may be casual passengers brought in with the food and thus not be authentic rumen bacteria,