Methanogenesis
Methane production in the reticulo-rumen is due to the presence of methanogens which are in a group of microorganisms known as archaea. They used to be known as archebacteria but recent phylogenetic analysis has put them in a class separate from other prokaryotes. They are strict anaerobes and generate ATP by anaerobic respiration.  The methane they produce can not be used by the host ruminant, nor by other microorganisms, and it is lost from the reticulo-rumen by eructation. This represents loss of feed carbon and reduction of methanogenesis is a high priority agribusiness objective.  Methane is a greenhouse gas and this is another basis for interest in metabolism in the rumen.