The rumen and hindgut as source of ruminant methanogenesis

The advantage of ruminants is their ability to convert fibrous biomass to high quality protein for human nutrition purposes. Rumen fermentation, however, is always associated with the formation of methane — a very effective greenhouse gas. Hindgut fermentation differs from rumen fermentation by a substantially lower methane production and the presence of reductive acetogenesis or dissimilatory sulfate reduction. Sulfate reduction and methanogenesis seem to be mutually exclusive, while methanogenesis and reductive acetogenesis may occur simultaneously in the hindgut. Although acetogenic bacteria have been isolated from the bovine rumen, methanogenesis prevails in the forestomachs. The substitution of acetate for methane as a hydrogen sink in the rumen should increase energy yield for the animal and decrease methane emissions into the environment. Differences in the major hydrogen sinks in both microbial ecosystems are discussed and mainly related to differences in substrate availability and to the absence of protozoa in the hindgut.