Attempts to induce reductive acetogenesis into a sheep rumen.

A rumen fistulated wether was used for continuous infusion of a 2-bromoethanesulfonic acid (BES) solution (2 g/d in 50 ml of water). The infusion was started after introduction of a pulse dose of BES (2 g) into the rumen. Immediately after introduction of the pulse dose, methane concentration in rumen gases was lowered from about 40 to less than 1%, with concomittant decreases and increases in the molar proportions of acetic and propionic acids respectively in the rumen volatile fatty acids. After 4 days of infusion however, and despite repeated pulse dosage of BES, methanogenesis adapted to BES and methane concentration in rumen gases reached 20%. Addition of BES to incubations of rumen contents with hay resulted in considerable inhibition of methanogenesis. Extra addition of methanol in such incubations increased both acetate and methane production, whereas addition of formate had no effect. In a second experiment using a second rumen fistulated whether, a 4 day control period was followed by 10 days of daily introduction of 11 of cattle cecal contents into the rumen. The cattle cecal contents were collected from slaughterhouse cattle, filtered and kept at -20 degrees C until use. Comparison of in vitro fermentation of thawed with fresh contents showed absence of methanogenesis but not of reductive acetogenesis after freezing and thawing. Evidence for the latter was sought by calculation of metabolic hydrogen recoveries from amounts of end products formed in incubations. In a similar way, evidence for induction of reductive acetogenesis was sought from incubations in vitro, carried out with rumen contents obtained before, during and after introduction of cecal contents into the rumen. No such evidence was obtained.