Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/2008
Publication Date: 6/6/2008
Citation: Weimer, P.J., Stevenson, D.M., Mertens, D.R., Thomas, E.E. 2008. Effect of monensin feeding and withdrawal on populations of individual bacterial species in the rumen of lactating dairy cows fed high-starch rations. Applied Microbiology and Biotechnology. 80(1):135-145. Interpretive Summary: The feed additive monensin is known to improve the efficiency of feed utilization in dairy cattle. Monensin is thought to work by suppressing a group of bacteria that produces hydrogen gas, a waste product cattle cannot use, but there is no evidence showing that specific bacterial groups are inhibited in the monensin-fed cow. We amplified specific genetic sequences for 13 well-characterized species of rumen bacteria in order to determine the relative amounts of each species present before and after monensin feeding, and again after removing monensin from the diet. Only a few of the bacterial species displayed population shifts with diet, and the shifts that were observed were not consistent with the inhibition of known hydrogen-producing bacteria. These results will stimulate a re-examination of how monensin works, and may aid in the development of strategies for improved feed efficiency in cows and other ruminant animals.
Technical Abstract: Monensin is known to improve ruminant animal production, purportedly by inhibition of H2-producing Gram-positive bacteria, yet there is no in vivo evidence for shifts in populations of specific microbial taxa. We used real-time PCR with relative quantification to assess the relative population size (RPS, calculated as the percent of the bacterial 16S rRNA gene copy number attributable to a given taxon) of the genera Prevotella and Ruminococcus; each of 13 classical, well-studied ruminal bacterial species; and the Domain Archaea in rumen contents from 2 lactating dairy cows fed a TMR containing primarily alfalfa silage, corn silage, and ground high-moisture corn. Diets averaged 30% NDF, 41.1% NFC (26.8% starch), and 17.4% CP (DM basis). PCR was conducted on DNA from rumen samples collected 6 h after feeding on 2 successive days prior to monensin feeding, after 28 d of monensin feeding (at 0.014 g/kg of diet DM), and at six weekly intervals after monensin withdrawal. Mean values of RPS attributable to genus Prevotella increased (P < 0.05) from 41.8% without monensin to 49.2% with monensin, and declined to 42.5% after monensin withdrawal. Less than 10% of the Prevotella were present as classical ruminal species P. rumincola, P. brevis, or P. bryantii. Mean values of RPS attributable to 4 cellulolytic species and 4 starch- or dextrin-fermenting species were not altered (P > 0.10) upon monensin feeding or withdrawal. Mean values of RPS attributable to two biohydrogenating species (Megasphaera elsdenii and Butyrivibrio fibrisolvens) were low (< 0.4%) and declined several-fold in response to monensin. Little change was observed in mean values of RPS for a third biohydrogenating species, Eubacterium ruminantium. The 13 bacterial species together contributed <10% of the bacterial 16S gene copy number. The 16S rRNA gene copy number contributed by Archaea, the domain containing methanogens, was ~100-fold lower than that contributed by the bacteria, and did not change in response to monensin treatment or withdrawal, suggesting no change in the availability of their H2 energy source. The data suggest that monensin in high-starch diets does not suppress populations of classical ruminal Gram-positive bacteria or the availability of H2, though it may affect bacteria involved in biohydrogenation of lipids that regulate bovine mammary lipogenesis.