ARS | Publication request: Effect of Monensin Feeding and Withdrawal on Ruminal Populations of Individual Bacterial Species in Cows Fed High-Starch Diets

Submitted to: Journal Dairy Science Supplement
Publication Type: Abstract
Publication Acceptance Date: March 8, 2007
Publication Date: July 9, 2007
Citation: Weimer, P.J., Stevenson, D.M., Mertens, D.R., Thomas, E.E. 2007. Effect of monensin feeding and withdrawal on ruminal populations of individual bacterial species in cows fed high-starch diets [abstract]. Journal of Dairy Science 90(Supplement 1):175.

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 fraction of 16S rRNA gene copy number (F) attributable to Prevotella and to each of 13 classical, well-studied ruminal bacterial species 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 F 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 removal. Less than 10% of the Prevotella were present as classical ruminal species P. rumincola, P. brevis, or P. bryantii. Mean values of F 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 F attributable to two biohydrogenating species (Megasphaera elsdenii and Butyrivibrio fibrisolvens) were low (< 0.4%) and declined several-fold in response to monensin, in accord with observed decreases in milk fat. No changes were observed in mean values of F for a third biohydrogenating species, Eubacterium ruminantium. The 13 species together contributed <10% of the bacterial16S gene copy number. The data suggest that monensin in high starch diets does not suppress populations of classical ruminal Gram-positive bacteria, though it may affect bacteria involved in biohydrogenation of lipids that regulate bovine mammary lipogenesis.