Fiber digestion, VFA production, and microbial population changes during in vitro ruminal fermentations of mixed rations by monensin-adapted and unadapted microbes

Mixed ruminal microbes were incubated for 24h in vitro with mixed forage and concentrate rations under conditions that differed in starch level (200 or 300g/kg dry matter (DM)), initial pH (6.3 or 6.7) and corn oil (0 or 10g/kg DM), in order to determine effects on fermentation (fiber digestibility, volatile fatty acid (VFA) production) and relative population sizes (RPS) of various bacterial taxa (expressed as a percentage of 16S ribosomal RNA gene copies determined by quantitative polymerase chain reaction (qPCR)). Within the range of in vitro conditions tested, monensin-adapted inocula incubated in the presence of monensin did not differ in in vitro fiber digestibility relative to inocula from the same cows prior to adaptation and incubated in the absence of monensin. Although total VFA production did not differ among in vitro treatments, a shift from acetate toward propionate production was generally observed at the higher starch level, lower pH, and presence of corn oil. Surprisingly, monensin cultures displayed slightly decreased proportions of propionate and increased proportions of butyrate and valerate. After 24h in vitro incubations containing 300g starch/kg DM and in the absence of monensin, the RPS of 13 of the 16 taxa examined differed (P<0.01) from that of the inoculum. In vitro incubation increased the RPS of the Ruminococcus albus, Selenomonas ruminantium, Succinivibrio dextrinisolvens, and Megasphaera elsdenii, and decreased the RPS of the genus Prevotella, and the species P. brevis, P. bryantii, P. ruminicola, Butyrivibrio fibrisolvens, Eubacterium ruminantium, Fibrobacter succinogenes, Ruminococcus flavefaciens, and Streptococcus bovis. The RPS of Ruminobacter amylophilus, the genus Ruminococcus and the Domain Archaea did not change after 24h in vitro incubation. Shifts in RPS during in vitro incubation were remarkably independent of initial pH or corn oil. The data indicate that in vitro conditions can substantially change the quantitative distribution of ruminal bacterial populations, and that these changes are both inherent in the in vitro method, and distinct from specific differences in divergent in vitro environmental conditions.