In cows fed diets with variable content of carbohydrates and protein, Belanche et al. (2012) analyzed the correlations
between the abundance of specific microbial groups and some fermentation parameters. In agreement with their results,
the abundance of bacteria, protozoa and methanogenic archaea showed no significant correlation (P > 0.05) with total VFA
production after 8 h of fermentation (results not shown), but Belanche et al. (2012) found a significant correlation between
VFA concentrations and fungal abundance at five h after feeding that was not detected in our study. In our study, the
abundance of F. succinogenes and R. flavefaciens in the inoculum was positive related (P < 0.05) to the amount of methane
produced (for the 3 forages and for the forage and mixed substrates), which is consistent with both the greater abundance
of methanogens and the greater methane production observed for STO compared with SQ and FL methods. The negative
relationship (P < 0.05) between the abundance of F. succinogenes and R. flavefaciens in the inoculum and the pH after 8 h of
incubation may reflect the greater fermentation (with associated lower pH values) produced by those inocula with greater
abundances of these cellulolytic bacteria, as the range of pH values (from 6.90 to 7.56) were adequate for the growth of both
of them. Finally, the lack of correlation between the abundance of methanogenic archaea and the production of methane
observed in our study agrees well with the results previously reported in the literature (Abecia et al., 2012; Romero-Perez
et al., 2014)