Results: Liquid and adherent fractions were obtained from the rumen of Jersey cattle fed hay alone and Holstein
cattle fed hay plus grain. The populations of cultured and uncultured bacteria present in each fraction were
quantified using specific real-time PCR assays. The population of total bacteria was similar between fractions or
diets, while total archaea was numerically higher in the hay-fed Jersey cattle than in the hay-grain-fed Holstein
cattle. The population of the genus Prevotella was about one log smaller than that of total bacteria. The populations
of Fibrobacter succinogenes, Ruminococcus flavefaciens, the genus Butyrivibrio, and R. albus was at least one log
smaller than that of genus Prevotella. Four of the six uncultured bacteria quantified were as abundant as F.
succinogenes, R. flavefaciens and the genus Butyrivibrio. In addition, the populations of several uncultured bacteria
were significantly higher in the adherent fractions than in the liquid fractions. These uncultured bacteria may be
associated with fiber degradation.