Composition of liquid-and particle-associated bacteria and their contribution to the rumen outflow

A method is described to estimate the composition and rumen outflow of microbes associated with liquid (LAB) and solid (SAB) digesta. Four rumen-cannulated Rasa Aragonesa ewes were given, in random order, the following 4 diets: (1) NaOH-treated barley straw, as a sole diet (700 g/day, TS); (2) NaOH-treated barley straw mixed (50 : 50) with 400 g/day of rolled barley grain (BS); (3) Diet 1 with addition of 8 g/day of urea; and (4) Diet 2 with addition of 16 g/day of urea. Co-EDTA was used as a marker for the liquid phase to estimate rumen outflow of liquid-associated purine bases (PB), and urinary purine derivatives were used as an indirect marker of total duodenal flow of PB. Solid-associated PB were calculated by the difference between both estimates. Urea infusion increased ammonia-N concentration in the rumen fluid from 4·8 to 15·9 mg/100 mL (P < 0· 05) and enhanced dry matter intake of TS diets (from 343±63· 5 to 556±41·2 g/day, P < 0·001). Significant differences were observed in the PB/N ratio of bacteria harvested from the liquid phase compared with that isolated from the solid phase (1·89±0·25 v. 1·66±0·32 mol/mg in LAB and SAB, respectively). Because of the differences observed between the liquid- and solid-associated bacteria, estimated values of bacterial N supply varied depending on which bacterial extract was used as reference. The fractional contribution of LAB and SAB to the postruminal bacteria was significantly influenced by the experimental diets, mainly through variations in the amount of LAB flowing out of the rumen.