Effect of diet and probiotic addition on chemical composition of free or particle-associated bacterial populations of the rumen

Eight dairy heifers (297 +/- 11 kg) fitted with a ruminal cannula were used in this study. The following diets were fed in a replicated, concurrently run 4 x 4 Latin square design: 1) 80% chopped timothy hay + 20% rolled barley (HF); 2) HF + 10 g head d(-1) of a mixture of Saccharomyces cerevisiae and Aspergillus oryzae (HF + SA); 3) 50% chopped timothy hay + 47.6% rolled barley and 2.4% soya supplement (HC); 4) HC + SA. Feed was offered every 2 h. A sample of whole rumen contents (liquid and solid) was collected 1 h after the 07:00 h feeding to isolate the bacteria associated with the liquid phase (BAL) and the bacteria adherent to the solid phase (BAS). Organic matter (OM) content of BAS was greater (P less than or equal to 0.0001) than that of BAL, independently of the type of diet fed and of the presence or absence of the probiotic mixture. Oppositely, total nitrogen content (N), expressed as g 100 g(-1) OM, was greater (P I 0.0001) in the BAL fraction than in the BAS fraction, the difference in N between the two bacterial fractions being more pronounced with the HF diet (P less than or equal to 0.05). Organic matter content in both bacterial fractions was greater (P less than or equal to 0.05) when heifers were fed the HC diet. Addition of probiotic had no effect on OM or N content of each bacterial fraction. Significant differences between amino acid composition of BAL and BAS were observed for 10 out of 16 amino acids studied. For most amino acids, differences in content observed between BAL and BAS were maintained independently of the probiotic supplementation whereas diet affected the relative composition of the bacterial fractions for five amino acids. Finally, addition of probiotics did not affect molar proportions of ruminal volatile fatty acids, which were affected by the diet.