EVALUATION OF DIFFERENT MARKERS FOR DETER MINATION OF THE MICROBIAL NITROGEN FLOW INTO THE DUODENUM OF DAIRY-COWS

2.6-Diaminopimelic acid (DAPA), ribonucleic acid (RNA), N-15, D-alanin e (D-ALA) and the amino acid profiles (AAP) were compared as microbial markers for determination of the microbial protein synthesis in the r umen. Three dairy cows (Schwarzbuntes Milchrind, LW 602 kg), each fitt ed with a rumen cannula and a re-entrant cannula in the proximal duode num, were offered four isoenergetic and isonitrogenous diets (mean dai ly intake 15.0 +/- 0.45 kg DM; forage: concentrate = 50:50) in a perio dic experiment. The diets contained soyabean extracted meal, meat and bone meal, pea meal and dried clover as major sources of protein. On t he 4th day after administration of 9 g N-15-labelled urea (95 atom-% N -15-excess) per day, samples of rumen fluid and duodenal digesta were obtained 3 h after feeding. The bacteria were isolated by differential centrifugation. Bacteria harvested from the rumen had significantly h igher N-15 enrichment and D-ALA: N ratio than 'duodenal' bacteria. How ever, DAPA: N ratio was higher in 'duodenal' bacteria compared to rume n bacteria. There were no differences in RNA: N ratio between rumen an d 'duodenal' bacteria. The source of the bacteria in the digestive tra ct has an influence on the ratio of microbial N: total N, especially w hen N-15, AAP, DAPA and D-ALA but not RNA were used as markers. The mo st reproducible method was D-ALA (C.V. 4.7 for rumen and 6.8 for 'duod enal' bacteria) followed by N-15(10.8 resp. 4.8) and RNA (9.7 resp. 8. 2). The results obtained with N-15 and D-ALA agreed closely at the sam e source of bacteria. The RNA method reached the level of these marker s (N-15, D-ALA) when the bacteria were isolated from the duodenum. It is concluded that D-ALA (bacteria isolated from rumen and duodenum) an d also N-15 (bacteria isolated from duodenum) were the best markers fo r estimation of the microbial protein synthesis.