Rumen microbial production estimated either from urinary purine derivative excretion or from direct measurements of N-15 and purine bases as microbial markers: effect of protein source and rumen bacteria isolates

Four ewes fitted with ruminal and duodenal T-piece cannulae were each given six diets in a 6 x 4 factorial design. Diets or experimental treatments consisted of two ratios of forage: concentrate (700 : 150 (LC) and 400 : 600 (HC)). Forage was ammonia-treated straw and the concentrate was formulated with barley supplemented with one of thr ee protein sources: sunflower meal, soya-bean meal or fish meal. Duodenal flows ofdigesta were estimated by the drralphase technique using Co-EDTA and Yb acetate as liquid and solid markers. Microbial nitrogen (N) was estimated front the digesta flow of purine bases and N-15 enrichment using as reference samples, bacterial isolates from the liquid (LAB) or solid (SAB) phase of rumen digesta. Duodenal flow of purine bases (mmol/day) was lower on LC (12.9) than HC (17.7) diets but ii? both treatments it teas depressed by fish meal (12.3) compared with either soya-bean (17.3) or sunflower meal (16.3) as supplements (s.e. 1.13). Urinary excretion of purine derivatives showed a similar trend, 8.6 v. 11.1 mmol/day in LC and HC respectively and 8.8 v. 10.4 and 10.5 mmol/day in fish meal, soya-bean and sunflower meal diets (s.e. 0.56), respectively. Variation in excretion of urinary purine derivatives was mainly associated with digestible organic matter intake with an average untie of 1.7 (s.e. 0.11) mmol per 100 g digestible organic matter intake. Irrespective of the microbial marker used, microbial yield teas higher in animals offered HC than in those offered LC and with soya-bean or sunflower meal compared with fish meal supplemented diets. The microbial purine bases/N (mmol/g) ratio varied between LAB (1.99, s.e. 0.092) and SAB (1.69, s.e. 0.071) isolates leading to different estimates of microbial-RT yield (g)from duodenal purine bases (7.76 (s.e. 2.84) v. 9.13 (s.e. 3.24)), urinary excretion of allantoin (5.57 (s.e. 2.0) v. 6.57 (s.e. 2.03)) or total purine derivatives (6.43 (s.e. 2.39) v. 7.56 (s.e. 2.77)). Urinary excretion of allantoin or total purine derivatives provided consistently lower estimates of duodenal microbial-N than duodenal purine bases or N-15, although it closely reflected the pattern observed in direct measurements.