QUANTIFICATION OF BACTERIAL BIOMASS IN TH E RUMEN OF STEERS WITH DIAMINOPIMELIC ACID AS MARKER - EFFECTS OF FORAGE STAGE OF MATURITY, TIME OF SAMPLING AND METHOD OF ISOLATING THE BACTERIAL FRACTION

Ln two separate experiments, groups of six and four ruminally cannulat ed Angler steers were twice daily fed diets consisting of silages of w hole-crop winter wheat (WW) and perennial ryegrass (PR) harvested at d ifferent stages of maturity. Experimental designs were a replicated 3 x 3 Latin square in experiment (Exp) 1 and a 4 x 4 Latin square in Exp 2. Silages deficient in crude protein were balanced to 13 % crude pro tein in diet dry matter. Diaminopimelic acid (DAPA) was used as a bact erial marker. After manual rumen evacuation, ruminal ingesta was sampl ed at six different times postfeeding. In Exp 1, ruminal fluid for iso lation of mixed-rumen bacteria (MB) was collected 2.5 and 10.5 h after the morning feeding and 6.5 h after the evening feeding. In Exp 2 MB, fluid (FAB) and particle (PAB) associated bacteria were isolated 1.5, 5.5 and 11 h after the morning and evening feeding, respectively. Tim e postfeeding markedly influenced the non ammonia-nitrogen (NAN) conte nt of ruminal digesta. The DAPA contents in bacteria and ruminal diges ta and the ratios DAPA:N in bacteria and DAPA:NAN in ruminal digesta h ad fewer postprandial variations. The composition of bacteria and rumi nal contents was affected by stage of WW and PR maturity at harvest. T he percentage of bacterial N of total NAN in the rumen was greatest fo r the WW silage diet from the early-dough stage of maturity (Exp 1; 68 .4 %), and for the PR silage stage diet from the end of the boot stage (Exp 2; 85.5 %). Differences were observed in the composition of MB a nd FAB compared with PAB. The percentages of bacteria N of total NAN i n the rumen for the three bacterial fractions were 71.7 (MB), 75.9 (FA B), and 76.4 (PAB). Ranking of PR silage diets was similar across bact erial fractions with respect to percentages of bacterial N of total ru minal NAN.