Effects of microbial colonization in the rumen on concentration and degradability estimates of fibre fractions

Errors due to microbial colonization in the rumen (determined using 15N techniques) on neutral (NDF) and acid (ADF) detergent fibres and on the N bound to both these fractions (NDIN and ADIN, respectively) were evaluated using in situ (experiment 1) and in vivo methods (experiment 2). In experiment 1, the ruminal degradation of Italian ryegrass (Lolium multiflorum Lam.) hay was studied in 6 and 72 h incubation residues, as well as in a composite sample (CS) representative of the rumen outflow of undegraded feed particles. The colonization errors of ruminal and duodenal digesta samples derived from a mixed diet were controlled in experiment 2. Three double-cannulated wethers (rumen and duodenum) were used in each study. In experiment 1, mean proportions of microbial-N in all tested fractions increased with the incubation time. The lack of correction for microbial colonization led to inaccuracies in the estimates of fibre fractions and its degradability. These errors were dramatic for NDIN, high for ADIN, moderate for NDF and low for ADF. The 15N enrichments recorded in experiment 2 showed large microbial colonization of NDF and ADF in ruminal and duodenal samples. However, estimates based on adherent bacteria probably overestimate this colonization, because evidence of a large presence of free bacteria was also found in these samples. Direct ADF extraction shows a lower efficiency in eliminating microorganisms than sequential NDF-ADF or NDF procedures. Results demonstrate the need for correcting the microbial colonization to obtain accurate conclusions about the ruminal digestion of cell wall proteins. These errors should also be considered when analyzing NDF using in situ and in vivo techniques.