Cell wall digestion and passage kinetics estimated by marker and in situ methods or by rumen evacuations in cattle fed hay 2 or 18 times daily

This study was conducted to determine the effects of timing rumen evacuations on kinetic parameter estimates of wall digestion and passage. Another objective was to estimate the rate constants of rumen neutral detergent fibre (NDF) kinetics in cattle fed grass hay using rumen evacuations, and to compare the results to those obtained by conventional methods (in situ incubation, marker kinetics). Four young cattle were used in a switch-back design and fed a grass hay diet either twice daily at 7.00 and 19.00 h or 18 times per day starting at 7.00 h. Rumen evacuations were made just before morning feeding (7.00 h) and 11.00 and 15.00 h. The ingested feed, rumen and duodenal digesta and faeces were divided into five particle size fractions by wet sieving. Particulate matter retained on the sieves were analysed for NDF which was further subdivided into digestible (DNDF) and indigestible NDF (INDF) by 12-day in situ incubation. The rate constants were derived from average rumen pool sizes by pooling the particle fractions into large, medium and small particles (three-pool) or into large and small particles (two-pool). Feeding frequency had no effect on ruminal or total digestibility, average rumen pool size or kinetic parameters. The results showed that kinetic parameters, estimated from a single evacuation, can be severely biased due to under- or overestimation of rumen pool size. The rate of particle breakdown based on rumen INDF kinetics was slower than the passage rate of small particles (0.033 h−1 versus 0.047 h−1). Digestion rate of medium particles was faster than that of large or small particles. The passage rate of both DNDF and INDF increased with decreasing particle size and it was slower for DNDF than for INDF irrespective of particle size indicating selective retention. Digestion rate of DNDF estimated by the in situ incubation was lower than that derived from rumen evacuation. Model calculations suggested that the values based on the in situ incubations were underestimated. Ytterbium labelled hay underestimated the residence time in ruminal compartments compared with INDF estimated by rumen evacuation technique (42.5 h versus 49.8 h). However, the passage rates of small particles estimated from the marker kinetics were similar to those derived from rumen evacuation.