Grass maturity effects on cattle fed silage-based diets. 2. Cell wall digestibility, digestion and passage kinetics

Four silages were harvested at approximately 1-week intervals from the same timothy-meadow fescue sward and studied in a 4 × 4 Latin square experiment with four ruminally and duodenally cannulated young cattle. The diets comprised silage and concentrate (7:3 dry matter (DM) basis) and were fed at a rate of 70 g DM kg−0.75 liveweight in two equal meals per day. l detergent fibre (NDF) digestibility was 0.757, 0.765, 0.692 and 0.686 on diets based on the four silages in order of harvest date. Increasing maturity of grass ensiled showed linear (PL < 0.001) and cubic (PC < 0.01) trends. NDF was separated into digestible (DNDF) and indigestible (INDF) fractions, which differed clearly in their rate of passage from the rumen (on average 0.0141 vs. 0.0258 h−1). The rate of digestion (kd) of DNDF was on average 0.076 h−1 when derived from the rumen evacuations but only 0.036 h−1 when calculated from the disappearance from nylon bags incubated in the rumen. Both methods detected decreased kd of NDF with increasing maturity of grass ensiled. f passage from the rumen increased with increasing maturity of grass both when determined for NDF with rumen evacuation technique and from the faecal excretion of ytterbium calculated with a two-pool model. Mean retention time (MRT) in the non-escapable pool of particles increased (PL < 0.01) with increasing grass maturity, the opposite being true for the escapable pool (PL < 0.05), resulting in no change in the total ruminal MRT. Pool sizes of ruminal DM PL < 0.01) and NDF (PL < 0.001) increased with increasing maturity of grass. Ruminal NDF digestibility was calculated by different methods. When digestion kinetic parameters were derived from rumen evacuations and two-pool models used for passage kinetics, estimated digestibilities were very close to the observed ones.