INFLUENCE OF STAGE OF MATURITY OF GRASS SILAGES ON DIGESTION PROCESSES IN DAIRY-COWS .1. COMPOSITION, NYLON BAG DEGRADATION RATES, FERMENTATION CHARACTERISTICS, DIGESTIBILITY AND INTAKE

In four change-over experiments, dairy cows were fed wilted grass silages (G1, G2, G3, G4 and G5) ad lib. In early lactation, silage was supplemented with 7 kg of concentrates and in late lactation with 1 kg. The silages were harvested at different growth stages, resulting in different cell wall contents (G1 44.6%, G2 54.7%, G3 54.8%, G4 64.1% and G5 with 67.3% neutral detergent fibre (NDF)). With an increase in cell wall content, crude protein (CP) content decreased (21.3% CP for G1 to 11.2% CP for G5), and organic matter (OM) digestibility decreased (P < 0.001) (76.2% for G1 to 63.7% for G5). The degradation rates of dry matter (DM), NDF and acid detergent fibre (ADF) were measured using nylon bags incubated in the rumen. The soluble fraction (f(S), %) decreased, the undegradable fraction (f(R), %) increased, and the degradation rate of the non-soluble, degradable fraction (k(d), % h) decreased with an increase in cell wall content of the silages. High correlation coefficients between the k(d) values of DM (r = -0.91, P < 0.001), NDF (r = -0.90, P < 0.001), ADF (r = -0.93, P < 0.001) and the NDF content of the silages were found. The in vivo DM digestibility was positively related to f(S) (r = 0.88, P < 0.01 ) and k(d) (r = 0.84, P < 0.01 ) and negatively to f(R) (r = -0.97, P < 0.001) When supplemented with 7 kg of concentrates (early lactation) rumen pH was lower, and VFA concentrations were higher as compared to supplementation with 1 kg (late lactation). Diurnal variations in ammonia concentrations were higher for the high concentrate level. On average, the ammonia concentrations were lower in combination with 7 kg of concentrates. Multiple regression analysis showed that the amount of concentrates consumed, the level of milk production, the nitrogen (N)/OM ratio of the silages and the f(R), could explain almost 70% of the variation in silage DM intake.