Grass maturity effects on cattle fed silage-based diets .1. Organic matter digestion, rumen fermentation and nitrogen utilization

Four silages were harvested at approximately one-week intervals from the same timothy-meadow fescue sward. Advanced maturity of the herbage was evidenced by increased neutral detergent fibre [409, 497, 579 and 623 g in 1 kg dry matter (DM)] and decreased nitrogen (N; 29.9, 26.7, 18.7 and 17.4) contents of the silages in the order of harvest date. The silages were studied using four ruminally and duodenally cannulated young cattle in a 4 x 4 Latin square experiment. On DM basis (g kg(-1)), the diet comprised grass silage (700), rolled barley (240) and rapeseed meal (60) and it was given at a rate of 70 g DM (kg live weight)(-0.75) per day. Organic matter digestibility decreased in a curvilinear manner (P-LINEAR(L) < 0.001, P-CUBIC((C)) < 0.01) the values being 0.821, 0.816, 0.758 and 0.747 for the diets based on the four silages in the order of harvest date. Rumen pH increased linearly (P-L < 0.05) and ammonia N concentration decreased curvilinearly (P-L < 0.01, P-C < 0.05) as the grass matured. The molar proportion of acetate in the rumen VFA increased (P-L < 0.001) and the proportion of butyrate decreased (P-L < 0.001) with increased grass maturity. The silage harvest date did not affect the proportion of propionate. The changes in rumen fermentation pattern were associated with a decrease (P-L < 0.05) in rumen protozoal number with increasing maturity of grass. N intake decreased significantly (P-L < 0.001, P-C < 0.01) with the maturity of grass from 167.5 to 118.0 g per day, but duodenal non-ammonia N decreased only from 111.3 to 97.3 g per day indicating greater N losses from the rumen with early-cut silages. The efficiency of microbial protein synthesis in the rumen was not affected by the maturity of grass ensiled. Apparent digestibility of N decreased (P-L < 0.001, P-C < 0.01) and the degradability of N in the rumen decreased (P-L < 0.05) as the grass matured. (C) 1997 Elsevier Science B.V.