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 t he 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. T he 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 rap eseed 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 curviline ar 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 r umen VFA increased (P-L < 0.001) and the proportion of butyrate decrea sed (P-L < 0.001) with increased grass maturity. The silage harvest da te did not affect the proportion of propionate. The changes in rumen f ermentation pattern were associated with a decrease (P-L < 0.05) in ru men protozoal number with increasing maturity of grass. N intake decre ased significantly (P-L < 0.001, P-C < 0.01) with the maturity of gras s 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 syn thesis 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 t he degradability of N in the rumen decreased (P-L < 0.05) as the grass matured. (C) 1997 Elsevier Science B.V.