EFFECTS OF STAGE OF GROWTH AND DURATION OF RUMEN INCUBATION-TIME ON INTESTINAL DIGESTIBILITY OF RUMEN-UNDEGRADABLE NITROGEN OF GRASS BY MOBILE-BAG METHOD IN COWS

The ruminal and intestinal degradability of the nitrogen constituents of timothy grass cut at five different stages of growth was studied in three non-lactating cows. The grass samples were incubated in nylon b ags in the rumen for 0, 6, 12, 24, 48, 72 and 96 h. The subsequent dig estion of rumen-undegradable N in the intestine was measured by the mo bile-bag method. The N content of grass declined, and the fibre conten t and the proportion of N bound in acid detergent fibre increased with increasing maturity. The values for rumen degradability of grass N co rrected for microbial contamination were generally high and they decre ased with increasing maturity. The intestinal N digestibility of rumen -undegradable N of grass ranged from 49% to 78%, being affected by gro wth stage and the duration of preceding rumen incubation time. These v alues may, however, be confounded by nonfeed N. Pre-incubation of fora ge samples in the rumen indicated that at least 48 h of rumen incubati on is needed for predicting intestinal N digestibility from the indige stible N fraction determined with the mobile-bag method.