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 bags in the rumen for 0, 6, 12, 24, 48, 72 and 96 h. The subsequent digestion of rumen-undegradable N in the intestine was measured by the mobile-bag method. The N content of grass declined, and the fibre content and the proportion of N bound in acid detergent fibre increased with increasing maturity. The values for rumen degradability of grass N corrected for microbial contamination were generally high and they decreased with increasing maturity. The intestinal N digestibility of rumen-undegradable N of grass ranged from 49% to 78%, being affected by growth stage and the duration of preceding rumen incubation time. These values may, however, be confounded by nonfeed N. Pre-incubation of forage samples in the rumen indicated that at least 48 h of rumen incubation is needed for predicting intestinal N digestibility from the indigestible N fraction determined with the mobile-bag method.