No abstract

No abstract

Three bulls were given twice daily a diet of grass silage and barley (70:30) on a DM basis. Manual evacuation of rumen contents was made before feeding , 3, 6 and 9 h after feeding. The interval between two subsequent evacuations was 27 h. Particle size distribution in the digesta was determined by wet-sieving. The greatest diurnal variation in ruminal NDF pool occurred in the 3-mm fraction. Small particles (0.04-0.2 mm) contained less NDF and cellulose and more ADL compared with larger particles. Potential NDF digestibility determined by 288 h nylon bag incubation declined as particle size decreased except for the 0.04-mm fraction. NDF digestibility was positively correlated with the proportion of cellulose in NDF and negatively with that of ADL and hemicellulose. Chemical composition of rumen particulate DM was comparatively unaffected by the time after feeding but NDF digestibility decreased with time when averaged across particle sizes. Particle-associated CM Case and xylanase activities increased gradually as the particle size decreased. The activities were 3 to 9-fold higher in the 0.04-mm fraction than in the other four fractions. There was a close positive relationship between enzyme activities and content of neutral-detergent solubles in particles.

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 ihcubated 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.