PASSAGE RATE AND TOTAL CLEARANCE RATE FROM THE RUMEN OF COWS FED ON GRASS SILAGES DIFFERING IN CELL-WALL CONTENT

Four non-lactating, rumen-fistulated cows were fed ad lib. on two gras s silages (first cut (FC) and second cut (SC)) harvested at different growth stages, resulting in different crude-protein (CP) and neutral-d etergent-fibre (NDF) contents (FC, 152 g CP/kg, 515 g NDF/kg and SC, 2 10 g CP/kg, 442 g NDF/kg). Voluntary intake and rumen contents, total as well as organic matter were higher for silage PC. Fractional passag e rate from the rumen, calculated from the logarithmic decline in Cr-N DF rumen pool, was higher for silage FC (0.0395/h and 0.0446/h for sil ages SC and FC respectively). When fractional passage rates from the r umen were calculated by dividing the intake of indigestible organic ma tter by the mean rumen pool of this fraction, the same differences bet ween silages were found, although the actual levels were much lower (0 .0258/h and 0.0300/h for silages SC and FC respectively). The results from the present experiment suggest that disappearance rate from the r umen of particles with a size between 1.25 and 0.071 mm is the rate-li miting step in the control of rumen fill.