The estimation of the rumen rate of passage of dietary NDF from degradability and digestibility data in cows

The average rumen rate of passage of dietary NDF (k(NDF)) of mixed forage diets for cows was estimated indirectly as the value which, together with the values of kinetic degradation measured for each ingredient under standardised conditions, allowed the calculation of the NDF effective degradability equal to the NDF digestibility. Forages were grown on a permanent grassland divided into equal plots and subjected to three cutting frequencies (two, three and four cuts per year, FC) and two types of fertilisation (shiny only or slurry plus mineral N). Within each FC, the forages from the individual cuts, produced from plots submitted to the same type of N fertilisation, were mixed in proportion to their seasonal yield. Each of these six mixed diets were fed ad libitum to two or three lactating cows (a total of 15 cows), with the proviso that three groups of five cows received the mixtures corresponding to 2, 3 and 4 FC, respectively. Another three similar groups received the same diets supplemented with 25% of dietary dry matter intake (DMI) of concentrate. The cows were kept in stalls and fed the diets for 12 weeks, using the fifth week to measure the dietary NDF digestibility. Samples of forages and concentrate were used to measure the NDF in situ degradability in standardised conditions. The type of grassland fertilisation caused no statistically significant effects. DMI increased with the inclusion of concentrate (111 vs 133 g/kg LW0.75, P < 0.001) and with the FC (103, 117 and 146 g/kg LW0.75, respectively for 2, 3 and 4 FC, P < 0.01). The dietary NDF digestibility was not affected by the presence of concentrate in the diets and increased regularly in diets composed of 2, 3 or 4 FC (53.6, 61.9 and 66.7%, respectively, P < 0.001). The estimated k(NDF) followed the DMI variations, with the highest values in correspondence with the highest DMI: the inclusion of concentrate caused a 25% increase in the estimated k(NDF) (from 1.49 to 1.84%/h, P < 0.001) and the FC also had a marked effect on k(NDF) (1.42, 1.56 and 2.03%/h, respectively for FC 2, 3 and 4, P < 0.01). The present work indicated that the approach used to estimate k(NDF) gave values comparable to those obtained in other experiments with direct measurements and demonstrated the sensitivity of the estimates of k(NDF) to variations of intake in lactating cows.