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 d iets for cows was estimated indirectly as the value which, together with th e values of kinetic degradation measured for each ingredient under standard ised conditions, allowed the calculation of the NDF effective degradability equal to the NDF digestibility. Forages were grown on a permanent grasslan d divided into equal plots and subjected to three cutting frequencies (two, three and four cuts per year, FC) and two types of fertilisation (slurry o nly or slurry plus mineral N). Within each FC, the forages from the individ ual 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 co ws), 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. Samp les of forages and concentrate were used to measure the NDF in situ degrada bility in standardised conditions. The type of grassland fertilisation caus ed no statistically significant effects. DMI increased with the inclusion o f 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 d iets 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). T he present work indicated that the approach used to estimate k(NDF) gave va lues comparable to those obtained in other experiments with direct measurem ents and demonstrated the sensitivity of the estimates of k(NDF) to variati ons of intake in lactating cows. (C) 1999 Elsevier Science B.V. All rights reserved.