Estimation of Ruminal Digestibility of NDF from in Sacco Degradation and Rumen Fractional Outflow Rate

The relation between effective degradation (ED) and ruminal digestibility of NDF was tested with 6 cows in two 3 × 3 latin square experiments, one with three different roughages fed alone and one with the same roughages fed together with concentrate. In vivo ruminal digestibility of NDF could be estimated (R 2=0.77) with the parameters from a degradation model including lag time and with rate of passage determined as rumen outflow of NDF divided by rumen NDF pool size. The largest deviations between ruminal digestibility and ED were found with medium to high contents of starch in the rations owing to a more profound depression of NDF degradation in nylon bags compared to in vivo. This was caused most probably by an underestimation of rate of degradation and an overestimation of lag time in the nylon bags.