Fermentation behaviour of the nylon bag washout and degradable fractions determined with the gas production technique

Most feed evaluation systems for ruminants are based on data obtained with the nylon bag technique. This technique divides the samples in a washout (W) fraction, passing the pores of the nylon gauze of approximately 40 μm, and a degradable (D) fraction, unable leaving the bags unfermented. Generally, it is assumed that the W-fraction is immediately and completely fermented in the rumen. Experiments were conducted to determine the percentage fast and moderate fermentable components in both the W- and the D-fraction, using the gas production technique. Ten starchy feed ingredients were separated in vitro by filtration over a nylon gauze of 40 μm. Gas production profiles were recorded of the obtained W- (mean 223 g/kg) and D-fractions (mean 777 g/kg). The results show that on average only 376 g/kg of the W-fraction, high in starch, can be regarded as fast fermentable and 624 g/kg as moderate fermentable. The moderate fermentable proportion showed a fermentation behaviour comparable with the D-fraction. On the contrary, also 143 g/kg of the D-fraction could be regarded as fast fermentable. The results obtained with the D-fractions obtained by an in vitro method were confirmed with D-fractions obtained according to the nylon bag protocol, using a washing machine with running tap water. It was concluded that the major part of the in situ W-fraction is not immediately fermentable and actually is a washout part of the D-fraction.