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 ¿m, 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 ¿m. 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.
- protein degradability