The Gamma Distribution-like Degradation and Passage Models (GnG1) was developed based on theoretical concepts and probability to generalize the rumen processes of fiber digestion, assuming a gamma distribution. It can be used to interpret fiber degradation and passage profiles. The GnG1 model was evaluated for quality of fit using in vitro fiber degradation profiles and in vivo fiber passage profiles. The integration of digestion and passage is based on the concept that fibrous digesta in the rumen is heterogeneous (Vieira et al., 2008a; Vieira et al., 2008b).