Comparison of mechanistic rumen models on mathematical formulation of extramicrobial and microbial processes

This study investigated the consequences of differences in applied concepts and individual mathematical formulations on steady-state behavior of three important mechanistic rumen models. In the models of Baldwin et al. (2) and Danfaer (6), the formulation of passage rate, nondietary inputs, defined rumen substrate pools, absorption rates, degradation rates, molecular weights, parameterization of VFA production, and physical compartmentalization were sequentially exchanged for the formulation of the model of Dijkstra et al. (9). Most of these adaptations had a considerable influence on model behavior, indicating large qualitative differences in formulation and sensitivity to concept choice. Because microbial substrate environments were similar after all adaptations, the microbial mechanisms could be compared objectively without being concealed by differences in extramicrobial formulation. None of the microbial functions were altered except for substrate degradation, which gave rise to a similar rate of substrate entrance to soluble rumen pools that are available for microbial utilization. Large differences remained in microbial functions of substrate fermentation, substrate incorporation, and microbial synthesis. Differences in extramicrobial rumen functions and microbial mechanisms had important consequences for simulated nutrient outputs from the rumen, illustrating the necessity for further validation of individual formulations.