A compartmental model to analyze ruminal digestion.

In contrast to digestion models that include a discrete lag phase, a compartmental digestion model was proposed. It assumed the existence of a lag compartment and a digestion compartment. Substrate present in the digestion compartment was subject to first-order kinetics digestion. Flow of substrate from the lag compartment to the digestion compartment was proposed to be a first-order process and likely was affected by hydration of substrate, bacterial attachment, and colonization. The proposed model was compared with models that assumed the existence of a discrete lag phase. Parameter estimates for these models were obtained either through logarithmic transformation of data or nonlinear regression. Statistically, there was no difference between the compartmental model and the nonlinear model with a discrete lag phase. Differences in parameter estimates between these two models were small. Residual mean squares were higher for the logarithmically transformed models. Differences in parameter estimates between these models and the compartmental model depended on the structure of the experimental data. In a number of cases, the nonlinear parameters of the compartmental model converged to the same value, resulting in a different interpretation of the model. Residual mean squares for predicting rate of disappearance were lowest for the compartmental model.