Estimating the extent of degradation of ruminant feeds from a description of their gas production profiles observed in vitro: comparison of models

An evaluation of general models that describe gas production profiles is pr esented. The models are derived from first principles by considering a simp le three-pool scheme and permit the extent of ruminal degradation to be cal culated, as described in the companion paper. The models evaluated were the generalized Mitscherlich, simple Mitscherlich, generalized Michaelis-Mente n, simple Michaelis-Menten, Gompertz, and logistic. Five sets of gas produc tion data consisting of 216 curves, obtained using a wide range of feeds (i ncluding straw, hay, silage, grain and various byproducts), were analysed t o study the performance of these gas production models. Application of the non-sigmoidal models (simple Mitscherlich and Michaelis-Menten) to the data resulted in convergence problems and these models were found to be inadequ ate in many cases. Based on results of a pairwise comparison between models (variance ratio test), ranking of residual mean squares, lack-of-fit test, and of analyses of residuals, the generalized Mitscherlich and the general ized Michaelis-Menten models seemed particularly suited because of their fl exibility to encompass sigmoidal and non-sigmoidal shapes of gas production profiles, whether symmetrical or not.