Multiphasic analysis of gas production kinetics for in vitro fermentation of ruminant feeds.

Recently developed time-related gas production techniques to quantify the kinetics of ruminant feed fermentation have a high resolution. Consequently, fermentation processes with clearly contrasting gas production kinetics can be identified. Parameterization of the separate processes is possible with a suitable multiphasic model and modelling method. A flexible, empirical, multiphasic model was proposed for parameterization of gas production profiles. This equation was fitted, using a two-step method, to four gas production profiles and the maximum fractional rate of substrate digestion (RM) was calculated for each phase. In the first step, the number of phases and starting values for parameters of the multiphasic model were derived from a combination of either (1) the measured cumulative gas production profile and its first (rate of gas production) and second (change in rate of gas production) derivative or (2) a fitted monophasic curve and the residuals between this curve and the observations. In the second step, the starting values were used to fit di- or triphasic models. Using Approach 1 was complicated by frequent fluctuations in the gas production rate. However, the combined graph of the fitted monophasic curve and residuals (Approach 2) gave good results for every profile. The multiphasic model fitted the profiles well. The robustness of the model declined when the number of phases was increased, underlining the importance of accurate estimations of starting values. It is argued that the model parameters and the calculated RM are useful for feed quality evaluation. The results show that mathematical description of gas production profiles requires a two-step approach and a multiphasic model.