Combining in vitro data and modelling to predict in vivo animal response

Abstract

SummaryExisting feed evaluation systems for ruminants assess the feed value in a rather empirical way, with a limited ability to integrate metabolism in a meaningful framework. For the quantitative description of the mechanisms, appropriate biological data can be obtained using in vitro methods. The aim of this paper is to examine the use of modelling and in vitro data to predict digestion processes in vivo. Suitable mathematical methods are required to describe and interpret substrate disappearance profiles or gas production profiles. The derivation of such models is important since this allows a clear definition of the underlying assumptions made. Such assumptions are related to the change in fractional rate of degradation (kd) during incubation that will determine the shape of the profile. Furthermore, the value of the fractional passage rate (kp) is of crucial importance in the prediction of extent of degradation in the rumen. The development and application of models, based on classic microbial growth equations, clearly shows that observed variation in microbial efficiency in batch cultures (including the gas production technique) is not necessarily related to that in vivo. Rather, kp is again a major factor contributing to explanation of variation in microbial efficiency. Similarly, the end products of fermentation (VFA) and the VFA molar proportions can be estimated in vitro, but its direct applicability to the in vivo situation is limited. It is concluded that some potential uses of in vitro techniques are ultimately misleading. Mechanistic models indicate that mechanisms governing microbial efficiency and VFA molar proportions in vitro are not necessarily valid for the in vivo situation. Therefore, the in vitro data cannot be used directly for a uniform system of feed evaluation to predict animal responses. Rather, the in vitro data obtained for substrate degradation may be used in whole rumen models as a basal input value to indicate the degradation potential.