techniques allow manipulation of parameters defining the state of the animal and, if properly evaluated against in vivo observations, can be appropriate to study the response of the animal when one factor is varied and controlled without the interaction of other related factors, which could conceal the main effect. | 88 S. Lopez techniques allow manipulation of parameters defining the state of the animal and if properly evaluated against in vivo observations can be appropriate to study the response of the animal when one factor is varied and controlled without the interaction of other related factors which could conceal the main effect. Thus in vitro and in situ techniques may be used to study individual processes providing information about their nature and sensitivity to various factors. Also a number of in vitro and in situ methods have been developed to estimate digestibility and extent of ruminal degradation of feeds and to study their variation in response to changes in rumen conditions. Such techniques have been used for feed evaluation to investigate mechanisms of microbial fermentation and for studying the mode of action of anti-nutritive factors additives and feed supplements. This chapter will review recent developments in feed evaluation with attention given to the role of in situ and in vitro methods in combination with mathematical modelling in predicting digestibility and extent of degradation in the rumen of feeds. In Vitro Techniques Methods to estimate whole tract digestibility An overview of methods in use to estimate whole tract digestibility is presented in Table . Solubility The objective of separating soluble and insoluble components by simple extractions is to differentiate fractions that are either readily digestible or potentially indigestible respectively Van Soest 1994 . This could explain why with some of these techniques and for some feeds a significant correlation between solubility and digestibility has been observed Minson 1982 . Nocek 1988 has reviewed some of the solubility techniques used to predict the digestibility of feeds. Different solvents have been used but with forages the best results have been obtained with the detergent system of fibre analysis Van Soest et al. 1991 which separates feeds into a combination of uniform and .
