AbstractDevelopment of mechanistic

Abstract
Development of mechanistic models of digestion is a means to account for interactions between different nutrients, and between nutrients and the animal. Although some digestion models for pigs have been proposed, quantitative information concerning the underlying processes is extremely limiting. The aim of this study was to determine the effect of dietary fibre on the transit of the solid and liquid phases of digesta in different segments of the gastro-intestinal tract (GIT) in growing pigs. Six barrows (initial BW 30 kg and fitted with two simple T-cannulas at the proximal duodenum and distal ileum) were used in a double 3 × 3 Latin square design. Pigs were offered diets differing in total dietary fibre content (17, 22 and 27%) at 4 h intervals. A single meal marked with YbO2 and Cr-EDTA was used to determine the passage kinetics of respectively the solid and liquid phases of digesta in different segments of the GIT.
The mean retention time (MRT) averaged 1 h from the mouth to the proximal duodenum (stomach), 4 h from the proximal duodenum to the distal ileum (small intestine), and 35 h from the distal ileum to the faecal excretion (large intestine). Increase in dietary fibre content had no effect on MRT in the stomach, tended to decrease MRT of solid phase in the small intestine (P = 0.07), and decreased MRT of both phases in the large intestine (43 to 29 h, P = 0.06). For all diets, transit of the liquid phase in the large intestine was 4–8 h less than transit of the solid phase.
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1. Introduction
Feed evaluation systems are currently based on classical and aggregate values such as the ileal or faecal digestibility, which are typically available as tabular values. Digestibility is the result of two competing processes: digestion (i.e., hydrolysis and absorption) and passage. Mathematical models of digestion have the potential to account for the dynamic release of nutrients, and may account for interactions that exist between nutrients and between nutrients and the animal. Although some models describing ileal or total tract digestion have been developed for pigs (Bastianelli et al., 1996; Rivest et al., 2000), there is little quan- titative information concerning the underlying digestive phenomena such as transit time and hydrolysis rate of dietary constituents. The usefulness of mathematical models as a tool for feed evaluation depends on the

Mean retention time (hour) of solid and liquid phases in segments of the gastro-intestinal tract
the different