This was observed at both time periods after the challenge and can be related to an increased gastric emptying rate. During intestinal inflammation, neu- rons of intrinsic and extrinsic innervation of the whole gut are involved (Collins, 1996). The very high ratio of intestinal length to BW at slaughter in susceptible pigs challenged and fed the basal diet may reflect pig immaturity. On the contrary, the increased intestinal density (g/cm), which was associated with Trp addi- tion to susceptible pigs, might indicate an increase in thickness of the intestinal wall. However, this was not confirmed by the histological measurements. To discuss the morphological characteristics of the small intestinal mucosa in the postweaning phase, the exact sampling location in the tract should be considered. In fact, dur- ing the immediate postweaning period, more changes of small intestinal mucosa in the upper tract are ob- served than in the lower tract, and those changes pre- sumably depend on the shortage of AA available from the gut lumen (Montagne et al., 2007). Consequently, the increased availability of Trp in nonsusceptible pigs fed the high Trp diet could have allowed the increase in villous height at the proximal small intestine. Con- versely, at the distal small intestine, where the adhesion of ETEC was maximal, the mucosa of susceptible pigs responded to Trp immediately after the challenged in- flammation, showing a compensatory increase in villous height as observed in some other challenge trials (Bosi et al., 2004b). In this instance, most of the response in susceptible pigs was due to the increase of villi in the high-Trp group, even though a statistically significant difference between the 2 diets was not reached.The trial indicated that the phenotype for the adhe- sion of the ETEC should be considered when the effects of dietary Trp are studied in weaning pigs. This is of practical relevance because of the variable frequency of the gene in pigs. Furthermore, it can be relevant for im- proved genotypes because of the association of suscep- tibility to ETEC with greater lean growth (Edfors-Lilja et al., 1986). The supplementation of Trp to a basal diet allows susceptible pigs to partially compensate for the effects of challenge with ETEC by increasing feed intake and maintaining an adequate body growth.
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