a b s t r a c tThe effects of inclusion of oat hulls (OH) and sugar beet pulp (SBP) in the diet on the developmentof the gastrointestinal tract (GIT) were studied in broilers from 1 to 18 days (d) ofage. A control diet based on rice that contained 16 g crude fibre (69 g dietary fibre)/kg wasdiluted with 25, 50 and 75 g of either OH or SBP/kg. Each of the seven treatments was replicatedsix times (a cage with 12 chicks). The weight of the digestive organs and the pH of thedigesta contents were recorded at d 6, 12 and 18, and the jejunal morphology at d 12 and 18.The inclusion of a fibre source in the diet affected in different ways the development of theorgans of the GIT. The relative weight of the GIT with digesta contents (g/kg body weight,BW) increased linearly (L; P≤0.001) as the level of fibre in the diet increased. The weightof the pancreas increased (L; P≤0.01 at d 6 and 12, and P<0.05 at d 18) with SBP inclusionbut little effect was observed with OH. The relative weight of the gizzard (P≤0.001) andits DM content was increased (P≤0.001), and gizzard pH was reduced (P≤0.001) with fibreinclusion at all ages. Broilers fed OH had heavier gizzards (P≤0.001) that had higher DMcontent (P≤0.001) and gizzard pH (P<0.05 at d 12 and 18) than broilers fed SBP. The pHof the digesta of the duodenum increased with OH or SBP inclusion at d 6 (L; P≤0.01) andwith SBP inclusion (L; P≤0.01) at d 12. Villus height at d 12 was reduced with SBP inclusion(L; P<0.05) but no effects were detected with OH. We conclude that the inclusion of up to75 g OH or SBP/kg in low fibre diets increased the relative weight of the GIT and reduceddigesta pH of the gizzard. The inclusion of high levels of SBP (75 g/kg) might have detrimentaleffects on the structure of the jejunal mucosa. Dietary fibre stimulates the developmentof the GIT in young birds but an excess might hinder intestinal mucosa structure. 1. IntroductionHigh quality ingredients are used in diets for young chicks to reduce the incidence of enteric disorders and increasegrowth performance (Mateos et al., 2002). These diets contain limited amount of fibre in order to maximize voluntary feedintake and increase nutrient digestibility. However, low fibre diets might affect the structure of the mucosal epithelium and compromise gastrointestinal tract (GIT) function and nutrient utilization (Jiménez-Moreno et al., 2011a, 2011b; Svihus,2011).Inclusion of whole grains and coarse grinding of ingredients are feeding practices often used to reduce feed cost and theincidence of digestive disturbances in broilers (Hetland et al., 2002; Svihus et al., 2004; Gabriel et al., 2008). Similar benefitsto those observed with these practices have been reported when certain amounts of structural fibre such as oat hulls (OH),pea hulls or wood shavings were included in the diet (Hetland et al., 2003; Amerah et al., 2009; Jiménez-Moreno et al., 2011a).Fibre inclusion reduced gizzard pH (González-Alvarado et al., 2007; Jiménez-Moreno et al., 2009a), increased digestive juicessecretion (Ikegami et al., 1990) and improved gizzard function and digesta flow, which might facilitate nutrient digestionand absorption (González-Alvarado et al., 2010; Svihus, 2011; Mateos et al., 2012).Solubility, water holding capacity, viscosity, bulk, fermentative capability and other physico-chemical properties offibrous ingredients affect the development and epithelial morphology of the GIT and consequently, they may have nutritionalimplications in poultry (Montagne et al., 2003; Jiménez-Moreno et al., 2009b, 2009c). In this respect, soluble fibrous fractionssuch as pectins from sugar beet pulp (SBP), are dispersible in water and might increase viscosity and bulk of the digesta(Bach Knudsen, 2001) delaying gizzard emptying. Also, an increase in digesta viscosity may reduce the rate of diffusion ofdigestive enzymes into the digesta, hampering nutrient absorption (Forman and Schneeman, 1980; Iji et al., 2001). On theother hand, insoluble fibrous fractions, such as those present in OH, stimulate gizzard activity and favour gastrointestinalrefluxes which may improve nutrient digestibility (Hetland and Svihus, 2001; Hetland et al., 2003; Jiménez-Moreno et al.,2010). The hypothesis of this research was that the inclusion of moderate amounts of fibre into low fibre diets may havepositive effects on the morphology, physiology and function of the GIT in young broilers, but that an excess could haveopposite effects. The aim of this study was to evaluate the effect of increasing levels of two fibre sources (OH or SBP) with different physico-chemical properties on digestive characteristics and mucosa morphology of broilers from 1 to 18 d of age.
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