This experiment was conducted to assess the effect of three levels of dietary sugar beet pulp (SBP), tallow (T) and soybean oil (SO) on the performance and morphology of the jejunum in the young broilers were applied central composite design (CCD; 3 levels and 3 factors) and response surface methodology (RSM). A total of 420 one-day-old male chicks Ross 308 were appropriated to 60 cages of seven birds each and fed with 15 diets, produced by CCD containing three levels of SBP (0.00, 1.75, 3.50%), T (0.00, 0.50, 1.00%) and SO (0.00, 0.50, 1.00%) from 0 to 14 days of age. The results showed that maximum average daily body weight gain (ADG, 31.59 g/bird/d) and minimum feed conversion ratio (FCR, 1.02) were obtained with diets comprising 0.51% SBP, 0.20% T, 0.38% SO and 0.50% SBP, 0.05% T and 0.10% SO, respectively. On day 7 of age, the linear effects for all models, the quadratic and interactions effects for the number of goblet cells (GC) model were significant (P<0.05). On day 14 of age, the linear and quadratic effects for all models and the interactions for the model of villus width (VW) were significant (P<0.05). Generally, aging and subsequent growth and expansion of the gastrointestinal tract can moderate the destructive impacts of soluble fibers to some extent and the RSM could provide an acceptable prediction for the relevance between variables and traits.

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