This study was conducted to evaluate the effects of xylanase and citric acid (CA) on growth, digesta pH, ileal populations of Clostridium perfringens and lactic acid bacteria, ileal nutrient digestibility, digestive enzyme activity, and mRNA expression of intestinal nutrient transporters in starter broilers challenged with C. perfringens. The experiment was conducted in a 2×2 factorial arrangement with two levels of CA (0 and 30 g/kg) and 2 levels of xylanase (0 and 200 mg/kg). Each of the four dietary treatments was fed to six replicate pens (15 birds/pen) between 0 and 21 d of age. Dietary CA significantly increased ADFI and ADG; meanwhile, xylanase addition led to a substantial reduction in FCR (P＜0.05). No differences in digesta pH, C. perfringens counts, or quantity of lactic acid bacteria were found between the treatments. Xylanase supplementation increased AME values (P＜0.01) and ileal digestibility of CP (P＜0.05) in challenged birds. The inclusion of CA also increased the AME (P＜0.01), and tended to increased ileal CP digestibility (P＝0.085). Xylanase supplementation increased α-amylase, trypsin, and sucrose activity in the jejunum (P＜0.01). Dietary CA significantly increased (P＜0.01) villi length as well as the villus length to crypt depth ratio in jejunum segments. The jejunal mRNA expression of sodium glucose co-transporter 1 (SGLT1) and H + -dependent peptide transporter 1 (PepT1) were upregulated by xylanase supplementation (P＜0.01). The results suggest that dietary CA can promote growth as well as improve intestinal morphology and AME in birds challenged with necrotic enteritis. This study shows that xylanase supplementation improved FCR and AME in birds independent of C. perfringens infection; it also elevated the apparent ileal digestibility of CP, digestive enzyme activities, and mRNA expression of nutrient transporters in challenged birds.
