Six dietary treatments were applied in a 42-d study to determine the effect of phytase, butyric acid, and S. boulardii on productive performance, intestinal microflora, immune response, stress indicators, and gut morphology in broiler chickens fed diets with reduced content of nonphytate P (nPP). On d 0, a total of 750 male broiler chickens were randomly allotted to 6 treatments with 5 pens of 25 broiler chickens for each diet: positive control (PC, adequate amounts of nPP), negative control (NC, 0.1% reduction in dietary nPP content), NC + 500 FTU phytase/kg (PHY), NC + 2 g butyric acid/kg (BA), NC + 1 × 108 cfu S. boulardii /kg (SB), and NC + butyric acid + S. boulardii (BA + SB). Considering the entire study period, body weight gain in broiler chickens fed any of the diets were greater (P < 0.05) than in broiler chickens fed the NC diet, where the broiler chickens fed diets PHY and BA + SB had the greatest value. Broiler chickens fed diets PC, PHY, SB, and BA + SB had greater (P < 0.05) relative thymus weight, primary and secondary antibody response against infectious bronchitis virus vaccine, and cutaneous basophil hypersensitivity to phytohaemagglutinin-P, but lower (P < 0.05) mortality rate and blood corticosterone concentration than those fed the NC diet at d 42. Feeding diet BA + SB increased (P < 0.05) cecal Lactobacillus spp. counts (d 35) and jejunal villus high:crypt depth (d 42), but decreased (P < 0.05) cecal Clostridium spp. counts (d 35) compared with those fed the NC diet. On d 35, the PHY, BA, SB, and BA + SB treatments had also the lower (P < 0.05) population of E. coli and Clostridium spp. in the ileum. No diets affected feed intake, relative weights of abdominal fat and liver, the population of total anaerobic bacteria, Bifidobacterium spp., and Coliforms in both ileum and ceca, serum insulin concentration, and duodenal morphology. In conclusion, the combination of butyric acid and S. boulardii may have synergistic effects on improving productiv
