The objective of the current study was to evaluate the effects of dietary zinc (Zn) supplementation on performance, immune responses, and gastrointestinal tract features of broilers fed wheat-soybean meal diet. Additionally, optimized dietary Zn level was estimated based on dose-response data. A total of 500 one-day-old male Ross-308 broiler chicks were randomly assigned to ten dietary treatments with five replicates and ten birds per replicate in a 5×2 factorial arrangement of five graded (+40 mg/kg) dietary Zn levels with or without phytase (500 FTU/kg of diet) supplementation. By increasing dietary Zn concentration, final live body weight (LBW), daily weight gain (WG), feed intake (FI), and European Production Efficiency Factor (EPEF), antibody titers against the sheep red blood cell inoculation, cutaneous basophils hypersensitivity to PHA-P injection, jejunum villus height, and apparent dry matter digestibility (ADMD) increased with a significant linear trend. Phytase supplementation improved WG and ADMD and decreased pancreas and intestine relative weights. The interaction effects of dietary Zn concentration and phytase supplementation on the whole of evaluated traits were not significant. Optimum dietary Zn levels were estimated by the regression models at 70, 82, 98, and 110 mg/kg of diet for optimized EPEF and immunity, LBW and WG, appetites, and ADMD, respectively. It is concluded, in the broiler chickens fed wheat-soybean meal diet, Zn and phytase supplementation can improve growth performance, immune response, digestibility, and intestinal features. The Zn requirement is higher than the NRC (1994) recommendation. The efficacy of dietary Zn was not affected by dietary phytase supplementation.

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