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A total of 288 32-wk-old Lohmann Selected Leghorn (LSL-Lite) laying hens were fed on corn-soyabean meal-based diets containing three concentrations of rice bran (RB; 0, 75 and 150 g/kg; equivalent to 1.8, 2.7 and 3.5 g/kg phytate phosphorus), two of non-phytate phosphorus (NPP; 2.5 and 3.5 g/kg) and two of microbial phytase (Phyzyme® XP 5000G; 0 and 150 FTU/kg) in a 10-wk trial under high temperature condition. The average minimum and maximum room temperatures were 26.0 ± 1.9 and 36.0 ± 1.8 °C, respectively, with relative humidity ranging from 42.3 ± 1.8% to 55.1 ± 2.1%. Each dietary treatment was fed to four cages (six birds/cage) from 32 to 42 wk of age. Egg production, egg weight and feed intake were recorded daily. Egg quality traits were recorded every 5 wk. Leukocyte subset counts and blood lipid profile were evaluated at the end of experiment. The results showed that inclusion of 150 g/kg RB negatively influenced ( P < 0.05) egg production (EP), feed conversion ratio (FCR), shell weight and shell thickness. Significant reductions ( P < 0.05) in the blood triglyceride and total cholesterol were observed with increasing dietary inclusion levels of RB. Although, dietary NPP levels did not influence any of the performance traits, the low-NPP diets caused a decrease ( P < 0.05) in shell weight and shell thickness when compared with normal-NPP diets. The H/L ratio, as a stress indicator, was also increased ( P < 0.05) by low-NPP diets. These adverse effects were partially overcome by the addition of microbial phytase (PHY). Supplemental PHY improved ( P < 0.05) EP, FCR and unmarketable eggs but the magnitude of the responses was greater in low-NPP and high-RB treatments, resulting in significant RB × NPP × PHY interactions. Egg mass, shell weight and shell thickness were also improved ( P < 0.05) by PHY addition but the response was greater at the highest concentration of RB, resulting in a significant RB × PHY interaction. Responses were also greater ( P < 0.05) in low-NPP diets as indicated by significant NPP × PHY interaction. It can be concluded that the positive effects of PHY supplementation were clearly evident for laying hens reared under the high ambient temperatures pertaining to this study. At high concentration of phytate (high-RB diet) and/or low level of NPP (2.5 g/kg), response to supplemental PHY appears to be increase in terms of performance and egg quality in heat-stressed laying hens. [ABSTRACT FROM AUTHOR]