Optimal phytase supplementation levels for broilers differ by thermal condition for phosphorus utilization but not for growth or bone mineralization

This study evaluated the effects of reducing dietary calcium (Ca) and phosphorus (P) concentrations and supplementing phytase on broilers reared under thermoneutral or cyclic heat stress (HS) conditions, and estimated the optimal phytase level for each environment. A total of 420 one-d-old male broiler chicks were assigned to a 2 × 5 factorial arrangement consisting of 2 rearing temperatures and 5 diets: a positive control (PC), a Ca- and P-deficient negative control (NC), and the NC supplemented with 500, 1,000, or 1,500 phytase units (FTU) per kilogram of diet. Birds were reared under identical temperatures from d 0 to 7 and then exposed to either thermoneutral or cyclic HS conditions from d 7 to 21. Growth performance, bone mineralization, and nutrient digestibility and utilization were measured, and one-slope broken-line analysis was used to estimate optimal phytase levels. Phytase supplementation linearly or quadratically increased (P < 0.05) ADG, ADFI, bone mineralization, and Ca and P utilization. Estimated optimal phytase levels for ADG, ADFI, tibia breaking strength, and nitrogen excretion did not differ between thermoneutral and HS conditions, were generally within the range of approximately 500 to 800 FTU/kg, and produced plateau responses that did not differ from the least squares means of broilers fed the PC diet under either environment. In contrast, the optimal phytase level for P utilization was greater (P = 0.029) under HS than under thermoneutral conditions (987 vs. 536 FTU/kg), and supplementing the NC diet with these estimated optimal phytase levels resulted in greater (P < 0.001) P utilization than in broilers fed the PC diet. In conclusion, phytase supplementation improved broiler performance, bone mineralization, and P utilization in NC diets under both environments, although the optimal inclusion level depended on the response criterion and environmental condition, indicating that supplementing an NC diet with the optimal level of phytase can restore most responses to the level of the PC diet and can even result in greater P utilization than the PC diet.