Two experiments were conducted to evaluate the feeding of higher levels of phytase on amino acid (AA) and energy digestibilities and broiler growth performance when fed diets adequate in calcium and nonphytate phosphorus supplemented with xylanase. Ross × Ross 708 broiler chicks were randomly distributed into battery cages (6 replicate cages per treatment) with 12 birds per cage at one day of age. Treatments evaluated consisted of six phytase treatments (0, 1,000, 2,000, 4,000, 8,000, or 16,000 phytase units (FTU)/kg) and three xylanase treatments (0, 6,000, or 32,000 birch xylan units (BXU)/kg) concentrations. Digesta contents of the terminal ileum were collected at 32 d of age in experiments 1 and 2, and growth performance was measured at 14, and 25 d of age in experiment 2.

There was no interaction of phytase and xylanase; only main effects of phytase were observed. In experiment 1, broilers fed diets supplemented with phytase at 1,000 FTU/kg had increased (P < 0.05) apparent ileal digestibility (AID) of all AA with the exception of Ala and Met. Diets fed to broilers supplemented with higher concentrations of phytase did not further increase AID of any AA. Phytase supplementation did not affect ileal digestible energy (P > 0.05). However, in experiment 2, supplementation with 2,000 FTU/kg of phytase increased (P = 0.05) ileal digestible energy by 36 kcal/kg compared with the basal diet. Broilers fed diets with 1,000 FTU/kg of phytase had improved growth performance (P < 0.05) in comparison with broilers fed diets containing 0 or 500 FTU/kg of phytase. The addition of 500 FTU/kg of phytase in diets fed to broilers did not affect ileal digestibilities or growth performance (P > 0.05). The research team concluded that data obtained from these experiments demonstrated improved performance of broilers fed diets supplemented with 1,000 FTU/kg of phytase and diminishing returns when higher concentrations were fed.