Two independentexperiments, one in pre-starter phase (1 to 7 days) and the other in the starterphase (8 to 21 days of age), were carried out with broilers to evaluate levelsof calcium and available phosphorus (aP) in diets supplemented with 500 unitsof phytase activity/kg of diet. Each experiment consisted of a performance anda metabolic assay, and was conducted in a factorial schedule (3 × 4) +1, with three aP levels (0.42, 0.37, 0.32% in the pre-starter phase and 0.39,0.34, 0.29% in the starter phase), four calcium levels (0.94, 0.84, 0.74, 0.64%in the pre-starter phase and 0.88; 0.78, 0.68, 0.58% in the starter phase) plusa control diet. The control corresponded to the only diet without phytase andcontained 0.47% of aP and 0.94% of calcium (pre-starter phase) or 0.44% of aPand 0.88% of calcium (starter phase). In the performance assays, at 7 and 21days of age the feed intake, weight gain, feed conversion and the ash contentin the tibia were evaluated. In the metabolic assays the apparent metabolizableenergy corrected for nitrogen balance (AMEn) of the feed, and dry matter digestibilitycoefficients (DMDC) were determined. Calcium levels influenced the feed intake,AMEn and DMDC in the pre-starter phase, and in the starter phase they influencedbone ash, AMEn and DMDC. The aP levels influenced the content of tibia ash,AMEn and DMDC in the pre-starter phase, and bone ash and DMDC in the starterphase. By adding 500 units of phytase activity/kg of diet, it is possible toreduce, respectively, the calcium and aP levels to 0.64% and 0.37% in the pre-starterphase, and to 0.58% and 0.29% in the starter phase, since this reduction doesnot cause a negative effect on performance or bone mineralization of broilersin the evaluated periods.
