An experiment was conducted to determine the minimum levels of nonphytate P (NPP) with and without phytase that can be continuously and safely used during the growing and laying periods for diminishing P excretion to the environment. The experiment consisted of a 2 × 8 factorial arrangement of the treatments with two strains (Babcock B-300 and Hy-Line W-98) and eight diets. The birds of the control group (T 1 ) were fed a determined NPP regimen of 0.34-0.30-0.23% for the periods of 0 to 6 wk

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... 34%), 6 to 12 wk (0.30%), and 12 to 18 wk (0.23%) of age and 0.39% determined NPP during the laying period (18 to 26 wk of age). The dietary NPP regimens during the growing and laying periods for T 2 to T 5 were as follows: T 2 , 0.25-0.20-0.16 and 0.29%; T 3 , 0.15-0.10-0.08 and 0.18%; T 4 , 0.12-0.07-0.08 and 0.12%; and T 5 , 0.08-0.07-0.08, and 0.08%. The birds of T 6 to T 8 were fed diets similar to T 3 to T 5 plus 300 U phytase/kg of diet. The growth parameters and egg production performance of birds of T 2 , which were fed the NPP regimen of 0.25-0.20-0.16% during the growing period and 0.29% (Body weight at 26 wk of age, egg production, egg weight, feed intake, egg mass, feed conversion, specific gravity, and mortality of birds fed a NPP regimen of 0.25-