Effects of continuous feeding of low-phosphorus diets with and without phytase during the growing and laying periods on performance of two strains of Leghorns.

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 x 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 (T1) were fed a determined NPP regimen of 0.34-0.30-0.23% for the periods of 0 to 6 wk (0.34%), 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 T2 to T5 were as follows: T2, 0.25-0.20-0.16 and 0.29%; T3, 0.15-0.10-0.08 and 0.18%; T4, 0.12-0.07-0.08 and 0.12%; and T5, 0.08-0.07-0.08, and 0.08%. The birds of T6 to T8 were fed diets similar to T3 to T3 plus 300 U phytase/kg of diet. The growth parameters and egg production performance of birds of T2, which were fed the NPP regimen of 0.25-0.20-0.16% during the growing period and 0.29% NPP during the laying period, and those of T6, which were fed a NPP regimen of 0.15-0.10-0.08% plus phytase during the growing period and 0.18% NPP plus phytase during the laying period were similar to the control group. However, their total P intakes were 16.1 and 44% lower, respectively, and their NPP intakes were 29.7 and 64.1% lower, respectively, than the control group during the entire growing period. During the digestion trial (wk 16), daily total P excretion of these two groups (T2 and T6) were 21.2 and 43.1%, respectively, lower than the control groups. The birds fed T3, T4, and T5 regimens had consistently inferior performance than the controls both during the growing and laying periods. Phytase completely alleviated the adverse effect of the low-NPP levels fed to T3 during the growing and laying periods and partially alleviated the adverse effects of low-NPP regimens fed to birds of T4 and T5 during the growing and laying periods. Additional experiments involving longer laying cycles are warranted prior to making practical recommendations.