Evaluation of modified dietary phosphorus levels with and without phytase supplementation on live performance and fecal phosphorus levels in broiler diets. 1. Full-term feeding recommendations

Based on preliminary studies conducted during periods of 0 to 3, 3 to 6, and 6 to 9 wk of age, four different phosphorus regimens were selected for use in long-term feeding trials. Using nonlinear regression, dietary levels of nonphytate phosphorus (NPP) were selected within each age period for 1) maximum body weight without phytase enzyme, 2) maximum body weight with phytase enzyme, 3) maximum tibia ash without phytase enzyme, and 4) maximum tibia ash with phytase enzyme. Two trials of identical design were conducted in which these regimes were compared to NRC recommendations for NPP. To evaluate the sensitivity of the P estimates; diets containing 0.05% above and below the estimates were also fed. Each diet was fed to six pens of 50 male chicks in each of two trials. Body weight, feed conversion; tibia ash, and fecal P content were determined at 21, 42, 53, and 63 d. Feeding levels estimated to maximize body weight, with or without phytase supplementation, supported body weight, feed utilization, and livability that did not differ significantly from birds fed NRC-recommended levels. Tibia ash was reduced in these birds, but no serious leg abnormalities were noted. Birds fed diets with NPP levels chosen to maximize tibia ash without the addition of phytase had tibia ash values that were significantly lower than those of birds fed the diets with,NRC levels of NPP; increasing this level by 0.05% resulted in tibia ash content that did not differ significantly from that of birds fed NRC levels. Birds fed diets with NPP levels chosen to maximize tibia ash with the addition of phytase did not differ significantly in tibia ash content from those fed the diets based on NRC levels of NPP. Implementation of the modified levels of NPP could result in marked reductions in fecal P content.