This experiment was conducted to evaluate the equivalency values of microbial phytase to inorganic phosphorus (Pi) in 22- to 42-d-old broilers fed a corn-soybean meal diet. Ten dietary treatments were designed in which diet 1 was the basal diet [calcium 0.90%, total phosphorus 0.35%, nonphytate phosphorus (NPP) 0.13%] and diet 4 was control (calcium 0.90%, total phosphorus 0.57%, NPP 0.35%). Diets 2 and 3 included 0.20 and 0.27% of NPP, and diets 5 to 10 contained graded levels of microbial phytase (125, 250, 500, 1,000, 2,000, and 4,000 U/kg of diet). Body weight gain, feed intake, phosphorus retained, plasma Pi concentration, tibia ash content, and tibia-breaking strength increased, but plasma calcium concentration decreased with increasing dietary NPP content (from 0.13 to 0.35%) or phytase activity. Microbial phytase caused an increase, whereas dietary NPP caused a decrease in phytate phosphorus (PP) retention. When 4,000 U of phytase was added to the basal diet with 0.13% NPP, 95% PP was released based on tibia ash content, and broiler BW gain and feed intake were equal to those of broilers fed the control diet (NPP 0.35%). Microbial phytase at 125, 250, 500, 1,000, 2,000, and 4,000 U/kg of diet was determined to be equivalent to the addition of 0.035 to 0.208% of Pi from dicalcium phosphate in 22- to 42-d-old broilers. These data suggest that high levels of microbial phytase could release almost all dietary PP and replace all supplemental Pi in growing-finishing broilers.
