Abstract: An experiment was conducted to determine the effect of high levels of microbial phytase supplementation in corn-soy-based diets on nutrient availability of broiler chickens.A totalof 576 one-day old male Arbor Acres broilers were assigned to 6 dietary treatments,consisting of 8 pens of 12 birds each,and were fed two periods experimental diets from 1 to 21 d and 22 to 42 d. The diets include a positive control (0.69% tP,0.45% nonphytate P for 1 to 21 d;0.59% tP,0.35% nonphytate P for 22 to 42 d)and a negative control(0.60% tP,0.35% nonphytate P for 1 to 21 d;0.49% tP,0.25% nonphytate P for 22 to 42 d),and 4 more negative diets with 500,2000,8000,32000 FTU/kg phytase. The results showed that ① High dose of phytase could significantly improve the availability of Ca at 17 to 21 d (P <0.05 ).②High dose of phytase could significantly influence the availability of P at 38 to 42 d (P<0.05 ),with the phytase levels increasing,the availability of P significantly decreased,while the availability of Ca tended to be increased then to be decreased.③There were significantly improvements in availability of crude protein and AME with phytase levels increasing(P<0.05)either at 17 to 21 d or at 38 to 42 d. This study suggest that higher levels of phytase can be used to improve the overall utilization of phytate P and possibly other nutrients in a tP-deficient diet. That current phytase supplementation and diet nutrients within the poultry industry may need to be reevaluated.

Key words: high levels; phytase; broiler; energy; protein; nutrient availability