The addition of microbial phytase to diets for broiler chickens has been shown to improve the availability of phytate phosphorus, total phosphorus, some other minerals, and amino acids. In this study, the effect of a novel microbial phytase expressed by synthetic genes in Aspergillus oryzae on amino acid and mineral availability was assessed. Phytase was incorporated (1,000 and 2,000 U/kg) into low-phosphorus corn-soybean meal-based diets for broilers. Broilers received the experimental diets for 3 weeks and excreta were collected from day 18 to 21 for the determination of AME and mineral retention. On the 22nd day, the broilers were killed and the left leg removed and ileal digesta collected. Ileal phytate phosphorus and total phosphorus absorption, ileal amino acid digestibility, as well as the bone mineral content and bone mineral density were determined.

During the starter phase, the interaction was significant for ADG; birds fed the LoPro-LoDig diet grew slower than birds fed the other three diets (P < 0.05). During the grower phase, the interaction was significant for ADFI; birds fed the LoPro-LoDig diet had the lowest ADFI compared with those fed the other three diets. The interaction between protein level and digestibility was significant for the SID of most of the AA and was significantly higher for birds fed the HiPro-HiDig diet compared with those fed the other three diets. Total breast meat yield was significantly higher in birds fed the HiPro diets than in those fed the LoPro diets, whereas birds fed the HiDig diets had significantly more abdominal fat than those fed the LoDig diets. The results suggest that low-protein diets can support growth performance equal to high-protein diets when highly digestible ingredients such as soybean meal and fish meal are used. However, maximum breast meat yield requires a high-protein diet and is not affected by ingredient digestibility. (Note-This research emphasizes the importance of using highly digestible protein feedstuffs when attempting to lower protein levels in practical broiler diets).