Simple SummaryKnowledge of the nutrients released by microbial phytase enzymes is critical for their economic and sustainable use and precise poultry feed formulation. Several factors, including assessing measurements, age, adaptation period, phytate content in the diet, phytase dose and source, as well as calcium (Ca) to phosphorus (P) ratio, particularly in low P diets, can affect the function of phytase and magnitude of responses. The present study aimed to investigate the influence of bacterial 6-phytase on growth performance, coefficient of apparent ileal digestibility of nutrients (CAID), blood parameters, and bone mineralization in broilers that were fed a diet with a constant ratio of calcium to available phosphorus (avP). Using a constant ratio between calcium and available phosphorus led to a decrease in the calcium level of the experimental diets. The results showed that the bacterial 6-phytase dosed in the range of 500–1000 FTU/kg was effective in replacing 1.5 g/kg avP and 3 g/kg Ca in broilers fed pelleted diets, using bone quality, body weight gain (BWG) and feed per unit gain (FCR) as outcome measures. Poultry nutritionists might need to consider Ca levels in diets supplemented with phytase in feed formulations.AbstractThe influence of a bacterial 6-phytase on growth performance, coefficient of apparent ileal digestibility (CAID) of nutrients, blood parameters, and bone mineralization in broilers was evaluated. A total of 630 one-day-old male broilers were allocated to 7 dietary treatments, including positive control (PC) diet containing dicalcium phosphate, the PC marginally reduced in available P (avP) by 0.1% and calcium (Ca) by 0.2% vs. PC (NC1) or moderately reduced by 0.15 and 0.3% vs. PC (NC2), respectively, and four further diets comprising the NC1 and NC2 supplemented with 500 or 1000 FTU/kg of phytase in starter and finisher phases. A constant Ca to avP ratio was maintained across all diets. The body weight gain (BWG) and feed per unit gain (FCR) of birds fed NC1 and NC2 diets supplemented with phytase (500 and 1000 U/kg) was equivalent to that of birds fed the PC diet at 35 days. Phytase supplementation in the NC1 diet linearly increased the CAID of nitrogen (N) (p< 0.01), phosphorus (P) (p< 0.01), and Ca (p< 0.05). Additionally, phytase reduced (p< 0.01) excreta P concentration by approximately 27%, improved (p< 0.001) toe ash, and tended to increase tibia ash (p= 0.08), comparable with the PC. In conclusion, the addition of bacterial 6-phytase dosed in the range of 500–1000 FTU/kg was effective in replacing 1.5 g/kg avP and 3 g/kg Ca in broilers fed pelleted diets, using bone quality, BWG, and FCR as outcome measures.