Requirement of digestible calcium at different dietary concentrations of digestible phosphorus for broiler chickens. 2. Broiler growers (d 11 to 24 post-hatch)

An experiment was conducted to determine the digestible calcium (Ca) and digestible phosphorous (P) requirements of 11 to 24 d old broiler chickens. Eighteen corn-soybean meal-based diets containing 1.80, 2.35, 2.90, 3.45, 4.00, and 4.55 g/kg standardized ileal digestible (SID) Ca and 3.5, 4.5, and 5.5 g/kg SID P were fed to broilers from d 11 to 24. Each experimental diet was randomly allocated to six replicate cages (8 birds per cage). Body weight and feed amount were recorded at the start and end of the experiment and the feed conversion ratio was calculated. On d 24, birds were euthanized to collect ileal digesta, tibia, and carcass for the determination of digestible Ca and P, the concentration of ash, Ca and P in tibia and the retention of Ca and P in the carcass, respectively. Titanium dioxide (5 g/kg) was included in all diets as an indigestible indicator for apparent ileal digestibility measurement. Total excreta output was measured during the last 4 d of the experimental period for the measurement of apparent total tract retention of Ca and P. Fixed effects of the experiment were dietary concentrations of SID Ca and SID P and their interaction. If the interaction or main effect was significant (P < 0.05), the parameter estimate for second-order response surface model was determined using General Linear Model procedure of SAS. The weight gain of broiler growers was optimized at the SID P concentration of 3.5 g/kg and SID Ca concentrations between 2.35 and 4.00 g/kg. At 3.5 g/kg SID P concentration, the required SID Ca for maximum weight gain was determined to be 3.05 g/kg, which corresponded to SID Ca to SID P ratios of 0.87. The concentration of SID Ca that maximized tibia ash at 3.5 g/kg SID P was 3.69 g/kg, which corresponded to SID Ca to SID P ratio of 1.05. Maximizing bone ash requires more Ca than maximizing weight gain. Carcass Ca and P retention were reflective of total tract Ca and P retention values. The estimated SID Ca requirements (at 3.5 g/kg SID P) for both maximized weight gain (3.05 g/kg or 6.11 g/kg total Ca) and bone ash (3.69 g/kg or 7.28 g/kg total Ca) are lower than the current Ca recommendation (8.70 g/kg total Ca equivalent to 4.03 g/kg SID Ca; Ross, 2019) for broiler growers, indicating possible oversupply of Ca in diets formulated based on the current recommendation.