Arbor Acres Plus male broilers (n = 1,152) were obtained at hatch and allocated to 1 of 6 diets from hatch to d 10 post-hatch. There were 16 replicate cages per diet and 12 birds per cage. Five of the diets were formulated to contain graded concentrations of standardized ileal digestible ( SID ) Ca at 0.60, 0.50, 0.40, 0.30, or 0.20%. A sixth reference diet was formulated using total Ca coefficients for each ingredient and contained 0.96% total Ca. Available P ( avP ) was maintained at 0.48% in all 6 diets. Data were subjected to an analysis of variance and the model included diet and block. Means were separated using contrasts to determine linear or quadratic effects of SID Ca and using the Dunnett's test to compare the reference diet to all SID Ca diets. There was no effect of graded levels of SID Ca on intake or gain. Birds fed diets containing 0.60, 0.50, 0.30, or 0.20% SID Ca ate ( P < 0.05) or gained ( P < 0.05) more compared with birds fed the reference diet. Mortality corrected FCR improved (linear, P < 0.05) as the SID Ca concentration in the diet increased from 0.20 to 0.60%. Tibia ash percent was greatest in birds fed 0.50% SID Ca and lowest in birds fed 0.20% SID Ca (quadratic, P < 0.05). Tibia ash percent was lower ( P < 0.05) in birds fed diets formulated to contain 0.20% SID Ca compared with birds fed the reference diet. No other differences in tibia ash were reported. Apparent ileal digestibility (AID) or retention of P was greater ( P < 0.05) in birds fed diets formulated using SID Ca compared with birds fed the reference diet. The AID of P increased (linear, P < 0.05) as the SID Ca content in the diet decreased from 0.60 to 0.20%. The AID or retention of Ca was similar in birds fed 0.60 or 0.50% SID Ca and increased as SID Ca decreased to 0.20% (quadratic, P < 0.05). Regression equations developed using bone ash and apparent P retention estimate the SID Ca requirement of Arbor Acres Plus broilers from hatch to d 10 post-hatch was 0.53 and 0.49%, respectively. This corresponds to a SID Ca to available P ratio of 1.1 to 1.02.

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