Estimation of standardized ileal digestible calcium and phosphorus requirements of broiler chickens aged 21-35 days.

This study investigated the standardized ileal digestible (SID) calcium (Ca) and phosphorus (P) requirements of broiler chickens using response surface methodology (RSM). A total of 464 male Ross 308 broilers, aged 21 days, were assigned to nine treatments in a completely randomized design, with six replicates per treatment and ten replicates for the central treatment. The experimental diets incorporated SID Ca and P at five concentration levels: 3.20 and 5.90, 3.84 and 4.70, 3.84 and 7.10, 5.40 and 4.20, 5.40 and 5.90, 5.40 and 7.60, 6.96 and 4.70, 6.96 and 7.10, as well as 7.60 and 5.90 g/kg, respectively. Body weights and feed intake were recorded on days 28 and 35 to assess growth performance. Tibias were collected to analyze tibial ash, Ca and P content, and bone mineral density (BMD). Additionally, tibia bone breaking strength (BBS) was measured on day 35. RSM-based statistical analysis was performed using Design-Expert® 13 software following a central composite design. From d 21 to 28, varying SID Ca and P content did not affect growth performance (P > 0.05), as no quadratic response was observed (P > 0.05); however, tibial ash concentration and BMD exhibited quadratic responses (P < 0.05). From d 28 to 35, significant quadratic effects in tibial ash concentration, BMD, and BBS were identified (P < 0.05). Multi-objective optimization analysis demonstrated that SID Ca and P at 6.96 and 5.53 g/kg, respectively, were necessary to optimize tibial ash concentration and BMD in broilers aged 21-28 days. Furthermore, SID Ca and P levels of 6.96 and 6.47 g/kg, respectively, were required to achieve optimal tibial ash concentration, BMD, and BBS in broilers aged 28-35 days. Based on the present results, the required SID Ca:SID P ratios were 1.26 and 1.08 for 21-28-day-old and 28-35-day-old broilers, respectively.