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.