261 Relationship between dietary calcium and available phosphorus in growing pigs

Abstract The amount of and relationship between dietary total calcium (tCa) and available phosphorus (avP) are critical for lean and bone growth in pigs. Our objective was to evaluate the relationship between tCa and avP in nursery-grower pigs. Mixed sex pigs n = 117; initial body weight (BW) = 11 ± 0.17 kg were blocked by BW to individual pens and assigned to 1 of 18 dietary treatments (n = 6 to 7 pigs/trt). Treatments were divided into 4 titrations containing 5 treatments each. Each titration had the same tCa:avP of 1.52, 1.77, 2.12, 2.64, and 3.50. In titration 1(T1), tCa was set at NRC recommendation of 0.68% and avP ranged from 0.46 to 0.20%. Titration 2(T2), tCa was set at 14% below NRC recommendation at 0.56% and avP ranged from 0.38 to 0.16%. Titration 3(T3), avP was set at NRC recommendation of 0.33% and tCa ranged from 0.49 to 1.12%. In titration 4(T4), avP was set at 0.26% with tCa ranging from 0.39 to 0.90%. Pig BW and feed disappearance were recorded at the beginning and the end to calculate average daily gain (ADG), average daily feed intake (ADFI) and feed efficiency gain to feed ratio (G:F) performance parameters over the 40-d study duration. Thereafter, whole body, femur, and 3rd metatarsal bone mineral density (BMD) and contents (BMC) were measured using a dual-energy X-ray absorptiometry (DXA). Within each titration, data were analyzed with pig as the experimental unit, random effect of block and the fixed effect of diet. In T1, performance parameters did not differ (P 0.10). However, body BMD decreased (0.843, 0.841, 0.804, 0.706, and 0.585 g/cm2) as tCa:avP increased (P 0.001). In T2, as tCa:avP increased, ADG decreased (0.85, 0.85, 0.77, 0.73, and 0.67 kg/d; P 0.001). Similarly, whole body BMD decreased as tCa:avP increased (0.800, 0.803, 0.668, 0.616, and 0.558 g/cm2; P 0.001). ADG, ADFI and whole and individual BMC did not differ in T3 and T4. Consequentially, body BMD ranged from 0.756 to 0.808 g/cm2 in T3 and 0.656 to 0.703 g/cm2 in T4. Breakpoint analysis reported tCa:avP of 2.321 and 1.849 or less was optimal for BMD in T1 and T2, respectively. No optimal breakpoints could be determined for whole BMD within T3 and T4. Breakpoint analysis for ADG reported tCa:avP of 2.311 and 2.341 or less was optimal in T2 and T3, respectively. No optimal breakpoints could be determined for ADG within T1 and T4.