Relative bioavailability of 25-hydroxycholecalciferol vs. cholecalciferol and vitamin D3 requirements in White Pekin ducks from 15 to 42 d of age

This study was designed to assess the effects of varying levels of vitamin D3 (VD3) or 25-hydroxycholecalciferol (25-OH-D3) supplementary in the diet on growth performance, VD3 status, plasma calcium (Ca) levels, and tibial mineralization in growing male White Pekin ducks, and to determine the dietary requirements and relative bioavailability (RBA) of these 2 compounds. The overall total of 728 male White Pekin ducks (15 d old) were allocated to 13 experimental groups, with 6 different VD3 supplementation levels (100, 200, 400, 800, 1,200, and 2,000 IU/kg), and 2 vitamin D3 sources (VD3 and 25-OH-D3), alongside a negative control group without vitamin D3 supplementation. Data were analyzed by 2-way ANOVA (2 × 6 factorial, excluding control) and 1-way ANOVA for cholecalciferol vs 25-hydroxycholecalciferol comparisons. Polynomial contrasts evaluated dose responses, with paired t-tests comparing sources at each level. Vitamin D3 requirements were estimated using quadratic broken-line model. RBA values (25-OH-D3 vs cholecalciferol) were determined via slope-ratio regression (using feed-adjusted vitamin D3 intake as independent variable). Ducks fed the basal diet had significantly lower body weight, average daily weight gain (ADG), plasma 25-OH-D3, calcium levels, and tibial mineralization (P < 0.05); however, all these parameters improved linearly or quadratically (P < 0.05) with increased VD3 or 25-OH-D3 levels in the diet. Plasma Ca, tibial weight, strength, density, mineral, and ash were higher (P < 0.05) in the 25-OH-D3 group in contrast with the VD3 group at 100 IU/kg, with higher plasma 25-OH-D3 concentrations when given 400, 800, 1,200, or 2,000 IU/kg (P < 0.05). Therefore, 25-OH-D3 was more efficient than VD3 in promoting Ca absorption and tibial mineralization. The optimal VD3 and 25-OH-D3 requirements of growing male Pekin ducks for growth and tibial development were 698 IU/kg and 352 IU/kg, respectively. The RBA of 25-OH-D3 compared to VD3 was 188%, based on slope-ratio comparisons from multiple linear regressions of plasma 25-OH-D3 concentration.