Abstract This study aimed to evaluate the effects of supplementing two vitamin D sources on reproductive performance, egg quality, and plasma biochemical indices of aged duck breeders, and to estimate vitamin D requirements and relative bioavailability from these two sources. A total of 180 laying Pekin ducks (64 wk of age) were randomly assigned to 9 different treatments, each containing 10 replicates with 2 birds per replicate. The birds were fed a basal diet with no vitamin D supplementation or supplemented with cholecalciferol (VD3) or 25-hydroxycholecalciferol (25-OH-D3) at 250, 500, 1000 or 2000 IU/kg of feed for 15 wk. The two-way ANOVA (2 × 4 factors, without a control group) and one-way ANOVA were employed to compare the differences between 25-OH-D3 and VD3. In comparison to ducks fed the basal diet, the egg production (9 to 15 wks), ovarian weight, and the number and weight of dominant follicles increased with increasing VD3 or 25-OH-D3 levels (linear and quadratic, P 0.05). The egg Haugh unit exhibited a linear increase with rising VD3 levels (P = 0.021) and a quadratic increase with rising 25-OH-D3 levels (P = 0.042). Additionally, the plasma calcium content increased linearly as dietary 25-OH-D3 levels increase (P = 0.001). Furthermore, diets supplemented with 25-OH-D3 resulted in higher plasma 25-OH-D3 concentration compared to those fed VD3 (P 0.001). According to the quadratic broken-line model, the VD3 requirements of duck breeders in terms of enhancing egg production, ovarian weight, and dominant follicle number were 906, 359, and 730 IU/kg, respectively, whereas the 25-OH-D3 requirements were 260, 324, and 308 IU/kg, respectively. Based on slope ratio comparison from multiple linear regressions of ovarian weight, dominant follicle number, and plasma 25-OH-D3 concentration, the bioavailability of 25-OH-D3 were 147%, 191%, and 211%, respectively, relative to VD3.
Li et al. (2026) studied this question.