Evaluation of Relative Bioavailability of 25-Hydroxycholecalciferol to Cholecalciferol for Broiler Chickens

This study was conducted to evaluate the relative bioavailability (RBV) of 25-hydroxycholecalciferol (25-OH-$D_3$) to cholecalciferol (vitamin $D_3$) in 1- to 21-d-old broiler chickens fed with calcium (Ca)- and phosphorus (P)-deficient diets. On the day of hatch, 450 female Ross 308 broiler chickens were assigned to nine treatments, with five replicates of ten birds each. The basal diet contained 0.50% Ca and 0.25% non-phytate phosphorus (NPP) and was not supplemented with vitamin D. Vitamin $D_3$ was fed at 0, 2.5, 5.0, 10.0, and $20.0{\mu}g/kg$, and 25-OH-$D_3$ was fed at 1.25, 2.5, 5.0, and $10.0{\mu}g/kg$. The RBV of 25-OH-$D_3$ was determined using vitamin $D_3$ as the standard source by the slope ratio method. Vitamin $D_3$ and 25-OH-$D_3$ intake was used as the independent variable for regression analysis. The linear relationships between the level of vitamin $D_3$ or 25-OH-$D_3$ and body weight gain (BWG) and the weight, length, ash weight, and the percentage of ash, Ca, and P in femur, tibia, and metatarsus of broiler chickens were observed. Using BWG as the criterion, the RBV value of 25-OH-$D_3$ to vitamin $D_3$ was 1.85. Using the mineralization of the femur, tibia, and metatarsus as criteria, the RBV of 25-OH-$D_3$ to vitamin $D_3$ ranged from 1.82 to 2.45, 1.86 to 2.52, and 1.65 to 2.05, respectively. These data indicate that 25-OH-$D_3$ is approximately 2.03 times as active as vitamin $D_3$ in promoting growth performance and bone mineralization in broiler chicken diets.