Abstract
This study aimed to evaluate the effects of cholecalciferol (vitamin D(3), VD(3)) or 25-hydroxyvitamin D(3) (25(OH)D(3)) supplementation in the diet of aged laying ducks on eggshell and bone quality. A total of 792 healthy Longyan laying ducks (60 weeks old) were randomly divided into 11 groups, each with 6 replicates of 12 birds. The 11 groups were fed for 16 weeks with diets containing either 0 (control), or varying levels of VD(3) or 25(OH)D(3): 800, 1600, 2400, 3200, and 4000 IU/kg, respectively. Compared to the control, supplementation with either VD(3) or 25(OH)D(3) increased (P < 0.05) the eggshell thickness, weight, and ratio throughout the trial period. Improvements were also observed in the eggshell ultrastructure, including increases in total thickness, effective thickness, and mammillary knob width (P < 0.05), as well as reductions in the scores of structures in the mammillary layer with VD(3) or 25(OH)D(3) supplementation (P < 0.05). The calcium (Ca) content in the eggshell increased, and the mammillary layer proportion and mammillae density of the eggshell decreased with 25(OH)D(3) (P < 0.05) but not VD(3) supplementation. Dietary supplementation with both VD(3) and 25(OH)D(3) increased (P < 0.05) the phosphorus (P) and 1,25-dihydroxyvitamin D(3) contents in the plasma, but the plasma Ca content only increased (P = 0.025) with 25(OH)D(3) supplementation. In addition, dietary 25(OH)D(3) supplementation decreased (P = 0.012) calcitonin levels, and increased (P < 0.05) procollagen type I C-terminal propeptide and bone Gla protein (osteocalcin) levels compared to VD(3) supplementation. Dietary VD(3) or 25(OH)D(3) supplementation increased (P < 0.05) the tibial Ca content and strength, as well as the femoral mineral content, Ca content, and strength. The potential for Ca transport and bone formation was improved by both supplements, as indicated by increased (P < 0.05) expression of essential genes in the uterus, duodenum, tibia, and femur, including vitamin D receptor, calbindin-D28k, plasma membrane calcium pump 1b, alkaline phosphatase, bone morphogenetic protein 2, and runt-related transcription factor 2. Beta-galactosidase staining revealed a significant reduction (P < 0.05) in beta-gal-positive cells in the femurs of aged ducks supplemented with VD(3), with an even more pronounced effect with 25(OH)D(3) supplementation. In conclusion, dietary VD(3) and 25(OH)D(3) supplementation improved the eggshell and bone quality of laying ducks aged 60 to 76 weeks. Supplementation of 25(OH)D(3) was more effective than VD(3) in promoting bone formation and quality, but they had similar effects on improving eggshell quality.