Metabolic and production responses to calcidiol treatment in mid-lactation dairy cows

The study of vitamin D in cattle has often focussed on its role in calcium and mineral metabolism. However, there is evidence of a wider role for vitamin D in bone and energy metabolism. Two studies were conducted to explore relationships between calcidiol supplementation, blood minerals and metabolites in mid-lactation dairy cows. In Experiment 1, a dose-response study was conducted in which 25 mid-lactation cows were fed one of five supplementary calcidiol doses (0, 0.5, 1, 2 or 4 mg calcidiol/day) for 30 days, with blood samples taken every 10 days. Increasing calcidiol dose increased plasma calcidiol (P=0.001), 24,25-(OH)(2)-D-3 (P=0.001) and serum phosphate (P=0.003) in a curvilinear manner, increased and then decreased plasma 25-OH-D-2 (P=0.004) and linearly increased 3-epi 25-OH-D-3 (P=0.001) and milk calcidiol concentrations (P=0.001). Calcidiol supplementation did not affect milk yield or composition, bodyweight or condition score. In Experiment 2, relationships between blood calcidiol and mineral and metabolite concentrations over time were explored using time-series analysis. Ten mid-lactation cows were fed either 0 or 0.5 mg calcidiol/day for 27 days, with blood samples taken every 3 days. Feeding calcidiol increased plasma calcidiol (P=0.001), 24,25-OH-D-3 (P=0.038)(,) and insulin (P=0.046), but decreased 25-OH-D-2 (P=0.008) concentrations. Positive associations were identified between blood calcidiol and concentrations of other metabolites, including cholecalciferol, calcium, osteocalcin, glucose, insulin, non-esterified fatty acids, -hydroxybutyrate, cholesterol, magnesium, phosphorus and total protein at varying lags (+/- 0, 3 or 6 days), while negative relationships were identified between calcidiol and 24,25-(OH)(2)-D-3,D- and phosphorus 3 days later. Importantly, strong positive associations between calcidiol and indicators of energy metabolism were identified. Overall, these experiments provide support for a positive effect of calcidiol treatment on dairy cow metabolism