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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-D3 (P = 0.001) and serum phosphate (P = 0.003) in a curvilinear manner, increased and then decreased plasma 25-OH-D2 (P = 0.004) and linearly increased 3-epi 25-OH-D3 (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-D3 (P = 0.038), and insulin (P = 0.046), but decreased 25-OH-D2 (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, β-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-D3, 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.

Long-term, declining fertility is a reported trend in high-production dairy herds. Nutrition management is identified as a key area where improvement is required. Most published literature relating to nutrition management of dairy cow fertility examines energy and protein status and peri-parturient calcium (Ca) homeostasis availability. This paper examines the current understanding of Ca homeostasis relevant to improving performance in the high-production dairy herd. In particular, this article reviews Ca and vitamin D nutrition during lactation, for achieving health and productivity improvements in subsequent lactations, with specific reference to pasture-based dairy herds. Vitamin D metabolites are essential for increasing the proportion of absorbed Ca when dietary Ca concentration is low or when the requirement for Ca is high. Vitamin D metabolites stimulate the synthesis of the proteins that control active intestinal Ca absorption. This has particular relevance to cows that may not be able to replenish Ca reserves before parturition, which may be a common occurrence in current pasture-based dairy systems. It is possible that due to reductions in Ca reserves, the effectiveness of the cow to liberate sufficient Ca into plasma at parturition will be compromised as she faces subsequent lactations. This has significant implications for the ability of the cow to progress through transition in a healthy state, maintain an optimum energy balance during early lactation and subsequently conceive. Suboptimal nutritional management of Ca status through successive lactations may be shortening the productive life of the high-producing dairy cow.

Phosphorus (P) is recommended as a supplement for beef cattle grazing rangelands and for dairy cows grazing pastures. However, existing supplementation mechanisms are wasteful, labour intensive, and are becoming increasingly expensive. Cattle have an inability to absorb or retain >70% of the P they consume. Therefore, the need for supplementation could be reduced if the utilisation of dietary P was increased. Vitamin D and its metabolites are involved in the absorption and metabolism of calcium (Ca) and P. However, the use of vitamin D for improving utilisation of P is novel. This review presents information gathered from various studies that demonstrate the potential for vitamin D metabolites to increase P utilisation. The concentration of dietary Ca is demonstrated to be the controlling element for utilisation of endogenous sources of vitamin D. Therefore, the use of exogenous sources of vitamin D in greater than normal concentration is required to overcome the suppression of active absorption mechanisms by normal calcaemia. Use of 25-hydroxyvitamin D in sufficient doses has been shown to increase active absorption of both Ca and P and therefore potentially provide a method of increasing utilisation of existing sources of P or increase the utilisation of P supplements for cattle.