Abstract/Summary

Milk enriched with vitamin D by supplementing dairy cow diets could provide a valuable dietary source of vitamin D, but information on the feasibility of this approach is limited. In the current study, the effects of supplementing dairy cows with either vitamin D3 or 25(OH) D3 over the transition/early lactation period on plasma and milk vitamin D concentrations were compared. Sixty dairy cows were randomly allocated to one of four dietary treatments from 14 days pre-calving to 56 days post-calving. Treatments were a control diet (Control) for both pre-calving and post-calving periods containing 0.625 mg/day vitamin D3; a pre-calving diet supplemented with 6 mg 25(OH) D3/day, but with a post-calving diet matching that of the control diet (25(OH) D3 pre-calving); the control diet pre-calving but with the post-calving diet supplemented with 2 mg vitamin D3/day (D3max), and the control diet pre-calving but with the post-calving diet supplemented with 1.5 mg 25(OH) D3/day (25(OH) D3 post-calving). No treatment effect on milk yield, composition or 25(OH) D3 concentration was observed. However there was an interaction of treatment and time for plasma 25(OH) D3 concentration; this increased within two weeks of supplementation for the 25(OH) D3 pre-calving treatment (peaking just after calving, 202 ng/ml), whereas that of the 25(OH) D3 post-calving group had a slower response following supplementation, continuing to increase at 56 days. There were correlations between plasma and milk 25(OH) D3 concentrations at days 4 and 14 of lactation, but not at later sampling times. The D3max treatment did not increase 25(OH) D3 concentration in plasma or milk. Overall, results from this study indicate that supplemental 25(OH) D3 is an effective means of enhancing dairy cow plasma 25(OH) D3 concentrations compared with vitamin D3 supplementation, but not necessarily milk concentrations.