Supplementing dairy steers and organically managed dairy cows with synthetic vitamin D3 is unnecessary at pasture during exposure to summer sunlight

Use of synthetic feed additives, including synthetic vitamin D-3 (D-3) in the feed for cows and other ruminants, is not consistent with the international principles of organic farming. If dairy farmers wish to produce in accordance with the organic principles, production animals would be left with only their enclogenous production of D-3 from summer sunlight as a source of D-3. To examine the impact of supplemental synthetic D-3 from the feed on the D-3 status of dairy cattle in organic production in Nordic countries, 20 high-yielding dairy cows and 30 dairy steers were divided into two groups: one supplemented with synthetic D-3 in the feed and one not supplemented with synthetic D-3. Vitamin D-3 status of the animals was assessed by measuring the concentration of the liver-derived 25-hydroxyvitamin D-3 (25OHD(3)) in plasma. Results showed that 25OHD(3) concentration in plasma from dairy cattle as well as from steers decreased during winter for both supplemented and unsupplemented groups. Unsupplemented cows and steers had approximately 2 ng 25OHD(3) per ml plasma during winter, whereas supplemented animals had between 10 (cows) and 30 (steers) ng/ml. During summer and autumn there was no additive effect of supplementing with synthetic D-3 since unsupplemented and supplemented animals had the same D-3 status at this time of year. In all cows summer concentrations of 25OHD(3) were 20-25 ng/ml and in all steers 40-50 ng/ml plasma. The decrease in vitamin D-3 status during winter indicates that cows and steers are able to store D-3 only to a limited extent. The results also show that cows or steers fed supplemental D-3 according to Swedish recommendation throughout the year are not able to maintain their summer value of 25OHD(3) during winter.