Use of synthetic feed additives, including synthetic vitamin D₃(D₃) 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 endogenous production of D₃from summer sunlight as a source of D₃. To examine the impact of supplemental synthetic D₃from the feed on the D₃status 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 D₃in the feed and one not supplemented with synthetic D₃. Vitamin D₃status of the animals was assessed by measuring the concentration of the liver-derived 25-hydroxyvitamin D₃(25OHD₃) in plasma. Results showed that 25OHD₃concentration 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 25OHD₃per 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 D₃since unsupplemented and supplemented animals had the same D₃status at this time of year. In all cows summer concentrations of 25OHD₃were 20–25 ng/ml and in all steers 40–50 ng/ml plasma. The decrease in vitamin D₃status during winter indicates that cows and steers are able to store D₃only to a limited extent. The results also show that cows or steers fed supplemental D₃according to Swedish recommendation throughout the year are not able to maintain their summer value of 25OHD₃during winter.

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