Supplementing dairy steers and organically managed dairy cows with synthetic vitamin D3is unnecessary at pasture during exposure to summer sunlight

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Abstract
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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