How hair-coated animals such as dairy cows synthesize endogenous vitamin D(3) during exposure to summer sunlight has been unclear since vitamin D(3) and its relation to sunlight was discovered. The fur of fur-bearing animals is thought to be comparable to clothing in humans, which prevents vitamin D(3) synthesis in the skin during exposure to sunlight. Different scenarios have been suggested but never tested in cows; for example, that vitamin D(3) is synthesized from sebum on the hair and ingested by cows during grooming or that body areas such as the udder and muzzle that have scant hair exclusively produce the vitamin. To test different scenarios, 16 Danish Holstein dairy cows were subjected to 4 degrees of coverage of their bodies with fabric that prevented vitamin D(3) synthesis in the covered skin areas. The treatments were horse blanket (cows fitted with horse blankets), udder cover (cows fitted with udder covers, horse blanket+udder cover (cows fitted with both horse blankets and udder covers), and natural (cows without any coverage fitted). The cows were let out to pasture daily between 1000 and 1500h for 4 wk in July and August 2009. Blood samples were collected 15 times during the study and analyzed for content of 25-hydroxyvitamin D(3) [25(OH)D(3)] indicative of the animals' vitamin D(3) status. Results showed that uncovered cows had a higher 25(OH)D(3) concentration in plasma after 28 d of access to sunlight compared with covered cows and that the plasma concentration of 25(OH)D(3) was strongly inversely correlated to the body surface area covered. These results are consistent with findings in humans, wherein the vitamin D(3) status of different individuals was inversely proportional to the amount of clothing worn during exposure to artificial sunlight. Hence, it appears that human clothing and cow hair are not comparable with respect to prevention of vitamin D(3) synthesis and that cows, like humans, synthesize vitamin D(3) evenly over their body surface. That vitamin D(3) should be synthesized from sebum on the hair and obtained by cows as a result of grooming is not supported by the findings in the present study either, because large differences were found between the treatment groups. If grooming were the source of vitamin D(3), then a relatively even 25(OH)D(3) concentration between treatments would be expected, because covered cows would obtain vitamin D(3) by grooming uncovered herdmates.

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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 D3 from summer sunlight as a source of D3. To examine the impact of supplemental synthetic D3 from the feed on the D3 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 D3 in the feed and one not supplemented with synthetic D3. Vitamin D3 status of the animals was assessed by measuring the concentration of the liver-derived 25-hydroxyvitamin D3 (25OHD3) in plasma. Results showed that 25OHD3 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 25OHD3 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 D3 since unsupplemented and supplemented animals had the same D3 status at this time of year. In all cows summer concentrations of 25OHD3 were 20-25 ng/ml and in all steers 40-50 ng/ml plasma. The decrease in vitamin D3 status during winter indicates that cows and steers are able to store D3 only to a limited extent. The results also show that cows or steers fed supplemental D3 according to Swedish recommendation throughout the year are not able to maintain their summer value of 25OHD3 during winter.

The number of dairy cows without access to pasture or sunlight is increasing; therefore, the content of vitamin D in dairy products is decreasing. Ultimately, declining vitamin D levels in dairy products will mean that dairy products are a negligible source of natural vitamin D for humans. We tested the ability of a specially designed UVB lamp to enhance the vitamin D3 content in milk from dairy cows housed indoors. This study included 16 cows divided into 4 groups. Each group was exposed daily to artificial UVB light simulating 1, 2, 3, or 4 h of summer sun at 56°N for 24 d, and the group with simulated exposure to 2 h of summer sun daily continued to be monitored for 73 d. We found a significant increase in 25-hydroxyvitamin D3 (25OHD3) levels in plasma as well as vitamin D3 and 25OHD3 levels in milk after daily exposure for 24 d in all treatment groups. Extending daily exposure to artificial UVB light to 73 d did not lead to an increase of vitamin D3 or 25OHD3 level in the milk. In conclusion, the change in production facilities for dairy cows providing cows with no access to pasture and sunlight causes a decrease of vitamin D levels in dairy products. This decrease may be prevented by exposing cows to artificial UVB light in the stable.