Access to outside areas during March and April in Denmark has negligible effect on the vitamin D(3) status of organic dairy cows

Use of artificial fat-soluble vitamins in feed for dairy cows does not conform to organic production principles. Therefore sunlight is the only natural source of vitamin D(3) for organic dairy cows. In order to establish the effect of sunlight on the vitamin D(3) status of high yielding dairy cows in spring when sunlight is scarce, 78 Danish Holstein cows from two different organic farms (farm 1 and farm 2) were blood sampled four times between November 2005 and April 2006. Use of artificial vitamin D(3) supplements in feed was terminated prior to first blood sampling and the cows in each herd were divided into two groups: one with access to outside areas during March and April (outdoor group), and one kept in confinement during the same period (indoor group). Blood samples were analysed for the major circulating metabolite of vitamin D(3), 25-hydroxyvitamin D(3) (25(OH)D(3)), as an indicator for the vitamin D(3) status of the cows. The effects of treatment and sampling time in March and April were analysed together with the interaction between treatment and sampling time. Results from this survey showed that sunlight in March and April in Denmark can not efficiently stimulate endogenous synthesis of vitamin D(3) in dairy cows.