Seasonal variation in serum 25-hydroxyvitamin D concentrations in sheep.

Seasonal changes in serum 25-hydroxyvitamin D were followed in pregnant and non-pregnant sheep kept indoors or at pasture. In housed sheep circulating levels depended on starting value, dietary intake and requirements. thus, in wethers given hay values declined steadily; in non-pregnant ewes on the standard diet values were constant; and in pregnant ewes given hay and nuts values were minimal at lambing. In sheep at pasture seasonal changes were aggravated by those changes due to pregnancy so that the lowest values were always observed at or near parturition.