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To evaluate the role of vitamin D3 during gestation and lactation of sows, 2 independent experiments were performed with the aim of investigating sow reproductive performance, milk composition (study 1 only), and changes in blood status of 25-hydroxycholecalciferol (25-OH-D3), 1,25-dihydroxycholecalciferol (1,25-(OH)2-D3; study 2 only), minerals, and bone markers of sows during gestation and lactation. Study 1 comprised 39 primi- and multiparous crossbred sows fed 1 of 3 barley meal-based diets fortified with 200 IU/kg vitamin D3 (NRC, 1998; treatment DL), 2,000 IU/kg vitamin D3 (cholecalciferol; treatment DN), or 50 μg 25-OH-D3 (calcidiol; treatment HD)/kg feed. This study was conducted over a 4-parity period under controlled conditions. Study 2, running over 1 parity only, was performed in a commercial farm with 227 primi- and multiparous sows allocated to 2 dietary treatments: control (CON), receiving 2,000 IU vitamin D3/kg (equivalent to 50 μg/kg) feed (114 sows), and test (HYD), supplemented with 50 μg 25-OH-D3/kg feed (113 sows). Blood samples of sows were collected at 84 and 110d postcoitum and 1, 5, and 33 d postpartum (study 1) and at insemination and 28 and 80 d postinsemination as well as d 5 and 28 postpartum (study 2). Colostrum and milk samples in study 1 were obtained at 1, 9, and 33 d of lactation after oxytocin administration. Plasma 25-OH-D3 concentrations were increased (P < 0.05) in sows receiving 25-OH-D3 (HD and HYD) at any time of sampling whereas circulating plasma concentrations of 1,25-(OH)2-D3, Ca, and P were not affected by treatment. Milk concentrations of Ca and P were similar, but 25-OH-D3 content (except in colostrum) was clearly increased (P< 0.05) when 25-OH-D3 was fed. Most characteristics of sow reproductive performance responded similarly to the 2 sources and levels of vitamin D3, but weight gain of piglets between birth and weaning was decreased (P< 0.05) in offspring of DL and HD sows compared with animals of treatment DN (study 1). In study 2 total litter weight and birth weight per piglet were increased (P< 0.05) with 25-OH-D3 supplementation in comparison with the control (CON). Overall, feeding sows with 25-OH-D3 was considered to improve maternal supply with vitamin D3 and thereby maintain Ca homeostasis during gestation and lactation.