Recent concerns for increased mortality of young pigs attributed to hypovitaminosis D have focused attention on response criteria used to assess vitamin D requirements. The objective of the current experiment was to determine the dose-response relationships between dietary vitamin D₃ and skeleton mineral accumulation and serum 25-OH D₃ in growing pigs. All pigs were fed diets with 95% of the recommended Ca and P, but no supplemental vitamin D for 1 wk post-weaning. At 1 wk after weaning (~25 d), 84 crossbred pigs (n = 4 pigs/pen, 3 pens/diet) were randomly assigned to 1 of 7 diets formulated to supply 95% of the recommended Ca and P concentrations with either 0, 50, 100, 200, 400, 800 or 1,600 IU D₃/kg. Sows that produced the litters were fed the UW Swine Research and Teaching Center standard gestation and lactation diets, both formulated to supply 325 IU vitamin D₃/kg. A segmented 2-phase linear model was applied to regress variables to vitamin D intake. Growth (ADG, kg/d), whole body bone mineral content (BMC, g) and bone mineral density (BMD, g/cm²) via dual energy x-ray absorpitometry (DXA) scans, femur mechanical properties (bending force, yBM, kg-cm; stress, kg/cm²) responses at the end of the 28-d trial (n = 6 pigs/diet, 2 pigs/pen) peaked at diet concentrations between 50 to 100 IU vitamin D₃ (P < 0.05), but serum 25-OH D₃ concentrations continued to increase as concentrations of dietary vitamin D₃ increased to 1,600 IU Intersections of the 2 regression slopes were 81, 84, and 77 IU vitamin D₃/d for ADG, BMD, and yBM respectively. Slopes for the upper range were not significantly different. No plateau was detected in serum 25-OH D₃ concentrations. Although limited by the number of groups in the lower range, diets with 100 IU vitamin D₃/kg appear adequate to meet requirements of young pigs for growth and skeletal traits. No benefit was attributed to the increased serum 25-OH D₃ concentrations. Table 1. Effects of supplemental vitamin D₃ on performance

Trait	Vitamin D3 (IU/kg diet)							SEM
0	50	100	200	400	800	1,600
ADG, kg/d	0.338	0.385	0.452	0.472	0.485	0.434	0.486	0.024
BMD, g/cm2	0.289	0.327	0.371	0.380	0.397	0.366	0.370	0.014
yBM, kg-cm	278	318	463	434	473	451	405	50
25-OH D, ng/mL	2.1	1.9	2.1	5.3	9.5	13.4	18.4	1.0