The time course for accumulation of bone mass was established in 49 pigs (~28 d of age) fed diets with either low (LCaP; 75% Ca; 95% P) or high (HCaP; 150% Ca; 120% P) Ca and P (expressed as a percentage of requirements) each formulated without (-D; 0 IU D/kg) or with (+D; 280 IU D/kg) supplemental vitamin D₃. Bone mass was measured by dual-energy x-ray absorpitometry (DXA). Accumulation of skeletal mass was calculated from DXA scans at weekly intervals for the first 3-wk period, then at 2-wk intervals for the next 4 wk. Animals were fed a recovery diet (+D HCaP) during the last 4 wk. Growth and skeletal mass traits were analyzed as randomized block design for each weekly interval with inferences about diet differences based on orthogonal contrasts for main effects of CaP or vitamin D and the CaP X D interaction. Differences (P < 0.01) in skeletal mass due to dietary Ca and P were detected after 1 wk, but differences (P < 0.01) due to vitamin D effects were not detected until 2 wk. However, during the recovery phase the initial dietary vitamin D treatments affected recovery responses, not dietary Ca and P. Pigs fed diets without vitamin D failed (P < 0.01) to regain skeletal mass over the 4-wk recovery period, but pigs previously fed diets supplemented with vitamin D recovered skeletal mass equal to that of control pigs. In conclusion, DXA provides an effective method to detect differences in bone mineral content gain (gBMC, g/d). Young pigs responded to dietary depletions of Ca and P earlier than to vitamin D, but recovery from a vitamin D depletion was not detected after a 4-wk period. Table 1. Whole-body bone mineral content gain, g/d

Days	−D LCaP	−D HCaP	+D LCaP	+D HCaP	SEM
0 to 8a	0.72	2.43	1.03	2.74	0.33
0 to 16abc	1.14	2.84	3.01	6.20	0.30
0 to 24abc	1.15	2.58	5.22	8.48	0.30
0 to 55b	11.98	12.39	17.92	18.22	0.66

a = CaP effect, P < 0.01; b = D effect, P < 0.01; c = CaP × D effect, P < 0.02.