Objectives were to determine the effects of dose and source of vitamin D (vitamin D₃ or 25-hydroxyvitamin D₃ [25OHD₃]) on mineral metabolism and bone mineral density (BMD) in calves. The hypothesis was that dietary 25OHD₃ would enhance Ca accretion and BMD. Forty-five Holstein male calves were enrolled within the first week of age in a randomized complete block design. Calves were assigned to 1 of 5 treatments: T1 (0.25 mg of vitamin D₃/kg BW), T2 (0.25 mg of vitamin D₃ + 1.5 mg of 25OHD₃/kg BW), T3 (0.25 mg of vitamin D₃ + 3.0 mg of 25OHD₃/kg BW), T4 (1.75 mg of vitamin D₃/kg BW) and T5 (3.25 mg of vitamin D₃/kg BW). Treatment were supplied from birth to 131 ± 9 d of age when calves were euthanized and tissues collected. Calves were fed milk replacer until 49 d of age and had ad libitum access to starter grain and water. Blood was sampled on d −15, −14, −13, −12, −11, −9, −7, −5, −3 and −1 relative to slaughter. Intake of DM was evaluated in the last 15 d before slaughter. Total fecal collection and spot urine was sampled on d −4 to −2 relative to slaughter. The BMD was determined in the right metacarpus. Data were analyzed by ANOVA with mixed models using the MIXED procedure of SAS. Contrasts evaluated included the effects of supplementing vitamin D (Sup: T1 vs. T2+T3+T4+T5), source of vitamin D (Source: T2+T3 vs. T4+T5), dose of vitamin D (Dose: T2+T4 vs. T3+T5), or the interaction between Source and Dose (Int: T2 + T5 vs. T3 + T4) (Table 1). Intake of DM or Ca did not differ among treatments. Supplementing vitamin D reduced blood Ca; however, supplemental 25OHD₃ increased blood Ca compared with vitamin D₃. Treatment did not affect digestibility or retention of Ca, but calves fed 25OHD₃ tended to have increased BMD. Table 1 (Abstr. M158).

Item	T1	T2	T3	T4	T5	SE
Intake
DM, kg/d	4.3	4.0	4.0	4.0	4.3	0.25
Ca, g/d	61.9	55.1	58.6	56.7	62.2	4.0
BW gain, kg/d	1.32	1.20	1.19	1.25	1.34	0.12
Blood
tCa, mM*,†	2.868	2.858	2.840	2.788	2.808	0.024
iCa, mM *	1.473	1.454	1.437	1.446	1.429	0.014
Calcitriol, pg/mL	51.4	53.8	55.2	53.4	57.0	3.2
BMD, g/cm2 †,‡	0.986	1.033	1.054	0.967	1.038	0.024
Ca digestibility, %	50.5	53.2	51.6	52.1	52.9	2.5
Ca retention, g/d	31.1	29.0	30.3	29.4	33.1	2.9

*Sup (P < 0.09); †Source (P < 0.10); ‡Dose (P < 0.07).