Effect of 25-OH-cholecalciferol on mineral metabolism of Holstein cows during the transition period.

High-yielding dairy cows around parturition are challenged by varying degrees of low calcium status (hypocalcemia), which is associated with increased risk of several metabolic disorders and diseases in early lactation. The aim of the present study was to investigate effects of dietary 25-hydroxycholecalciferol (25-OH-D3; calcidiol) supplementation during the transition period. Forty-two multiparous Holstein cows were fed 1 of 2 treatments: either cholecalciferol (D3) or 25-OH-D3 (HyD) as dietary vitamin D supplements. From 21 d before the expected calving until parturition, the D3 group received 0.475 mg/d (0.035 mg/kg of DM) of D3, and the HyD group received 1.45 mg/d (0.108 mg/kg of DM) of 25-OH-D3. From parturition until 21 DIM, the D3 group received 0.625 mg/d (0.024 mg/kg of DM) of D3, and the HyD group received 0.625 mg/d (0.024 mg/kg of DM) of D3 plus 1.0 mg/d of 25-OH-D3 (0.056 mg/kg of DM). Arterial blood samples were collected by puncture of a. auricularis on d -21, -7, -3, 0 (calving day), and +1 relative to calving for analysis of blood ionized Ca. Blood samples were taken from the tail on d -21 (22 ± 5 d), -7 (6 ± 2 d), -3 (2 ± 1 d), 0 (d 0; 13 ± 8 h after calving), 1 (36 ± 11 h after calving), 10 (9 ± 2 d), and 20 (16 ± 2) relative to calving to analyze plasma 25-OH-D3 level, plasma minerals, and other plasma metabolites. During the entire experiment, cows were fed TMR, and DMI, productive performance, metabolic disorders, and other diseases were recorded. Milk yield, milk protein percentage, milk lactose percentage, BCS, and BW were not affected by treatment. There was an interaction between treatment and time for DMI. The DMI in HyD was numerically lower compared with D3 at d -3, -2, and -1 relative to calving; however, the DMI in HyD was numerically higher compared with D3 at d 0, 1, and 2 postpartum. Plasma 25-OH-D3 was higher in HyD compared with D3 both pre- and postpartum; however, plasma 1,25(OH)2D3 was not affected by treatment. Arterial blood ionized Ca was affected by treatment, and it was higher in the HyD (1.08 ± 0.02) compared with the D3 on the day of calving (0.99 ± 0.02 mmoL/L; lsmeans ± SEM). There was a tendency for lower incidence of mastitis in HyD compared with D3. Plasma BHB level (lsmeans ± SEM) was significantly lower in HyD (0.61 ± 0.02) compared with D3 (0.70 ± 0.02 mM). In conclusion, feeding 25-OH-D3 to transition dairy cows increased arterial blood ionized Ca on the day of calving compared with cows supplemented with D3. The increased arterial blood ionized Ca in HyD was apparently mediated by 25-OH-D3 without affecting plasma 1,25-(OH)2D3. Additionally, the results showed a lower incidence of mastitis in HyD.