Effect of vitamin D source and amount on serum 25-hydroxyvitamin D levels in pregnant Holstein cows

Abstract

Vitamin D plays essential roles beyond calcium and bone metabolism, including regulation of immune function, reproduction, and mammary gland physiology. Its two primary forms, cholecalciferol (CHOL) and calcidiol (CAL) differ in origin and bioavailability. While CHOL is synthesized in the skin under ultraviolet B (UVB) exposure and converted to CAL in the liver, modern dairy systems limit sunlight exposure, increasing reliance on dietary supplementation. This study aimed to evaluate the effects of vitamin D source (CHOL vs. CAL) and supplementation amount (0.2 vs. 1.0 mg/day) on serum 25-hydroxyvitamin D 25(OH)D concentrations in pregnant Holstein cows. Twenty-four cows at approximately 63 days of gestation were blocked by parity and performance, assigned to a randomized complete block design with a 2 × 2 factorial arrangement of treatments, and analyzed using repeated measures over time. Treatments included: 0.2 mg CHOL, 1.0 mg CHOL, 0.2 mg CAL, and 1.0 mg CAL. Cows were housed indoors, individually fed a common TMR, and received daily top-dress supplementation for 56 days. Blood samples were collected on days 0, 28, and 56. Serum 25(OH)D concentrations were analyzed using a commercial ELISA kit. Data were analyzed using the MIXED procedure in SAS and least squares means (LSMeans) were used to assess treatment effects. A significant interaction between vitamin D source and amount (P < 0.001) was observed. Cows supplemented with 1.0 mg/day of CAL (HCAL) had the highest serum 25(OH)D₃ concentrations (110.31 ± 4.37 ng/mL), while increasing CHOL from 0.2 to 1.0 mg/day did not affect serum concentrations (76.91 ± 5.06 vs. 76.80 ± 4.74 ng/mL). These findings demonstrate that CAL is more effective than CHOL in raising circulating 25(OH)D concentrations.