Background: Vitamin D plays a key role in metabolism, particularly during the transition period in dairy cows. Various forms of vitamin D are administered orally or via injection to support metabolic processes influenced by this vitamin.
Objectives: To evaluate the effects of vitamin D injections on Ca, P, and Mg metabolism in transitional period of dairy cows.
Methods: Sixty-four multiparous dairy cows, entering their second or greater lactation, were randomly selected and assigned to one of three groups: a control group (CON), two treatment groups that one group receiving a single intramuscular dose of 7.5 × 10⁶ IU cholecalciferol (7.5VitD group) on days 21, and other group receiving two intramuscular doses of 7.5 × 10⁶ IU cholecalciferol (totaling 15 × 10⁶ IU; 15VitD group) on days 21 and 7 before the expected calving date. Blood samples were collected at five time points: 21 and 7 days prepartum, and at 24–48 hours, 7 days, and 21 days postpartum. Samples were analyzed for calcium, phosphorus, and magnesium by commercial kite and chemical autoanalyzer. All date analysed using repeated measures ANOVA.
Results: The relationship between different dosages of cholecalciferol injections with blood calcium and phosphorus, and magnesium levels varied among the groups. Statistical Analysis showed, time, group and the interaction between time and group has an effect (P<0.001), on serum calcium; time has a significant effect (P<0.001), group has no effect (P>0.05), and the interaction between time and group has an effect (P<0.05). on serum phosphorus; and time and group has a significant effect (P<0.001), and the interaction between time and group has an effect (P<0.05) on serum magnesium.
Conclusion: Different dosages of cholecalciferol injection during the dry period had effects on the blood calcium, phosphorus, and magnesium levels of cows during the transition period compared to the time before injection, and there was significant difference was observed between a single injection and two injections in general. Considering the complications of two injections such as stress and cost, it is recommended that if vitamin D injection is needed before parturition, a single injection may be sufficient.