To evaluate the effect of administration of insulin and/or dexamethasone during early lactation on reproductive performance of dairy cows in a randomized controlled clinical trial, two hundred Holstein cows in a commercial dairy farm were enrolled in this study. The cows were randomly assigned to receive 1 of 4 treatments at day 3 or 10 of lactation (half of the cows received treatments at day 3 postpartum and the second half at day 10 postpartum): 1. i.m. injection of 20 mg of dexamethasone and 100 units of insulin , 2. s.c. injection of 100 units of insulin, 3. i.m. injection of 20 mg of dexamethasone , and 4. 10-mL i.m. injection of sterile water (control). To assess reproductive performance, the following outcomes were measured: interval from calving to first insemination, calving to conception interval, first service pregnancy risk (%), pregnancy rate at 120 days in milk , and cumulative pregnancy risk (%) till 9 month. There was no statistically significant effect of time of interaction (day 3 or 10 of lactation) on the outcomes. Survival analysis was showed no difference among groups in median days to first service. Dexamethasone treated cows had significantly lower days to pregnancy than controls. In addition, dexamethasone treated cows had a significantly greater pregnancy rate at 120 days in milk than other treatment groups and controls. The odds of the development of pregnancy at 120 days were 2 times greater in cows that received dexamethasone in comparison to controls (P=0.02). This study suggests that a single dose of dexamethasone injection in early lactation, might improve reproductive performance.

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