The Effect of Delaying the Administration of Second GnRH in the Ovsynch Protocol on Pregnancy Rate in Dairy Cows

The aim of this study was to evaluate the effect of delaying the second GnRH injection on pregnancy rates in the Ovsynch protocol (OVS) in cows. In the study, a total of 40 primiparous, Holstein-Friesian dairy cows treated with 10 μg GnRH on day zero and 500 μg PGF2α on day 7 and the cows were then randomly divided into two groups. The 2nd GnRH injection was performed at the 48 hours in the OVS 48 (n = 21) and at the 56 hours in the OVS 56 (n = 19) after the PGF2α injection. Artificial insemination (AI) was performed 16 hours after the protocols. Blood samples were taken at day 7 and at the time of insemination to evaluate progesterone (P4). Pregnancy diagnosis was determined at 40-45th day after the AI by transrectal ultrasonography. No statistically significant differences were determined between groups for the mean serum P4 values on day 7 and at the time of insemination (OVS 48: 9.11 ± 7.60 and 0.89 ± 1.31 ng/ml vs. OVS 56: 6.00 ± 5.98 and 1.10 ± 1.79 ng/ml, respectively) (P 0.05). Pregnancy rates were determined as 66.66 % in group OVS 48 and 47.36 % in group OVS 56. However, no significant difference was observed between the groups (P 0.05). In conclusion, the findings suggest that delaying the second GnRH injection for 8 hours (56th hour instead of 48th hour) in conventional Ovsynch protocol had no effect on pregnancy rate in dairy cows.