Timing of final GnRH of the Ovsynch protocol affects ovulatory follicle size, subsequent luteal function, and fertility in dairy cows

Abstract

Synchronization of ovulation (Ovsynch) is an effective method for controlling time of first and subsequent AI in lactating dairy cows. However, validation of the original Ovsynch program did not include testing the optimal time to deliver the final treatment of GnRH. In Experiment 1, the effect of administering the final dose of GnRH on the same day as prostaglandin F 2α (PGF 2α) administration was tested. Lactating dairy cows ( n=218) were randomly assigned to receive either Ovsynch (OV; cows were given 100 μg GnRH, then 7 days later cows were administered 25 mg PGF 2α followed by a subsequent treatment of 100 μg GnRH 2 days after the PGF 2α) or the modified version of Ovsynch (MOV; cows were given 100 μg GnRH, then 7 days later cows were administered 25 mg PGF 2α followed immediately with 100 μg GnRH). In both treatment groups, AI took place 16 h after the final administration of GnRH. In Experiment 2, cows ( n=457) were randomly divided into four treatment groups that were administered GnRH 0, 12, 24 and 36 h following PGF 2α. The 36 h treatment group served as control. Pregnancy diagnoses were performed by palpation per rectum 36 days post-AI in Experiment 1 and by ultrasonography on Day 28 in Experiment 2. In Experiment 1, pregnancy rate/AI (PR/AI) was greater ( P<0.025) in OV versus MOV. In a subset ( n=85), percentage of cows with both synchronized ovulations and regressed CL following administration of PGF 2α were similar ( P>0.1) between OV and MOV, respectively. All cows that became pregnant in the MOV subset group showed regression of the CL in response to the PGF 2α. Diameter of the ovulatory follicle at the time of final GnRH administration was greater ( P<0.05) in OV versus MOV. In Experiment 2, the synchronization rate was once again similar among treatments ( P>0.28). There was a linear effect of treatment on follicle size ( P<0.05) and PR/AI ( P<0.0001) as time increased between administration of PGF 2α and GnRH, with the greatest PR/AI at 36 h. There was a trend for a greater percentage of cows with short luteal phases in the 0 h group ( P<0.10). In summary, delivering the final treatment of GnRH of the Ovsynch program at the same time as PGF 2α, or in the 24 h following PGF 2α, resulted in lower fertility compared to controls.