Strategies to optimize reproductive efficiency by regulation of ovarian function in yak (Poephagus grunniens L.)

Ovulatory response to the first GnRH of Ovsynch is the critical determinant for successful synchronization of ovulation in animals. An attempt was made in this study to design a pre-Ovsynch hormonal strategy in yaks to increase the ovulatory response to the first GnRH injection of Ovsynch so that overall synchronization rate to Ovsynch could be improved. Non-lactating cyclic yak cows (n = 33) were assigned to receive either no treatment before Ovsynch (control) or 0.375 mg of PGF2α (PreP) followed 2 d later by 10 μg of GnRH (PreG), administered 4 (G4G), 5 (G5G), or 6 (G6G) d before initiating the Ovsynch protocol. Rectal palpation was performed to assess ovulation and blood samples were collected to measure progesterone concentrations during pre-treatment, treatment and post-treatment periods. All the animals received timed AI 12 and 24 h after the final GnRH of Ovsynch. Diagnoses for pregnancy were performed by rectal palpation and profiles of plasma progesterone 35 d after AI. Percentage of yak cows that ovulated in response to the first GnRH injection of Ovsynch, synchronized to Ovsynch treatment, had a functional CL at PGF2α of Ovsynch, had circulating concentrations of P4 at PGF2α of Ovsynch and likely to be pregnant after 35 d after AI, were greater in G6G and G5G compared with control, whereas G4G did not differ from controls. In addition, animals that ovulated in response to first GnRH of Ovsynch had greater response to PGF2α of Ovsynch and greater synchronization rate to the overall protocol than those that did not ovulate. In summary, PGF2α-and-GnRH-based pre-Ovsynch strategies consisting of 5 or 6-d interval between PreG and first GnRH of Ovsynch resulted in a greater ovulatory and luteolytic response to first GnRH andPGF2α of Ovsynch, respectively, compared with control animals. These, in turn, optimized synchronization rate to Ovsynch in yaks.