The effect of pretreatment with different doses of GnRH to synchronize follicular wave on superstimulation of follicular growth in dairy cattle

The objective of this study was to evaluate the efficiency of gonadotropin releasing hormone (GnRH) and GnRH doses in synchronizing follicular wave emergence as a pretreatment for superovulation in cattle. Fourteen Holstein-Friesian cows 6 days from estrus were randomly assigned to receive 100 mug (n=4), 50 mug (n=5), or 25 mug (n=5) of GnRH. Superovulation was induced with injections of porcine FSH (pFSH) twice daily, decreasing the dose (total 42 AU) over 5 days beginning 2.5 days after receiving GnRH. On the 7th and 8th injections of pFSH, 750 mug of PGF2alpha was also given. With the exception of one cow that was given 50 mug of GnRH, ovulation was induced in all cows from the three groups and the new follicular wave emergence was observed. The total number of follicles for the 25 mug GnRH group was less than that observed for the 100 mug GnRH group (P0.05), although there were no differences between the 100 mug, 50 mug and 25 mug GnRH groups with respect to the number of preovulatory follicles (- 10 mm) and CL. The numbers of normal embryos were greater for the 25 mug GnRH group than the 100 or 50 mug GnRH groups (P0.01); however, the numbers of ova/ embryos did not differ significantly between the three groups. These results suggest that 25 mug of GnRH was sufficient to induce ovulation and follicular wave emergence. On day 6 of the estrous cycle, a reduction of the dose of GnRH to synchronize follicular wave emergence as a pretreatment for superstimulation promotes transferable embryos.