Altering Conception of Dairy Cattle by Gonadotropin-Releasing Hormone Preceding Luteolysis Induced by Prostaglandin F_2α

The objective of these experiments was to determine the effect on fertility of GnRH when used in conjunction with one or two injections of PGF_2α. In Experiment 1, GnRH was administered 7 d before the second of two injections of PGF_2α (14 d apart). The control group received two injections of PGF_2α without GnRH. Conception was reduced from 63.5% for 74 controls to 48.7% for the 79 heifers and cows that had been treated with GnRH, but estrus detection and pregnancy rates were similar. In Experiment 2, 85 heifers and cows received GnRH at a random stage of the estrous cycle, followed in 7 d by PGF_2α. Thirty to 32 h after PGF_2α, a second dose of GnRH was given to induce ovulation of the preovulatory follicle, followed by one fixed-time insemination 18 to 19 h later (treatment designated as GnRH, PGF_2α, and GnRH). Controls (n =85) were given PGF_2α and inseminated at estrus. Although conception rate was not different, one fixed-time insemination after the GnRH, PGF_2α, and GnRH treatment tended (35.3%) to reduce fertility compared with effects of the control (47.1%). It is unclear how an injection of GnRH during the intervening week between two injections of PGF_2α reduced fertility in Experiment 1. However, in Experiment 2, when GnRH was given 7 d before one injection of PGF_2α and when ovulation was induced with a second GnRH injection, one fixed-time insemination seemed to produce acceptable fertility in dairy cows but probably less than that when inseminations were based on detected estrus.