Altering conception of dairy cattle by gonadotropin-releasing hormone preceding luteolysis induced by prostaglandin F2 alpha.

The objective of these experiments was to determine the effect on fertility of GnRH when used in conjunction with one or two injections of PGF2alpha. In Experiment 1, GnRH was administered 7 d before the second of two injections of PGF2alpha (14 d apart). The control group received two injections of PGF2alpha 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 PGF2alpha. Thirty to 32 h after PGF2alpha, 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, PGF2alpha, and GnRH). Controls (n = 85) were given PGF2alpha and inseminated at estrus. Although conception rate was not different, one fixed-time insemination after the GnRH, PGF2alpha, 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 PGF2alpha reduced fertility in Experiment 1. However, in Experiment 2, when GnRH was given 7 d before one injection of PGF2alpha 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.