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1 Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201
2 Department of Animal, Dairy, and Veterinary Science, Utah State University, Logan 84322-4815
The objective of these experiments was to determine the effect on fertility of GnRH when used in conjunction with one or two injections of PGF2
. In Experiment 1, GnRH was administered 7 d before the second of two injections of PGF2
(14 d apart). The control group received two injections of PGF2
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 PGF2
. Thirty to 32 h after PGF2
, 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, PGF2
, and GnRH). Controls (n = 85) were given PGF2
and inseminated at estrus. Although conception rate was not different, one fixed-time insemination after the GnRH, PGF2
, 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 PGF2
reduced fertility in Experiment 1. However, in Experiment 2, when GnRH was given 7 d before one injection of PGF2
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.
Key Words: gonadotropin-releasing hormone conception rates progesterone prostaglandin F2
Submitted on June 26, 1995
Accepted on December 7, 1995
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