Two experiments were conducted to measure conception and pregnancy rates in lactating dairy Follicular development must be controlled cows after various treatments followed by artifiand synchronized with the regression of the cial insemination (AI) after detected estrus or at corpus luteum after PGF in order to reduce one fixed time. In Experiment 1, Holstein cows variation in the intervals to estrus. Precise conin one herd were assigned randomly to four trol of follicular development with the regression treatments every 3 wk (27, 3-wk cluster groups): of the corpus luteum should allow improved 1) Ovsynch33, 2) GnRH+PGF , 3) 2×PGF , conception rates associated with one fixed-time 2 2 and 4) 2×PGF +GnRH. In Experiment 2, Holinsemination. Such a synchronized ovulation 2 stein cows in one herd were assigned randomly protocol (OvSynch) has been tested. A first to two treatments every 3 wk (14, 3-wk cluster injection of GnRH is administered 7 days before groups): 1) Ovsynch48 and 2) GnRH+PGF . In PGF , and a second injection of GnRH is given 2 both experiments, frozen-thawed semen from 36 to 48 hr after PGF to ovulate the dominant multiple sires was used, one technician perfollicle via GnRH-induced release of luteinizing formed >95% of all inseminations, and preghormone (LH). The objective of these experinancy was diagnosed by palpation per rectum ments was to compare conception and pregnancy between 38 and 52 d after first insemination. rates in lactating dairy cows after various proAlthough actual conception rates resulting from grammed breeding systems used to synchronize inseminations after detected estrus were consisestrus before first postpartum inseminations. tently greater, pregnancy rates of cows were Specifically, we wished to determine the: 1) superior after fixed-time inseminations because effect of incorporating GnRH in a two PGF of poor rates of detected estrus in treatments that injection scheme (administration of GnRH after relied solely on observation of sexual behavior. the second PGF injection), 2) actual pregnancy