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Abstract
Three experiments were conducted to evaluate the effects of development of a persistent ovarian follicle and associated elevation in 17$\beta$-estradiol on: (1) fertility at the subsequent estrus if artificial insemination or embryo transfer was performed; and (2) recruitment of additional follicles during superstimulation of follicular development with FSH.
The objective of first experiment was to determine if development of a persistent ovarian follicle and associated increase in 17$\beta$-estradiol during estrous synchronization decreases conception rates after artificial insemination. Development of a persistent ovarian follicle resulted in elevated circulation concentrations of 17$\beta$-estradiol for 6-10 days compared to 1-2 days preceding behavioral estrus which is typically observed. Interval from simulated luteal regression until observation of estrus was decreased by development of a persistent ovarian follicle. Pregnancy rate was decreased when a persistent follicle is ovulated and cows were inseminated. In addition, pregnancy rate was negatively correlated (r = $-$0.39) to circulating concentrations of 17$\beta$-estradiol preceding estrus.
The objective of the second experiment was to determine if elevated 17$\beta$-estradiol preceding ovulation associated with a persistent ovarian follicle alters pregnancy rates after transfer of an embryo on day 7 of the estrous cycle. Development of a persistent follicle during estrous synchronization resulted in elevated 17$\beta$-estradiol preceding ovulation similar to experiment 1. However, elevated 17$\beta$-estradiol did not alter pregnancy rate when an embryo was transferred on day 7 of the estrous cycle.
The objective of the third experiment was to determine if development of a persistent ovarian follicle suppresses recruitment of additional follicles during FSH treatment. Development of a persistent ovarian follicle for 5 days but not 2 or 8 days preceding initiation of FSH stimulation was able to suppress recruitment of subordinate follicles and reduce the number of corpora lutea, total ova and transferable embryos observed on day 7 after estrus.
Results from these studies indicate that pregnancy rates could be improved if estrous synchronization programs more precisely controlled timing of behavioral estrus and follicular development.
