Effect of Ovulatory Follicle Size and Standing Estrus on Circulating Hormone Concentrations and Interval to Ovulation

In postpartum cows, ovulatory follicle size at time of insemination (GnRH/TAI) influenced pregnancy rates following TAI, but had no effect on pregnancy rates when cows spontaneously ovulated. Furthermore, cows that exhibited estrus (± 24 h of GnRH/TAI) had higher pregnancy rates compared to cows not in estrus. The objective was to assess the relationship between ovulatory follicle size and estradiol concentrations, timing of the LH surge, timing of ovulation, and subsequent progesterone concentrations. Cows were synchronized with the CO-Synch (n = 64; induced ovulation) or the Select Synch (n = 20; spontaneous ovulation) protocol. Cows that exhibited estrus and were induced to ovulate medium (11.5-14 mm) or large (>14) follicles had preovulatory concentrations of estradiol similar (P > 0.05) to cows that spontaneously ovulated and higher (P < 0.05) than cows not exhibiting estrus. Cows not exhibiting estrus had lower (P < 0.05) preovulatory concentrations of estradiol compared to cows that spontaneously ovulated. There was no effect (P > 0.36) of follicle size or estrus on concentrations of LH. Among cows induced to ovulate, cows detected in estrus had a shorter (P < 0.01) interval from GnRH to the LH surge and ovulation compared to cows not exhibiting estrus. Cows that spontaneously ovulated had an intermediate interval from onset of estrus to the LH surge, but a shorter (P = 0.02) interval to ovulation compared to cows not exhibiting estrus. Cows that ovulated medium follicles had a longer (P = 0.03) interval to ovulation compared to cows that ovulated large follicles, with cows that ovulated small follicles