estrus, ovulatory follicle size, estradiol concentrations, cows
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 (≤ 11 mm) intermediate. The rate at which concentrations of progesterone increased following ovulation was similar (P > 0.30) among cows that spontaneously ovulated, cows detected in estrus and cows not detected in estrus and induced to ovulate medium and large follicles. Concentrations of progesterone were lower in cows not detected in estrus and induced to ovulate small follicle compared to cows not detected in estrus and induced to ovulate medium or large follicles (P < 0.08), cows that spontaneously ovulated (P < 0.07), and cows detected in estrus and induced to ovulate medium follicles (P < 0.01). In summary, concentrations of estradiol, timing of the LH surge, timing of ovulation, and rate subsequent progesterone rose could explain the increased pregnancy rates in cows that exhibit estrus and are induced to ovulate compared to cows that do not exhibit estrus.
Number of Pages
South Dakota State University
Copyright © 2005 South Dakota State University
Perry, George A., "Effect of Ovulatory Follicle Size and Standing Estrus on Circulating Hormone Concentrations and Interval to Ovulation" (2005). South Dakota Beef Report, 2005. 27.