Relationship of pre-ovulatory follicle size, estradiol concentrations and season to pregnancy outcome in dairy cows

This study was carried out to evaluate effects of pre-ovulatory follicle size, plasma concentrations of estradiol and progesterone, and season on pregnancy outcomes in dairy cows. Holstein cows (n = 144) were synchronized and inseminated (Ovsynch/TAI protocol) in two distinct periods (cold versus warm season). Blood samples were collected daily from AI (day 0) to day 8 and on days 15, 22, 29, 36 and 64 to measure progesterone and estradiol. Pregnancy diagnosis was performed at days 29, 43 and 64. The pre-ovulatory follicle size was larger and the plasma estradiol concentrations on the day of AI were greater in animals that became pregnant. Plasma progesterone concentrations diverged and became greater after day 5 post-AI, in cows diagnosed pregnant, as compared to non-pregnant cows. The overall pregnancy rate (33%) or late embryonic/early fetal losses (23%) did not differ between seasons, but plasma estradiol concentrations on the day of AI and plasma concentrations of progesterone in pregnant cows were lower in the warm season. Reduced CL function, measured as plasma progesterone concentrations, from days 22 or 29 post-AI onward for cold and warm season, respectively, was associated with subsequent late embryonic/early fetal mortality. Overall, pregnancy was related to diameter of the pre-ovulatory follicle and plasma E2 on the day of AI, but embryonic/fetal losses were not. Season did not affect these outcomes, even though it influenced luteal function after AI.