Comparison between lactating and non-lactating dairy cows on follicular growth and corpus luteum development, and endocrine patterns of ovarian steroids and luteinizing hormone in the estrous cycles

The dynamics of ovarian follicle, corpus luteum (CL), and peripheral plasma ovarian steroids were compared between lactating and non-lactating cows, and a possible association of pulsatile luteinizing hormone (LH) secretion with the dynamics was examined. Lactating (n = 5) and non-lactating (n = 5) cows were monitored daily for follicle and CL throughout two consecutive estrous cycles (Day 0: day of ovulation). Blood samples were collected daily and at 15 min intervals for 8 h on Days 2, 4, 6, 8, and 14 of the second cycle. Lactating cows had larger CL (25.4 ± 1.8 mm vs. 23.5 ± 1.5 mm, P < 0.01) and greater progesterone concentrations (4.6 ± 1.0 ng/ml vs. 3.9 ± 0.9 ng/ml, P < 0.01) during mid-luteal phase compared with non-lactating cows. Maximal diameters of the first wave dominant follicle (17.2 ± 1.8 mm vs. 15.5 ± 0.8 mm) and the ovulatory follicle (17.9 ± 1.2 mm vs. 15.2 ± 0.8 mm) were greater (P < 0.05) in lactating cows than in non-lactating cows during the estrous cycles with two follicular waves, but no significant differences were detected between the groups during the estrous cycles with three follicular waves. Plasma estradiol concentrations did not differ between the groups throughout the experiment. Lactating cows had more LH pulses from Days 2 to 14 than non-lactating cows. These results imply that differences in ovarian dynamics may exist between lactating and non-lactating cows, for which the increased number of LH pulses observed in lactating cows may have responsibility.