Accessory corpus luteum induced by human chorionic gonadotropin on day 7 or days 7 and 13 of the estrous cycle affected follicular and luteal dynamics and luteolysis in lactating Holstein cows.

Our objective was to determine the effect of inducing an accessory corpus luteum (CL) with human chorionic gonadotropin (hCG; 3,300 IU) on d 7 (hCG7) or 2 accessory CL with hCG on d 7 and 13 (hCG7+13) of the estrous cycle in noninseminated lactating Holstein cows. Cows (n = 86) between 39 and 64 DIM were pretreated with an Ovsynch + CIDR protocol, and only synchronized cows were used (n = 64). The day of the last GnRH of Ovsynch was considered d 0 of the estrous cycle. Follicular and luteal dynamics of cows were evaluated daily during an entire estrous cycle by ovarian ultrasonography. Blood samples were collected daily to measure serum concentration of progesterone (P4). Cows were randomly assigned to CON (n = 22, no treatment), hCG7 (n = 20), or hCG7+13 (n = 22) treatments. Two cows from hCG7+13 failed to ovulate after hCG and were removed from the analyses post-hCG treatment. The first day of luteolysis was considered the day that P4 declined to more than 2 SD of the mean for the 4 consecutive P4 concentrations with the greatest mean in late diestrus for each individual cow. The P4 cut-off for complete luteolysis was <1.0 ng/mL. Mean P4 on d 7 (3.23 ± 0.16 ng/mL) did not differ among treatments. Cows treated with hCG had greater total luteal and original CL volume and serum P4 during diestrus than CON. Cows treated with hCG7+13 had greater serum P4 after d 13 of the cycle than hCG7. Cycles were classified as having atypical cycles if the dominant follicle or future dominant follicle at the time of luteolysis did not ovulate (delayed ovulation; CON, n = 2; hCG7, n = 4; hCG7+13, n = 3), had a short cycle (CON, n = 1), delayed (CON, n = 2) or incomplete luteolysis (CON, n = 1; hCG7, n = 4; hCG7+13, n = 5). The remainder of cycles with normal complete luteolysis followed by ovulation were considered to be typical. Based on blood perfusion, the CON cow with incomplete luteolysis had 2 original CL remaining functional after first onset of luteolysis. The rest of the cows with incomplete luteolysis (9/10) had one or more CL regressing and at least one remaining functional after first onset of luteolysis. No specific pattern for CL side (ipsilateral vs. contralateral to a CL with complete regression) was observed for nonregressed CL. Cows with incomplete luteolysis had a second onset of luteolysis to undergo complete functional luteolysis. The proportion of cows with typical cycle was 73% (16/22) for CON, 60% (12/20) for hCG7, and 55% (11/20) for hCG7+13. Cows with typical cycles treated with hCG (hCG7 and hCG7+13) had a later onset of luteolysis, prolonged time to undergo complete luteolysis, and greater proportion of cows with 3 follicular waves than CON, resulting in a longer interovulatory interval for hCG7 and hCG7+13 than CON. In summary, accessory CL induced by hCG during diestrus not only altered follicular and luteal dynamics but also deferred and prolonged the luteolytic process.