Human chorionic gonadotropin-induced alterations in ovarian follicular dynamics during the estrous cycle of heifers

This experiment was designed to characterize hCG (3,000 IU)-induced alterations in ovarian follicular and corpus luteum (CL) dynamics during the estrous cycle (EC) in heifers. Following synchronization of estrus (norgestomet implant for 7 d with injection of PGF2alpha 1 d before implant removal), 13 heifers were treated with either hCG (n = 6) or saline (control group; n = 7) on d 5 of the EC (d 0 = day of estrus). Blood sampling from the jugular vein and ultrasonography of both ovaries were conducted daily until confirmation of ovulation following a detected estrus. Treatment with hCG at d 5 induced formation of an accessory CL in all hCG-treated heifers. Subsequent plasma progesterone (P4) concentrations were higher (P < .01) between d 9 and 17 for the hCG group than for the control group. More (P < .05) hCG-treated heifers had EC with three waves of follicles than control heifers. The second-wave dominant follicle (DF) emerged earlier in hCG than in control heifers (7.3 vs 10.4 d; P < .01). A group x wave interaction (P < .01) was detected for duration of the second and third wave follicles; hCG treatment decreased duration of the second wave (6.3 vs 9.3 d) and increased duration of the third wave (9.2 vs 5.3 d). The hCG-treated heifers had preovulatory follicles that lasted longer (P < .01) on the ovary than those in control heifers. However, plasma estradiol concentrations did not differ (P > .10) between hCG-treated and control heifers during the preovulatory period. A wave x duration interaction (P < .01) was detected for P4 concentrations, between d 13 and 22, for heifers with two vs three spontaneous-wave EC. Higher P4 concentrations during the luteal phase may contribute to a greater rate of follicular turnover or frequency of three-wave cycles, and lower P4 concentrations were associated with two-wave cycles.