CHANGES IN OVARIAN FOLLICULAR DYNAMICS AND LUTEINIZING-HORMONE PROFILES FOLLOWING DIFFERENT PROGESTAGEN TREATMENTS IN CATTLE

Twelve cyclic non-lactating cows were allocated at random to three tre atment groups (IN, NP and 2N). Cows in the IN group (n = 4) received a n ear implant containing Norgestomet at pro-estrus (day 0 = day of Nor gestomet-implant insertion). Cows in the NP group (n = 4) received a p rogesterone-releasing intravaginal device 3 d after a Norgestomet impl ant at pro-estrus, and cows in the 2N group (n = 4) received a second Norgestomet implant 3 d after the first implant at pro-estrus. Treatme nts were terminated on day 9. Ultrasonic examination of the ovaries wa s carried out before, during and after treatment to monitor follicular dynamics. Serial blood samples were collected every 15 min for 8 h fr om all cows on days 3 and 6 after initiation of treatments to characte rize the luteinizing hormone (LH) profiles. The dominant follicle was maintained for 9 d in all IN cows, and upon implant removal all cows o vulated. In both NP and 2N cows, the dominant follicle regressed and a new cohort of small follicles appeared before the end of the treatmen t. The dominant follicle from this pool ovulated after implant removal in all NP cows. In the 2N group, only one cow ovulated after implant removal; the dominant follicles in the remainder became cystic and did not ovulate. Decreases in pulse frequency, amplitude and mean plasma LH were observed in the NP cows, compared with IN cows (P < 0.05), but no change in LH-pulse frequency was observed in the 2N cows. This stu dy shows clearly that maintenance and regression of the dominant folli cle are associated with changes in progesterone and LH environment.