OVARIAN FOLLICULAR DOMINANCE IN CATTLE - RELATIONSHIP BETWEEN PROLONGED GROWTH OF THE OVULATORY FOLLICLE AND ENDOCRINE PARAMETERS

In cattle, the development of ovarian follicles 5 mm or larger occurs in either two or three consecutive follicular waves per estrous cycle. When the luteal phase is artificially lengthened with an intravaginal progesterone-releasing device (CIDR) that maintains subluteal levels (i.e. levels of progesterone that are below normal luteal levels, but higher than basal follicular phase levels), prolonged development of the ovulatory follicle is observed. To study the endocrinological correlates of prolonged follicular dominance and to test the hypothesis that it is mediated by effects of plasma progesterone on LH pulse frequency, heifers (n = 6/group) were treated with blank CIDRs (no progesterone, control group), with one CIDR for 14 days from day 14 of the cycle (1 CIDR group), or with one CIDR for 14 days from day 14 plus a second CIDR during days 24-28 (2 CIDR group). Cycle length was significantly longer in the 1 and 2 CIDR groups than in the controls (30.2 +/- 0.2 and 31.8 +/- 0.5 vs. 21.6 +/- 0.4 days, respectively; P < 0.0001). Follicular dynamics were normal in the control heifers. In the 1 CIDR group, the ovulatory follicle grew larger than in control or 2 CIDR animals, was maintained as the largest follicle on the ovaries for a much longer time, and ovulated after CIDR removal. In the 2 CIDR group, a similar growth pattern was observed until day 24; after insertion of a second CIDR, however, prolonged dominance was reversed, a new wave was recruited, and the dominant follicle of this wave ovulated after CIDR removal. The size of the ovulatory follicle and the length of the dominance phase in the 2 CIDR group were similar to those in control animals. Reversal of prolonged dominance in the 2 CIDR group was associated with changes in progesterone. Progesterone remained at subluteal levels (1.5-2.3 ng/ ml) in both CIDR groups until day 24, when insertion of the second CIDR in the 2 CIDR group restored progesterone concentrations to normal luteal levels (3.5-6 ng/ml). Pulsatile LH secretion was assessed by frequent blood sampling every 12 min for 6 h (0800-1400 h) on selected days of the treatment cycle. LH pulse frequency was not different among groups before treatment started (days 12 and 13). However, LH pulse frequency was significantly higher in the 1 CIDR than in the 2 CIDR group on both day 26 (P < 0.03) and day 28 (P < 0.05), i.e. during the reversal of prolonged dominance in the 2 CIDR group. On day 29, the day after CIDR removal, LH pulse frequency was similar in 1 and 2 CIDR groups and comparable to that on the day after luteolysis in the controls. Peripheral 17beta-estradiol levels were significantly higher in the 1 CIDR than in the 2 CIDR group at times that coincided with days of increased LH pulse frequency. Heifers were inseminated artificially during the estrus after treatment. Only 14% of heifers that ovulated follicles with prolonged dominance were pregnant on day 28 after estrus, whereas the pregnancy rate was 73% in heifers with normal ovulatory follicles. In conclusion, these results indicate that slight increases in LH pulse frequency promote prolonged follicular growth and dominance associated with increased plasma estradiol and suggest that the demise of nonovulatory dominant follicles during normal estrous cycles occurs through feedback effects of luteal progesterone, which maintain low LH pulse frequency and estradiol production. This study also shows that prolonged follicular growth is associated with lower fertility.