EFFECT OF MID-LUTEAL PHASE PROGESTERONE LEVELS ON THE 1ST WAVE DOMINANT FOLLICLE IN CATTLE

A wave-like pattern of follicular growth and regression during the luteal phase has been described in the bovine. The factors responsible for inducing the onset of regression of nonovulatory dominant follicles are unknown. The present study was designed to examine the effect of progesterone (P4) administration early in the estrous cycle on the first wave dominant follicle. Nine heifers were administered P4 on day 3 (200 mg), day 4 (100 mg) and day 5 (50 mg) of the estrous cycle (Day 0 = day of estrus) and seven heifers received vehicle to serve as controls. All heifers received a luteolytic dose of prostaglandin F2alpha (PGF2alpha) on day 7. Follicular dynamics were monitored by daily ultrasonography. All seven control heifers ovulated the first wave dominant follicle. In four P4 treated heifers, the first wave dominant follicle regressed prior to PGF2alpha administration and a dominant follicle from the second pool of follicles ovulated. The remaining five P4 treated heifers ovulated the first wave dominant follicle. However, in these heifers the growth of the first wave dominant follicle was slower (0.65 +/- 0.13 mm day-1 between days 3 to 7 for treated vs 1.46 +/- 0.23 min day-1 for control; P < 0.05) and estrus and ovulation were delayed compared to controls (3.8 +/- 0.3 vs 2.4 +/- 0.2 and 5.2 +/- 0.4 vs 3.9 +/- 0.2 days after PGF2alpha, respectively; P < 0.05). The results indicate that P4 administered early in the estrous cycle to mimic the mid luteal phase levels alters follicular dynamics and is capable of inducing premature regression of the first wave dominant follicle.