Ovarian Follicular Dynamics in Heifers during Early Pregnancy1

Daily ultrasonic monitoring of individual follicles was used to compare follicular wave characteristics of nonbred (n = 6) and pregnant heifers (n = 6). The dominant follicle of the first wave (Wave 1) did not differ significantly between reproductive statuses for any endpoint. The dominant follicle of Wave 2 was the ovulatory follicle in all nonbred heifers. The maximum diameter of the dominant follicle of Wave 2 was greater (p < 0.05) for the nonbred heifers (14.8 mm) than for the pregnant heifers (13.0 mm). The dominant follicle of Wave 3 was detected later (p < 0.003; Day 19.7 vs. Day 17.3) and reached a greater diameter (p < 0.05; 16.6 mm vs. 12.0 mm) in the nonbred than in the pregnant heifers. On the mean day of onset of luteolysis (Day 15.2) in the nonbred heifers, the dominant follicle was similar in diameter for the two groups. Within a few days, the follicle began to regress in the pregnant heifers but maintained or increased in diameter in the nonbred heifers so that a greater maximum diameter was attained. During Days 0 70 of pregnancy, the interval from emergence of a wave to the emergence of the next wave was constant (not significantly different; mean intervals, 8.5 9.8 days). The mean maximum diameter attained by the dominant follicles differed significantly among the first 6 follicular waves; diameter was greatest for Wave 1 (15.7 mm), smallest for Waves 2 (13.1 mm) and 3 (12.6 mm), and intermediate for Waves 4 (14.0 mm), 5 (13.7 mm), and 6 (14.5 mm). The number of subordinate follicles differed significantly among waves, due primarily to more subordinates for Waves 6 and 7 than for Waves 2,3, and 4. Results indicated that waves of follicular activity occurred periodically until the corpus luteum began to regress (nonbred heifers). The dominant follicle present at the onset of luteolysis became the ovulatory follicle, and the emergence of the next wave was delayed until near the day of ovulation. In the absence of luteolysis (pregnant heifers), the periodic emergence of waves continued. The length of the interval from emergence of a wave to the regression of the previous dominant follicle was approximately 3 days and was not affected by reproductive status. For all wave relationships in all heifers, the previous dominant follicle did no: begin regression (progressive decrease in diameter) until after a new wave emerged. The interval from wave origin to cessation of growth of subordinate follicles was similar (approximately 3 days) for all waves in both reproductive statuses. These temporal relationships suggest that the newly developing dominant follicle may be an important factor in the regression of its subordinates and the previous dominant follicle.