Ultrasonographic Analysis of the Number of Large, Middle and Small Follicles of Japanese Black Cattle during the Estrous Cycle and Early Pregnancy

Abstract

Daily ultrasonic monitoring was conducted to examine the dynamics of bovine ovarian follicular development (large: ≥10 mm, middle: ≥5 mm and <10 mm, small: ≥2 mm and <5 mm) during the estrous cycles and early-pregnancy. Two Japanese Black cows were used in this study. One (No.2167) was observed daily for successive 90 days including two estrous cycles and subsequent early-pregnancy, and the other (No.2249) was done for consecutive 112 days over five estrous cycles. Most cycles were consisted of three waves of middle size follicles with only one large follicle (dominant follicle). The number of middle size follicles tended to decrease from around the day of the emergence of dominant follicle, and a new wave of middle size follicles emerged concomitantly with either the regression of a dominant follicle or its ovulation. The dominant follicle in the third wave was the ovulatory follicle. Until 21 days of pregnancy, the pattern of the growth and regression of middle size follicles was similar to that of the estrous cycle, but after that the wave of middle size follicles became smaller. Additionally, the maximal size of the dominant follicle in the third wave during the estrous cycles tended to be greater than those of first and second waves. During the estrous cycles, most (13 out of 14 waves) of the dominant follicles in the first and second waves appeared in the ovaries ispilateral to the corpus luteum (CL), but the ovulatory follicles in the third follicle waves always (6 out of 6 waves) appeared in the ovaries contralateral to the CL. In contrast, each dominant follicle during early-pregnancy appeared in the ovaries randomly, and had a tendency to be smaller than the maximal size of the dominant follicle in the third wave during the estrous cycles.