After the cows were superovulated, the imported cows were observed by B-ultrasound, and the number of large, medium and small follicles showed a certain dynamic change process. Under the intervention of exogenous hormones, the number of medium follicles in the previous normal estrus cycle decreased rapidly, and the number of large follicles showed a certain upward trend 2 days later. At the same time, the number of small follicles also showed the first peak 2 days after superovulation, forming the first follicle wave.

According to the observation of imported cows by B-ultrasound, the large number of large follicles at this time provided the necessary conditions for the dominance of follicles. During the superovulation process, due to the continuous action of the exogenous hormone FSH, the dominant follicles in the same period matured and ovulated 12-24h after estrus.

In each estrus cycle, the development of cattle follicles must go through the process of recruitment, selection, dominance, maturation and degeneration. The dynamic changes in the number and diameter of large, medium and small follicles can be observed on imported cattle by B-ultrasound, forming 2-4 follicle waves in the estrus cycle, and each follicle wave is marked by the rapid appearance of a large number of small follicles.

Among the 2-4 follicle development waves, the last follicle wave in the estrus cycle is also called the ovulatory follicle development wave. In this follicle development wave, the recruited follicles develop and form dominant follicles, which continue to grow into mature follicles. After the mature follicles rupture and ovulate, the first follicle development wave of the next estrus cycle is initiated.

In the superovulation test, due to the continuous effect of exogenous hormone levels, only one follicle wave appeared before the first estrus from superovulation, and it disappeared one day before estrus, and the duration was much shorter than the normal follicle wave of 6.48d. The reason is that in addition to the gradual EVOlution of some small follicles into medium follicles, it may be that in the process of more dominant follicles transforming into mature follicles, the imported cattle B-ultrasound can see that the ovarian luteinization is tending to form, which increases the progesterone concentration in the local blood environment of the ovary and decreases the estradiol level. As a result, the small follicles are atretic.

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