As early as 1979, Palmer et al. used a veterinary B-ultrasound scanning device to observe the ovaries, uterus and early embryonic sacs of horses. Subsequently, Inther et al. (1983) conducted a systematic study on the development characteristics of cow follicles, corpus luteum formation, and the relationship between corpus luteum and follicle development. According to the size of the follicles, a reasonable breeding time can be selected, and measures to accelerate estrus can be taken according to the changes in the corpus luteum. So far, many researchers at home and abroad have reported the ovarian structure of humans, cattle, horses, and pigs. The sonogram of the follicles is an echo-free area and a dark area. The follicle wall is a strong echo with smooth edges. Due to the squeezing of adjacent follicles or ovarian tissues, the follicles are irregularly round (horse) or round (cattle, sheep, pigs), and multiple follicles can be observed in pigs at the same time. Generally, the intracavitary probe with a frequency of 5.0° can monitor the follicles of livestock with a diameter of ≥2. Chen Zhaoying in China once reported that before ovulation, 84% of mature follicles in horses changed from elliptical to conical or pear-shaped, and 16% remained elliptical. Most of the mature follicles of Mongolian horses become non-elliptical, pear-shaped or flat on the top. 40% of the follicles feel soft. The follicle wall thins 2 days before ovulation, from an average of 3.5 to 2.0 μm, the echo is enhanced, and the boundary with the ovary is clearer151. Lun Shichun et al. (1998)inl used real-time B-type ultrasound imaging to preliminarily observe the development of follicles and corpus luteum formation during estrus in 16 donkeys and 17 mares. The follicle is a fluid organ with a complete outline, and the follicular fluid is echo-free (dark area). The follicle wall thins before ovulation, the echo is enhanced, and the outline is clearer. 75% of the horse follicles change from elliptical to pear-shaped or cone-shaped 2 days before ovulation, and most donkey follicles remain elliptical. There are multiple follicles developing on the ovaries of donkeys or horses, and multi-follicle development is slower than single follicle development in ovulation. The largest follicle of a donkey begins to grow rapidly from 5 days before ovulation, reaches its maximum size 1 day before ovulation, and the follicle on the second day begins to shrink gradually 6 days before ovulation. The largest follicle of a horse begins to grow rapidly from 5 days before ovulation, reaches its maximum size 1 day before ovulation, the follicle on the second day develops slowly, and begins to shrink 3 days before ovulation.

The normal follicle of a cow is filled with homogeneous liquid and does not reflect ultrasound. On the ultrasound image of veterinary B-ultrasound, it appears as a clear and very regular dark area of anechoic liquid, and the follicle wall and surrounding tissues have strong echoes relative to the follicular fluid. According to the dynamics of cow follicle development, the diameter of a cow follicle greater than 10 is the second and third stages of the growth period, which appear on the 3rd to 9th day and 10th to 16th day of ovulation, respectively. The 17th to 21st day is the ovulation period, at which time the growth and degeneration of the follicle proceed at a rate of 1.6 to 2.4 and 1.1 to 2.9 milliliters per day, respectively. When a cow is in estrus, there is generally at least one follicle with a diameter of ≥10 mm on the ovary. Therefore, for cows with no obvious external estrus, veterinary B-ultrasound can be used to detect the development of follicles to determine whether they are in estrus so that they can be bred in time. In the application of modern reproductive biotechnology, veterinary B-ultrasound diagnostic technology can be used to monitor the growth and development process, number, and size of superovulated follicles of ≥2 mm, predict and determine ovulation. The correlation coefficient between veterinary B-ultrasound examination and sectioning method for detecting follicles of different diameters is 0.80-0.92. The number of follicles ≥2-3 mm in each ovary monitored by veterinary B-ultrasound is slightly higher than that by sectioning method. This proves that the data of veterinary B-ultrasound monitoring of follicles ≥2 mm is accurate and reliable.