Rectal ultrasound scanning is an effective method for determining and measuring cow follicles,so cattle ultrasound is a good tool to detect follicles development in dairy cows.Cows have follicular follicles that are non-reflective and can be distinguished based on their vascular extension characteristics. Follicular ultrasound images show that bovine follicles develop in a clear, very regular pattern. Each follicular wave consists of a series of 5 mm or larger follicles that appear simultaneously, with dominant follicles occurring within a few days. There are 2 to 3 follicular waves in each estrus cycle of young cows.
Dominant follicles have a growth phase and a stationary phase, lasting about 5 to 6 days. The first wave of dominant follicles maintains a 4 to 5 day advantage and loses its advantage and degenerates until 11 to 12 days of the estrus cycle, lasting about 5 to 7 days. At the same time, the second wave occurs, selecting its dominant follicle and developing to ovulation. However, in the three-wave cycle, the second wave is replaced by the third wave, and the third dominant follicle ovulation occurs. Dominant follicles maintain a morphological advantage (the largest follicle in both ovaries) longer than maintaining a functional advantage (inhibiting the growth of other follicles).