Abstract
The aim of the current study was to use ultrasound imaging to follow the follicular dynamics in dairy cows and to determine the relationship between the diameter of the dominant follicle (DF), the life span of the corpus luteum (CL), interovulatory interval (IOI), and the number of follicular waves. Twenty nine Holstein normal cyclic cows 60 days postpartum were used in this study. The ovaries of the experimental cows were scanned for the DF and CL every three days from ovulation to ovulation. Cows undergo the Ovsynch program to unify the beginning of the ovulation per cycle.
The results revealed that, cows that exhibit the two wave patterns have a longer time of IOI than a cow with a one wave pattern (P<0.05), and a shorter length of IOI than cows with three follicular wave patterns (P<0.05). The duration of the DF growth phase in the first follicular wave (Wave 1) was prolonged in a cow that exhibited a one-wave pattern than in the cows that exhibited the two or three-wave patterns (p<0.05). The maximum diameter of DF for a single follicular wave pattern (17.6 ± 0.67 mm). The maximum diameter of the anovulatory follicle in the two-wave patterns (12 ± 2.53 mm) was larger than the maximum diameter of the anovulatory follicle (10.75 ±1.33 mm, p<0.05) of wave one in cows that exhibited three-wave patterns. The lifespan of the corpus luteum in the one-wave pattern was significantly shorter (p<0.05) than in two-wave patterns and three-wave patterns. The length of the IOI was positively increased with the number of the follicular wave and the length of the luteal phase but was negatively related to the diameter of the first dominant follicle in the wave and the duration of the (DF) growth phase in the first wave (p<0.05).
In Conclusion, these results demonstrate that the length of the IOI in cows exhibiting the two-wave patterns is shorter than in cows exhibiting three-wave patterns due to the larger diameter of the dominant follicle in wave one, the longer duration of the DF growth phase in the first wave, and the shorter life span of the CL in the cows exhibiting two-wave patterns than in cows exhibiting three-wave patterns.

Key words: bovine ovary; estrous cycle; Corpus luteum; follicular dynamics; dairy cattle; Ultrasound.