Ovarian follicular wave emergence after treatment with progestogen and estradiol in cattle

An experiment was designed to evaluate the effects of treatment with progestogen and estradiol-17β (E-17β), in combination, at different stages of development of the dominant follicle, on follicular development and subsequent wave emergence. Cross-bred beef cows (n = 12) and heifers (n = 25) were randomly allocated to one of four treatment groups on Day 0 (ovulation). Treatment groups were untreated control animals and those that were given progestogen ear implants on Day 2, 5 or 8 and injected intramuscularly with 5 mg E-17β in sesame oil on Day 3, 6 or 9, respectively. Ovarian ultrasonography was performed daily from Day 0 until progestogen implant removal, 4 days after the emergence of the post-treatment follicular wave. The mean (± SEM) time (days) to cessation of growth and onset of regression of the dominant follicle of the first wave were earlier (P < 0.05) in the Day 3 treatment group than in the control group (3.6 ± 0.2 vs. 5.6 ± 0.4 and 7.0 ± 0.3 vs. 11.0 ± 0.6, respectively). Characteristics of the dominant follicle of the first wave in the Day 6 and Day 9 treatment groups were not different from those of the control group. However, treatment on Day 9 resulted in earlier (P < 0.05) cessation of growth and onset of regression of the dominant and largest subordinate follicle of the second wave than in the control group (9.6±0.2 vs. 13.4±0.7 and 11.5±1.3 vs. 17.1±0.9, respectively). Follicle wave dynamics was affected by early emergence of the second follicular wave in heifers treated on Day 3 (Day 7.3 ± 0.3) and delayed emergence in those treated on Day 6 (Day 10.7 ± 0.2) compared with the control group (Day 8.6 ± 0.3; P < 0.05). Furthermore, treatment on Day 9 resulted in earlier (P < 0.05) emergence of the third wave (Day 13.1 ± 0.3) than the control group (Day 15.3 ± 0.6). The interval from treatment to wave emergence was not different among treatment groups (4.3 ± 0.2 days after E-17β) and supported the hypothesis that treatment with progestogen plus E-17β results in consistent emergence of a new follicular wave, regardless of the stage of development of the dominant follicle at the time of treatment. It was concluded that treatment with progestogen and E-17β in combination can be used effectively to control and synchronize follicle wave development.