184 Effects of cetrorelix on the growth pattern of the dominant follicle and follicular wave emergence in cattle

Oestradiol-based protocols are widely used for breeding management in cattle. However, the use of oestradiol in food animals is banned in many countries. Our long-term aim is to develop an alternative oestrogen-free fixed-time AI protocol that is simple, short, and effective at all phases of dominant follicle development and endocrine milieu. In this study, we examined the effects of a GnRH antagonist, cetrorelix, on ovarian dynamics and synchrony of follicular wave emergence in cattle. We tested the hypotheses that cetrorelix will (1) suppress the growth and function of the dominant follicle of a follicular wave, (2) induce emergence of a new follicular wave at a consistent time after treatment, and (3) reduce the function of the corpus luteum (CL). Ovaries of heifers (n = 32, 13 months of age) were examined daily by transrectal ultrasonography to record the number and diameter of both follicles and CL. Heifers were given two IM doses of cetrorelix (3 mL, 0.5 mg mL⁻¹ in 5% D-mannitol) 24 h apart starting on Day 1 (pre-selection phase of the dominant follicle; Cetro1), Day 3 (selection phase; Cetro3), or Day 6 (early static phase; Cetro6; Day 0 = wave emergence), or two 3-mL normal saline treatments at a 24-h interval (control, n = 3 starting on Day 1, n = 2 on Day 3, and n = 3 on Day 6). Data were analysed by analysis of variance and t-test. Four days after treatment, the diameter of the dominant follicle was smaller in the Cetro1 and Cetro3 groups than in the control group (8.2 ± 0.6 mm, 9.9 ± 1.6 mm vs 11.1 ± 0.6 mm, respectively; P ≤ 0.05), but did not differ from control and the Cetro6 group (11.8 ± 0.9 mm). The diameter of the CL 4 days after treatment was smaller in Cetro1 and Cetro6 than in control (P < 0.04) but did not differ between control and Cetro3. The CL life span was not affected by cetrorelix treatment. Interwave interval was shorter (P < 0.01) in Cetro1 (6.9 ± 0.4 days) compared with control (9.1 ± 0.58 days) but did not differ between control and Cetro3 or Cetro6 (8.9 ± 0.44 and 10.3 ± 0.65 days). The subsequent interwave interval did not differ among groups (P = 0.32). Wave emergence occurred, on average, 5.3 ± 0.3 days after the first cetrorelix treatment (Cetro1 = 5.9 ± 0.4 days, Cetro3 = 5.8 ± 0.4 days, Cetro6 = 4.3 ± 0.6 days; P = 0.06). Consistent duration between cetrorelix treatment and next wave emergence indicates functional demise of pre-selection, growing as well as static-phase dominant follicles. In conclusion, results supported the stated hypotheses; cetrorelix inhibited the form and function of the dominant follicle resulting in emergence of the next follicular wave at a consistent time post-treatment, and cetrorelix treatment can be an effective method to synchronize ovulation.

Research support was provided by grants from the Natural Sciences and Engineering Research Council of Canada and Saskatchewan Agriculture Development Fund.