175 The effect of cetrorelix on follicular dynamics and ovulation in beef cattle in high versus low progesterone milieu

Estradiol-based ovulation synchronization protocols are effective for breeding management in cattle, but legislation in many countries prohibits the use of estrogens in food-producing animals. In an effort to develop an alternative protocol for fixed-time AI that is simple, steroid-free, and effective during different stages of the estrus cycle, the present study was done to determine if cetrorelix, a GnRH antagonist, could consistently induce a new follicular wave emergence during preovulatory and luteal phases in cattle. We hypothesized that treatment with cetrorelix would cause regression of the extant dominant follicle during preovulatory and luteal phases (Experiment 1) and cause the extant dominant follicle to lose its ovulatory potential within 48 h after treatment (Experiment 2). Sexually mature beef heifers were examined daily by transrectal ultrasonography to detect ovulation (Day 0 = assumed day of wave emergence), record the number and diameter of follicles, and monitor the diameter and vascular flow dynamics of the corpus luteum (CL). In Experiment 1, heifers (n = 12) were assigned randomly to two groups and given either a luteolytic dose of cloprostenol on Day 4 and 4.5 (PreOv group) or allowed to maintain a functional CL (Luteal group). Heifers were given 3 mg of cetrorelix i.m. on Day 5, and the ovaries were examined daily until the next ovulation or wave emergence. In Experiment 2, beef heifers (n = 24) were synchronized by ultrasound-guided follicular ablation in combination with an intravaginal progesterone-releasing device for 5 days and PGF at the time of removal. Heifers in PreOv and Luteal groups were given either cetrorelix (Cetro groups) or normal saline (Control groups) on Day 5 (2 × 2 factorial). Heifers were treated with 1500 IU of human chorionic gonadotrophin i.m. on Day 7 to determine the effect of cetrorelix on the ovulatory ability of the dominant follicle. Blood samples were taken at −24 h, 24 h, and 48 h from treatment to determine plasma progesterone concentration. Data were analyzed by ANOVA for repeated measures, t-test, and generalized linear mixed models. In Experiment 1, the diameter profile of the dominant follicle did not differ between PreOv and Luteal groups (P = 0.33), nor did the interval to emergence of a new follicular wave (3.1 ± 0.3 days after cetrorelix treatment; P = 0.28). The dominant follicle present at the time of cetrorelix treatment failed to ovulate in both groups (PreOv = 1/6, Luteal = 0/6; P = 0.99). In Experiment 2, ovulation rates did not differ among groups (PreOv-Cetro = 4/6, PreOv-Control = 6/6, Luteal-Cetro = 5/6, Luteal-Control = 6/6; P = 0.22). Cetrorelix did not affect the CL diameter, vascularity, or plasma progesterone concentration. In conclusion, cetrorelix treatment inhibited ovulation and induced new wave emergence at a consistent time after treatment in both luteal and preovulatory phases of the estrus cycle. Dominant follicle demise was not complete by 48 h after cetrorelix treatment.

This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada and Saskatchewan Agriculture Development Fund.