A new cetrorelix-based estrogen-free ovarian synchronization protocol for fixed-time artificial insemination in beef cattle.

The objective was to test the hypotheses that treatment with a GnRH antagonist, cetrorelix, in combination with progesterone will i) induce synchronous emergence of a new follicular wave and ovulation in cattle at random stages of the ovarian cycle, and ii) result in a pregnancy rate similar to that of an estradiol-based protocol. In Experiment 1, heifers at unknown days of the cycle (Day 0) were given a progesterone-releasing device (CIDR) and either 3 mg cetrorelix im (Cetrorelix; n = 19) or 2 mg estradiol benzoate (Estradiol; n = 19). CIDR devices were removed on Day 8 and heifers were given prostaglandin. Heifers were artificially inseminated on Day 10 and given gonadorelin. Heifers that had not yet ovulated on Day 11 were inseminated again. In Experiment 2, the above protocol was replicated in suckled post-partum beef cows at random stages of the ovarian cycle (n = 106 per group) but only one insemination was performed at 54 h after CIDR removal. In Experiment 1, new follicular wave emergence occurred earlier (P < 0.01) in the Cetrorelix group (2.4 ± 0.2 days) than the Estradiol group (3.8 ± 0.2 days). The dominant follicle was larger in the Cetrorelix than the Estradiol (P ≤ 0.03) on Day 8 (11.3 ± 0.4 vs 9.8 ± 0.3 mm) and Day 10 (13.1 ± 0.3 vs 11.9 ± 0.4 mm). Pregnancy rates were not different (P ≤ 0.48) between treatment groups in Experiment 1 (Cetrorelix 78.9%, Estradiol 68.4%) or Experiment 2 (Cetrorelix 42.5%, Estradiol 45.3%). We conclude that a simple cetrorelix-based synchronization protocol is effective in heifers and suckling beef cows, and is a viable alternative to estradiol-based protocols.