Cetrorelix suppresses the dominant follicle and synchronizes follicular waves and ovulation in cattle†.

To determine the effects of a GnRH antagonist on ovarian function in cattle, we tested the hypotheses that cetrorelix will (1) cause regression of the extant dominant follicle by altering gonadotrophin secretion and (2) induce a new follicular wave emergence at a consistent time after treatment. In Experiment 1, heifers were given 1.5 mg cetrorelix im on Days 1-2 (Day 0 = wave emergence), Days 3-4, and Days 6-7, or normal saline (Control, n = 8 per group). The dominant follicle was smaller and regressed earlier in groups treated with cetrorelix on Days 1-2 or Days 3-4 than in Controls (P = 0.01). Plasma LH concentrations were lower (P = 0.04) for 3 days after treatment, and the pre-wave surge in FSH occurred earlier in groups treated with cetrorelix on Days 1-2 or Days 3-4 than Controls (P = 0.01). Corpus luteum diameter was smaller (P = 0.01) in the cetrorelix groups, but luteal vascularity, life-span, or progesterone production was not affected. In Experiment 2, heifers were treated with a single 3 mg dose of cetrorelix on Day 1, 3, or 6, or normal saline (Control, n = 8 per group), given prostaglandin F2a and inseminated. Wave emergence occurred synchronously 3.6 ± 0.3 days after cetrorelix treatment, ovulations occurred 12.4 ± 0.3 days later, and 13/23 heifers became pregnant. In Experiment 3, cetrorelix given at random days of the cycle induced a new wave 3.4 ± 0.3 days later and wave emergence was more synchronous than in saline-treated cows (Barlett test P = 0.04). Results supported hypotheses and provide rationale for the development of new cetrorelix-based protocols for breeding management in cattle.