Development of the "waveless" bovine model.

Development of a bovine model without ovarian follicular waves (waveless) and transient increases in gonadotropin secretion during estrous cycles may lead to new methods to more consistently regulate ovulatory follicle growth thereby improving efficiency of embryo transfer. We hypothesized that the GnRH antagonist acyline would inhibit gonadotropin secretion thereby blocking follicular waves, ovarian function and ovulation during estrous cycles of cattle. To test this hypothesis, beef heifers (n = 5 per group) were treated twice daily with vehicle (control) or 25 or 50 µg/kg acyline beginning 12 h after GnRH-induced ovulation and ending 21 days later. Each animal was subjected to ovarian ultrasonography for 25 days to monitor number and growth of follicles ≥3 mm in diameter and growth of the corpus luteum (CL). Blood samples were taken at various intervals to determine circulating concentrations of FSH, LH, anti-Müllerian hormone (AMH, marker of small follicle growth) and progesterone. Results show that acyline treatment decreased or blocked: circulating concentrations of LH, transient FSH spikes associated with emergence of follicular waves, preovulatory FSH/LH surges, follicular waves, dominant follicle development, CL growth, and progesterone production. In contrast, the largest acyline dose increased AMH concentrations. In conclusion, long-term acyline treatment blocks follicular waves but not growth of preantral and small antral follicles (≤ 3 mm). Future studies will determine if the waveless bovine model, which has enhanced development of preantral and small antral follicles, can be used to develop new methods to improve predictability of response of cattle to superovulation.