INCREASED OVARIAN RESPONSES IN THE ABSENCE OF A DOMINANT FOLLICLE IN SUPEROVULATED COWS

Dairy cows (n = 35) were given a single dose of equine chorionic gonadotrophin (eCG or PMSG) between Days 9 and 12 after a previously synchronized estrus, and after their ovaries had been examined daily by ultrasound scanning from Day 4 or 5 to assess the presence of a dominant follicle and to monitor follicular development before the superovulatory treatment. Two different classification criteria for follicle dominance were tested: 1) a follicle entered the dominance phase when its diameter exceeded 8 mm and it stayed dominant until 3 days after it stopped growing; 2) a follicle entered the dominance phase when it exceeded 9 mm in diameter and it stayed dominant until 4 days after it ceased to grow. Under both classifications the number of transferable embryos recovered nonsurgically on Day 6 after insemination was significantly higher in cows that did not have a dominant follicle on the day they received their PMSG injection. Under Classification 1, the total numbers of embryos and oocytes recovered, and the concentration of progesterone in the milk on Day 6, were also higher in the group of cows without a dominant follicle. The results suggest that the presence of a dominant follicle at the time of gonadotrophic stimulation decreases the superovulatory response. However, more precise criteria for determining follicular dominance are required in order to improve the predictability of embryo yield in future.