Abstract:The objective was to determine effects of manual transrectal ablation of the largest (dominant) ovarian follicle on days 7 or 8 of the cycle (estrus = day 0) to synchronize wave emergence before superovulation. Superovulation was performed in 167 donors, 32 nulliparous, 36 primiparous and 99 multiparous Holsteins. They were randomly allocated into 3 groups: control group (n=64, including 15 cows where the follicle failed to rupture by light digital pressure), dominant follicle ablation on day 7 (n=57) and dominant follicle ablation on day 8 (n=46). Superstimulatory treatments started on day 10 for control or 1 day after ablation. Superovulation was induced by 9 im injections of decreasing dosage of gonadotrophins at 12-hour intervals over 4.5 days. PGF2α was administered in 2 im injections at the time of the seventh and the eighth injections of gonadotrophins. All females were artificially inseminated with frozen-thawed semen. Seven days after estrus embryos/ova were recovered by flushing the uterine horns, classified for stage and quality according to the IETS guidelines, and transferred fresh or frozen. Treated cows were subsequently monitored for adverse effects that could affect reproductive parameters or reproductive culling rate at 270 days post-partum. Total numbers of recovered structures and transferable embryos were lower (P<0.05) in control (7.8 ± 4.5 and 5.1 ± 3.3, respectively; mean ± SD) compared to ablation 7 (9.0 ± 2.5 and 7.0 ± 1.9) and ablation 8 (9.6 ± 3.4 and 6.9 ± 2.5) groups. There was no difference (P>0.05) among groups in mean grade of transferable embryos, but percentage of Grade-I embryos were higher in ablation 8 group (47.3%) than in control (37.9%), whereas Grade-II embryos were lower in ablation 8 group (37.5%) compared to control (45.6%) (P<0.05). Percentage of unfertilized oocytes was higher (P<0.05) in control (14.4%) than in ablation 8 group (9.8%), whereas degenerating embryos were lower (P<0.05) in ablation 7 group (12.2%) than in control (20.0%). Conversely, percentage of morulae was higher (P<0.05) in ablation 7 group (32.8%) than control (25.5%). Reproductive parameters and culling rates were similar among groups (P>0.05). In conclusion, improvements in embryo number and quality were achieved by manually ablating the dominant ovarian follicle on days 7 or 8. Furthermore, there was no evidence that judicious manual ablation could have any detrimental effect on subsequent reproductive performance and culling rate.