Factors influencing equine embryo outcomes in OPU‑ICSI systems: Insights from a large‑scale program in Brazil

Equine OPU‑ICSI supports commercial IVEP, yet substantial variability persists. We retrospectively analyzed a large dataset generated in Brazil spanning Quarter Horses, Warmbloods, and Mangalarga to evaluate whether predictors previously identified in the literature show comparable patterns under distinct ecological and breeding conditions. Overall oocyte and embryo outcomes were aligned with broadly reported trends from previous studies. Oocytes retrieved from estrous mares bearing a dominant follicle exhibited higher maturation and blastocyst rates. Stallion effects segregated into distinct phenotypes, including a subset displaying low cleavage, but normal blastocyst development among cleaved embryos, indicative of impaired oocyte activation, and others showing normal cleavage, but reduced blastocyst formation, suggesting limitations at embryo genome activation, genome integrity, or epigenetic regulation. Practically, performing OPU during estrus in the presence of a dominant follicle and implementing stallion‑specific optimization of semen handling or activation‑support strategies appear to be promising approaches for improving blastocyst yield in equine OPU‑ICSI programs.