Follicular growth and atresia at the time of oocyte aspiration differentially affect in vitro embryo production in mares

Ovum pick-up (OPU) followed by intracytoplasmic sperm injection (ICSI) is increasingly used in horses, yet embryo production rates remain variable, potentially reflecting differences in oocyte developmental competence at the time of aspiration. We investigated whether the stage of the follicular wave, growth phase (GP) versus atresia phase (AP), and associated endocrine profiles in mares, influence oocyte developmental competence. Mares underwent follicular ablation to synchronize follicular waves, and oocytes were aspirated during GP or AP for in vitro production of embryos. Serum concentrations of progesterone (P4), luteinizing hormone (LH), follicle-stimulating hormone (FSH), inhibin A, inhibin B, and anti-Müllerian hormone (AMH) were measured daily. Oocyte maturation rate was higher in AP than in GP (62.8% vs. 17.1%; P < 0.01). Blastocyst formation per recovered oocytes was numerically higher in AP than GP, although the difference was not significant (20.0% vs 7.3%; P = 0.34). At the mare level, 4/6 AP mares (66.7%) vs 1/7 GP mares (14.3%) produced ≥1 blastocyst, but not statistically significant (P = 0.091). Chromatin analysis revealed a higher proportion of non-matured oocytes in post-germinal-vesicle stages in AP than in GP (30.8% vs. 2.9%, P < 0.01). Endocrine analyses revealed lower P4 and higher inhibin A concentrations in AP mares (P < 0.01). LH, FSH, and inhibin B exhibited significant group-by-day interactions (P < 0.05), characterized by higher LH and inhibin B and lower FSH in AP mares. These findings demonstrate that follicular physiological status at the time of aspiration influences oocyte competence in mares and suggest that targeting follicles entering early atresia may improve outcomes of equine OPU-ICSI programs.