A Follicle Size Window of Competence for In Vitro Embryo Production in High-Producing Dairy Cows: Evidence from OPU-IVP Performance and Follicular Fluid Profiling

A key objective of the dairy industry is to balance genetic progress with reproductive efficiency. Ovum pick-up followed by in vitro embryo production (OPU-IVP) is a pivotal technology for accelerating genetic gain. However, the relationship between follicle size and oocyte developmental competence in high-producing dairy cows under hormonal stimulation remains to be fully elucidated. This study systematically evaluated the effects of follicle diameter ovum pick-up on OPU-IVP outcomes and the underlying follicular fluid (FF) microenvironment. A total of 109 high-yielding Holstein cows were subjected to ovarian stimulation and OPU. Follicles were categorized as small (2.0-5.9 mm), medium (6.0-9.9 mm), or large (10.0-20.0 mm). Oocyte recovery, quality, and developmental competence were assessed. FF was analyzed for hormonal profiles, including anti-Müllerian hormone (AMH), estradiol (E2), follicle-stimulating hormone (FSH), and progesterone (PROG); oxidative stress markers, including malondialdehyde (MDA), glutathione peroxidase (GPx), superoxide dismutase (SOD), and total antioxidant capacity (T-AOC); and untargeted metabolomics (<i>n</i> = 10 per group). Consistently, oocytes from medium follicles exhibited superior developmental competence, achieving the highest maturation (89.93%), cleavage (72.19%), and blastocyst rates (41.88%). In contrast, large follicles had a low recovery rate (32.64%), a high proportion of degenerated oocytes (32.00%), and reduced embryonic efficiency. Metabolomic profiling revealed distinct microenvironmental differences, with medium follicles enriched in pathways like pyruvate metabolism and arachidonic acid metabolism indicating an optimal metabolic state. Hormonally, AMH decreased while E2 and PROG increased with follicle size. Large follicles exhibited significantly elevated MDA levels, indicating oxidative stress, without a concurrent rise in antioxidant capacity. In conclusion, while small follicles provide an abundant source of morphologically good oocytes, medium follicles (6.0-9.9 mm) represent a distinct "window of competence" for OPU-IVP, characterized by a follicular microenvironment most conducive to embryo production. Excessive reliance on large follicle aspiration should be avoided due to signs of over-maturity and oxidative damage. These findings provide a physiological basis for optimizing OPU strategies to enhance IVP efficiency in high-producing dairy cows.