Season and oxygen tension during maturation and fertilization impact the developmental competence of bovine oocytes

Seasonal heat stress (HS) can reduce the developmental competence of bovine oocytes used for in vitro embryo production. Atmospheric oxygen tension (20% O₂), commonly used for in vitro maturation and fertilization (IVM/IVF), may also influence embryo development and quality. This study investigated whether oxygen tension during IVM/IVF affects the developmental competence of bovine oocytes collected in summer and winter. Oocytes were matured and fertilized under physiological (5% O₂) or atmospheric oxygen tension. Summer conditions showed a significantly higher temperature-humidity index than winter (72.9 ± 4 vs 51 ± 3; P < 0.0001), consistent with HS risk. Oocyte maturation and normal fertilization rate did not differ significantly among experimental groups. Physiological oxygen during IVM/IVF increased cleavage and blastocyst rates in both seasons, although developmental rates of summer oocytes remained lower than those of winter oocytes under the same oxygen conditions. Regarding embryo quality, total cell number was affected by season, whereas apoptotic index was affected by both season and oxygen tension. The BAX/BCL2 ratio was affected by oxygen tension, and a significant season × oxygen interaction was detected only for BCL2 expression (P < 0.05). In conclusion, physiological oxygen tension during IVM/IVF improved embryo development but did not eliminate seasonal differences in developmental competence. These findings indicate that oxygen tension and seasonal conditions independently influence bovine in vitro embryo production.