In vitro maturation of class I oocytes of bovine during different tropical seasons.

The aims of this study were to determine the effects of seasonality on yields and quality of bovine abattoir-derived oocytes and their in vitro maturation (IVM) competence in Thailand, a tropical country. Ovaries were collected monthly from a slaughterhouse, from January to June 2017, separated into cool (January to February), summer (March to April), and rainy (May to June) seasons. Cumulus oocyte complexes (COCs) were obtained by follicular aspiration, and subsequently, oocyte yield and quality were examined. Selected class I and II COCs were cultured under IVM condition, and their maturation capacity was also evaluated. Results showed that average numbers of COCs and class I COCs per ovary were highest in summer. However, no significant seasonal difference in oocyte maturation rates was found. The higher numbers of COCs and class I COCs in summer might be caused by their follicular development during the cool season, and might be a reason why the pregnancy rate in summer is better than in the rainy season.