Effects of season and reproductive phase on the quality, quantity and developmental competence of oocytes aspirated from Japanese black cows.

The aim of the present study was to determine whether the season (hot and cool) and reproductive phase (pregnant and non-pregnant) of the cow affect follicular recruitment and oocyte development. Follicular oocytes were aspirated from Japanese black cows by the ovum pick-up (OPU) method, which was performed 2 to 6 times within 1.5 months in pregnant cows and 2 to 4 times within 2 months in non-pregnant cows, during the hot (July to September) and cool (October to November) seasons. After follicular aspiration, the number and morphology of cumulus-oocyte complexes (COCs) and the developmental competence of oocytes after in vitro maturation (IVM), in vitro fertilization (IVF) and in vitro culture were evaluated. The quality of aspirated COCs did not differ between the hot and cool seasons, irrespective of the reproductive phase of the donor cows. In the pregnant cows, the season did not affect follicular recruitment, early embryonic development or the quality of embryos. In the non-pregnant cows, however, the mean number of aspirated follicles and collected oocytes decreased during the hot season as compared with the cool season. When the data for the 2 seasons were combined to assess the effects of reproductive phase on oocyte development, the total proportions of cleavage, development into blastocysts and freezable embryos were higher for embryos obtained from pregnant cows (P<0.05) than those obtained from non-pregnant cows. In conclusion, the season did not have any apparent effects on the quality of aspirated COCs and the developmental competence of oocytes after IVM-IVF, but it may affect follicular recruitment in non-pregnant cows. Moreover, the reproductive phase may influence the developmental competence of the recovered oocytes.