129. REDUCED OOCYTE DEVELOPMENTAL COMPETENCE DURING THE PERIOD OF SEASONAL INFERTILITY IN PIGS

Abstract

Reduced farrowing rate due to early pregnancy loss is a manifestation of seasonal infertility in pigs. It has been hypothesised that the early disruption of pregnancy is due to poor oocyte developmental competence. The aim of this study was to determine if there are seasonal differences in oocyte developmental competence. Ovaries were collected from sows slaughtered 4 days after weaning. Cumulus-oocyte complexes (COCs) recovered from small (3–4mm) and large (5–8mm) antral follicles were morphologically graded and subjected to parthenogenetic activation following in vitro maturation (IVM) during the winter (n = 1419) and summer (n = 2803). Cumulus expansion was assessed subsequent to IVM. Data were analysed using a generalised linear mixed model in GenStat release 10. There was an effect of season on oocyte grade, with a larger proportion of oocytes collected in summer being graded suitable for IVM, compared with winter oocytes (P<0.05). A larger proportion of COCs had expanded cumulus during the winter than in the summer, which suggested that the preovulatory LH surge had already occurred. There was a season x follicle size interaction affecting cumulus expansion (P<0.05). There were no seasonal effects on the proportion of oocytes reaching metaphase II or cleaving after parthenogenetic activation. However, the proportion of oocytes from large follicles that developed to the blastocyst stage was higher in winter than in summer (55% vs 23%; P<0.05). There was no effect of season on the proportion of oocytes developing to the blastocyst stage from small follicles. However, during summer there was a reduction in the cell number of blastocysts derived from small follicles (P<0.05). Our results suggest that porcine oocytes are less able to reach their full developmental potential during the period of seasonal infertility, and that the associated pregnancy losses are due to reduced oocyte developmental competence.