Seasonal effects on oocyte developmental competence in sows experiencing pregnancy loss

Recently, oocyte quality in sows culled for reasons unrelated to fertility was found to decline during the period of seasonal infertility. Wean-to-service interval (WSI) has also been associated with pregnancy loss in sows mated during the period of seasonal infertility. The aims of this study were to determine whether WSI and season are associated with changes in oocyte developmental competence in sows experiencing early (before Day 35 of gestation) and late (after Day 35 of gestation) pregnancy loss. Ovaries were collected in pairs from sows sourced from commercial piggeries that were culled for reasons related to infertility after being mated in summer and winter/spring. Sows were grouped according to their pregnancy loss type, their previous WSI and the presence or absence of corpora lutea (CL) on their ovaries. Oocyte developmental competence was assessed following in vitro maturation, artificial activation and parthenote development in vitro. In sows culled for early-pregnancy loss, there was a greater number of CL present on ovaries collected in spring compared to those collected in summer (11.57 ± 3.3 vs. 9.26 ± 0.99; P < 0.05). Also, the proportion of oocytes developing to the blastocyst stage was greater in summer than in spring (55.9 ± 5.2% vs. 31.2 ± 6.4%; P < 0.05). In sows culled for late-pregnancy loss, a greater proportion of oocytes developed to the blastocyst stage in winter compared with late-spring (64.3 ± 7.0% vs. 34.1 ± 6.6%; P < 0.05). In addition, the blastocyst formation rate of oocytes was lower in sows that displayed a WSI ≤ 6 days than in sows that displayed a WSI > 6 days (37.8 ± 7.3% vs. 62.2 ± 6.9%; P < 0.05). The results of the present study indicate that sows culled for pregnancy loss exhibit seasonal changes in oocyte developmental competence. The mechanism which causes WSI to be prolonged does not appear to result in reduced oocyte developmental competence. While poor oocyte quality and the mechanism that increases WSI may contribute to pregnancy loss during the seasonal infertility period, the findings suggest that these factors are not the main drivers of early and late pregnancy loss throughout the year.