304 EFFECT OF THE SEASONAL INFERTILITY PERIOD ON OOCYTE COMPETENCE IN PIGS

Abstract

Photoperiod is the principal regulator of seasonal breeding; however, effects of photoperiod on the fertility of the domestic sow are inconclusive. Some evidence indicates that the modern sow exhibits a period of impaired reproductive performance during the late summer and early fall. Seasonal variation in oocyte developmental competence has been described as a contributing factor. Alterations in oocyte quality, along with reductions in blastocyst rates and cell numbers in embryos from summer-sourced oocytes, may be attributed to an alteration in follicular fluid (FF) composition. The objectives of this study were to determine whether seasonal variations in blastocyst development rates are associated with changes in cumulus-oocyte complex (COC) morphology and oocyte developmental competence in sows. This study also compared the effect of FF collected in spring v. summer during in vitro maturation (IVM) on oocyte competence. In experiment 1, oocytes from 3- to 8-mm follicles were aspirated from sow ovaries during 1 calendar year for a total of 77 replicates. Only oocytes with homogeneous dark cytoplasm and at least 2 layers of cumulus cells underwent IVM. Mature oocytes were electrically activated and the resulting embryos were cultured for 6 days. In experiment 2, a total of 1256 good quality COC were divided into 2 groups and cultured in IVM medium containing 10% FF collected in either spring or late summer. Metaphase II oocytes were electrically activated and cultured to generate diploid embryos. Differences between experimental groups were assessed using Student's t-test or X². The percentage of ovaries exhibiting good-quality follicles and the number of COC per ovary remained constant during the entire calendar year (60% and 6.2 COC/ovary, respectively). However, oocyte quality decreased significantly from 3.6 to 3.2 during late August throughout early October in a 1 to 4 scale. The percentage of good-quality COC decreased significantly during late summer and early fall compared with the rest of the year (54.5 v. 65.5%). However, maturation, cleavage, and blastocyst rates did not show significant differences between the summer and the other seasons (85.5 v. 87.6, 87.8 v. 87.7, and 27.8 v. 27.0%, respectively). The presence of FF collected in either spring or summer in the IVM medium did not affect maturation, cleavage, or blastocyst rates (88.9 v. 87.7, 90.7 v. 90.5, and 42.1 v. 43.7%, respectively). Blastocyst cell numbers (Day 6) did not differ when FF from spring and summer antral follicles was used for supplementing IVM medium (43.6 v. 46.1 cells, respectively). In summary, impaired reproductive performance of domestic sows during late summer and early fall is coincident with a decreased in the number and quality of COC. However, efforts to use strict selection criteria for COC during this time period may result in maturation and development rates comparable to the rest of the seasons. Additionally, the presence of FF collected in either spring or summer in the IVM medium does not seem to affect oocyte maturation and subsequent embryo development.