271. Effect of season on sow ovarian morphology

Abstract

Reduced farrowing rate caused by embryonic mortality is a manifestation of seasonal infertility in pigs. The ability of the oocyte to mature, be fertilised and sustain embryonic development is acquired gradually by the oocyte throughout folliculogenesis. This study was undertaken to determine if seasonal differences in ovarian morphology are associated with reduced reproductive performance displayed during seasonal infertility. Sows culled after weaning were sourced from two genetically distinct herds (Farms A and B). Pairs of ovaries were collected from sows 4 days post-weaning during winter (n = 131) and summer (n = 275). Ovarian weight (Farm A only) and the numbers of small (3–4 mm) and large (5–8 mm) follicles were assessed (Farms A and B). Data did not follow normality and was analysed using the Mann–Whitney test. Mean ovarian weight per sow during winter (20.0 ± 1.3 g) was significantly heavier than that during summer (15.3 ± 0.8 g; P < 0.05). Farm A ovaries had a greater total number of antral follicles in winter compared with summer, and a greater number of antral follicles in winter compared with Farm B ovaries (P < 0.05). In Farm A ovaries, the proportion of follicles that were large was greater in summer compared with winter (78% v. 66%; P < 0.05), but the follicular distribution did not change with season in Farm B ovaries. While the findings demonstrate that ovarian weight was greater in winter compared with summer, they suggest that this difference was not the result of changes in the number or distribution of surface antral follicles. The difference in ovarian weight is possibly due to differences in ovarian tissue resulting from regressed corpora lutea. Further studies are being undertaken to assess the effect of season on oocyte developmental competence and the steroid content of follicular fluid isolated from small and large follicles.