Seasonal effects on reproduction in the domestic sow in Finland--a herd record study.

Seasonal effects on fertility of the domestic sow were assessed by retrospective analysis of the Finnish national computerised data management system covering 1081 herds in 1993. Multivariate analyses were used, where the reproductive parameter of interest (repeat breeding, weaning to oestrus interval, age of gilts at first farrowing, litter size, culling due to anoestrus or no conception) was designed as the response variable. The months of the year (each month compared with January) and all herds and breed were included in the models as explanatory variables. The study demonstrated clear seasonal effects on various aspects of fertility in the domestic sow. The poorest reproductive performance was consistently observed in late summer and autumn and was demonstrated in a number of ways. Firstly, the gilts born between December and April were older (> 5 days) at farrowing than those born during the rest of the year (p < 0.01). Secondly, the risk that a culled sow would be culled due to anoestrus was significantly increased during the autumn months (Odds Ratio (OR) ranged from 1.10 to 1.36). Thirdly, the risk of a repeat breeding was higher from July to November (OR = 1.16). Risk of a prolonged weaning-to-oestrus beyond day 10 was the highest from August to October (OR ranged from 1.70 to 1.77). Risk of a sow to be culled due to no conception was the highest in January and February (weaned in October-November). In addition, descriptive data gathered in a slaughterhouse in 1993 (a subpopulation of the sows included in the herd records) suggest that incidence of inactive ovaries is increased in summer-autumn (p < 0.05). In conclusion, a marked reduction in fertility of the domestic sow in Finland is reported between July and November.