A study on primiparous sows of the ability to show standing oestrus and to ovulate after weaning. Influences of loss of body weight and backfat during lactation and of litter size, litter weight gain and season.

The ability to show standing oestrus and to ovulate within 10 days of weaning was studied in 240 purebred Swedish Yorkshire primiparous sows, fed according to a conventional feeding regime during lactation. The sows were weighted and backfat depth was recorded at farrowing and at weaning. Oestrus control was performed daily and blood samples for determination of plasma progesterone were drawn regularly in 205 sows. The distribution among the sows of the first standing oestrus after weaning had 2 peaks. The first peak occurred within 10 days of weaning and the second 24-30 days after weaning. Twelve per cent of the sows ovulated without showing standing oestrus within 10 days of weaning and 4% had an anovulatory first oestrus within the same time. Significant differences in age at farrowing and in loss of weight and backfat during lactation were found between sows which both showed standing oestrus and ovulated within 10 days of weaning and sows which neither showed standing oestrus, nor ovulated within the same time. The season during which weaning occurred significantly influenced the ability to show standing oestrus and ovulate within 10 days of weaning. Among the sows which both showed standing oestrus and ovulated within 10 days of weaning, significant positive correlations were found between weight loss, litter size, litter weight gain and the interval from weaning to first standing oestrus.