Effects of nursing a large litter and ovarian response to gonadotropins at weaning on subsequent fertility in first parity sows

Post-weaning fertility failures occur more often in parity 1 (P1) sows due to high metabolic demands for lactation and their inability to meet energy requirements for maintenance, growth, and reproduction. We hypothesized that body condition loss occurs more frequently in P1 sows nursing a large litter, resulting in impairment of ovarian follicle development during lactation and post-weaning, which can negatively impact estrus and subsequent fertility. At 24 h post-farrowing, P1 sows (n = 123) were assigned to treatment (TRT) based on sow weight and the number of functional teats to receive a high number (HN, 15 to 16) or low number (LN, 12) of nursing piglets. At weaning, sows in each TRT were assigned to receive PG600 or None (Control). During lactation, sow body measures were obtained and ovarian follicles were assessed in mid-lactation and post-weaning. Lactation data were analyzed for the effects of TRT, and fertility data after weaning were assessed for TRT x PG600, but there were no interactions (P > 0.10). During lactation, 22.2 % of HN sows lost ≥ 4 piglets due to death or removal, and so these sows were excluded from further analysis. The HN sows were lighter (-6.2 kg), had less backfat (-1.0 mm), had lower body condition score (-0.4), and lost more nursing piglets (-1.2) than LN sows (P < 0.05). However, HN sows weaned more pigs (14.0) than LN sows (11.0). There was no effect of TRT on wean to estrus interval (4.2 d), but the interval was 0.5 days shorter for PG600 (P = 0.004) than control. There were no effects of TRT or PG600 on estrus within seven days after weaning (87.3 %), but PG600 induced smaller (P = 0.002) follicles at estrus (6.7 mm) than control (7.3 mm). In the subsequent parity, there were no effects of TRT or PG600 on farrowing rate (93.9%) and total born (13.2). Overall, HN sows lost more piglets and body condition but still weaned more pigs without any detrimental effects on subsequent reproductive performance.The motivation for this study was to determine if assigning parity one (P1) sows to nurse a large number of piglets (15 to 16) would cause substantial body condition loss and have detrimental consequences for fertility after weaning. The results showed that when P1 sows nurse 15 to 16 piglets, they lose more body condition and more piglets than P1 sows nursing 12 piglets. However, there was no impact on the fertility of these sows. This study demonstrates that P1 sows can nurse a high number of piglets and still have a high potential to be fertile after weaning their piglets. Still, there is potential to improve management to avoid excessive weight loss in sows and piglet losses.