Parturition and subsequent uterine health and fertility in sows

There has been a steady increase in litter size, and consequently farrowing duration, in modern pig production during recent decades. At the same time, annual removal rates due to reproductive failure have increased and an association with litter sizedemonstrated. Underlying physiological mechanisms for the associations between these phenomena are not well established. It is plausible that the increase in farrowing duration is causally linked with the increase in litter size because it takes longer to farrow a larger litter. Therefore, it is possible that prolonged farrowing negatively affects subsequent reproductive performance of sows.Postpartum dysgalactia syndrome (PDS), the major puerperal disease complex of sows, can cause subfertility, but it is not clear that this is linked with prolonged farrowing. However, PDS and prolonged farrowing represent similar risk factors regarding increased backfat, constipation, low dietary fiber, and restricted movement in farrowing crates. In some studies there was evidence of a connection between prolonged farrowing and increased incidence of general clinical symptoms such as fever and anorexia. Therefore, it is feasible that prolonged farrowing increases the risk for PDS, which in turn increases the risk of subsequent subfertility. In order to understand better these tentative relationships it makes sense to look at the different symptoms dividually, it being most likely that prolonged parturition in particular affects the incidence of metritis. Puerperal metritis is an acute inflammation and infection of the uterus that occurs during the first week after parturition and is characterized by an enlarged uterus that results from fluid accumulation. In other species, including the cow, a long and difficult parturition increases the risk of puerperal metritis, which in turn delays uterine involution and subsequently reduces fertility by disturbing postpartum ovarianfollicular growth and ovulation. Furthermore, long and difficult parturitions can cause retained placenta, which is of major consequence in the etiology of postpartum metritis. We hypothesized that prolonged farrowing decreases subsequent fertility, that is, pregnancy rate. Furthermore, we hypothesized that prolonged farrowing causes retention of placentae and metritis, delays uterine involution and perturbs follicular growth after weaning. We also hypothesized that sows that undergo prolonged farrowing release less oxytocin at subsequent estrus in response to boar stimulation. It is also known that prolonged parturition is linked with reduced oxytocin concentrations. We therefore speculated that this is the case also at subsequent breeding. At that time, oxytocin is needed for sperm transport to the site of fertilization and formation of the sperm reservoirs and thus, impairment of transport may have a negative effect on the establishment of pregnancy. Furthermore, corpus luteum function is important for establishment and maintenance of pregnancy. The primary function of the corpus luteum is secretion of progesterone, thepregnancy hormone, and this function depends on other hormones active before embryonic attachment and maternal recognition. Whether corpus luteum function is compromised in sows with prolonged farrowing was not investigated in this study, but validation and testing of a transvaginal ultrasound-guided biopsy method for luteal tissue was done. This represents an easy and reliable tool for future investigations of corpus luteum function. In the first part of the study we determined the farrowing duration (time between expulsion of first and last piglet) of sows and explored whether there is a negative effect on subsequent post-weaning pregnancy rate (n = 148). In the following part, we explored the relationship between farrowing duration and placenta expulsion (n = 142), and postpartum uterine size and intrauterine fluid (n = 99). For that, placenta expulsion was observed until 24 h after birth of the last piglet and ultrasonography was used during the first week postpartum to examine the uteri of the sows. Uterine size and intrauterine fluid were used as indicators for initial uterine involution and puerperal metritis. Furthermore, wedetermined the farrowing duration of sows (n = 30) and monitored the subsequent follicular development using transrectal ultrasound twice a day between the third day after weaning and ovulation. At estrus, blood samples were collected in the presence of a boar in order to determine the endogenous oxytocin concentrations and release patterns. The results show that sows with a prolonged parturition (> 300 min) were 3.4 (odds; P = 0.027) more likely not to be pregnant. Farrowing duration was highly ssociated with retained placentae. Sows that experienced total (no expulsion of placental parts; 3%) and partially retained placentae (no expulsion of placental parts after birth of the last piglet; 3%) had longer farrowing durations (1009 ± 275 and 734 ± 136 min) than sows without retained placentae (369 ± 202 min; P = 0.021 and P = 0.004). Otherwise, farrowing durationconformed to a quadratic relationship with the number of expelled placental parts (P = 0.001), placental expulsion duration (time between expulsion of first and last placental part; P = 0.002) and time between expulsion of last piglet and last placental part (P =0.024). On the other hand, use of oxytocin increased number of expelled placental parts (LSmeans ± SD, 3.8 ± 0.2 vs. 2.9 ± 0.3, P = 0.035), decreased the placental expulsion duration (172 ± 44 vs. 328 ± 26 min, P = 0.011) and time between expulsion of last piglet and last placental part (148 ± 48 vs. 300 ± 24 min, P = 0.025). Furthermore, prolonged farrowing (β ± SE, Wald χ2, Odds; 2.0 ± 0.5, 13.1, 7.6; P = 0.001), obstetrical intervention (1.5 ± 0.7, 4.4, 4.3; P = 0.036) and two or more stillborn piglets (1.4 ± 0.7, 3.8, 3.9; P = 0.052) increased the risk of having enlarged uterine size whereas oxytocin administration (- 1.5 ± 0.7, 4.7, 0.2, P = 0.040) decreased the risk. Two or more stillborn piglets (2.6 ± 0.9, 8.7, 13.7; P = 0.003), obstetrical intervention (1.8 ± 0.8, 5.0, 6.0; P = 0.025), prolonged farrowing (1.7 ± 0.8, 4.3, 5.7; P = 0.039) and impaired placenta expulsion (3.3 ± 1.7, 4.0, 26.9; P = 0.044) were associated with intrauterine fluid. After weaning, OT concentrations were higher in sows with prolonged farrowing than in sows with shortened farrowing (LSmeans ± SD, 28.0 ± 7.7 vs. 20.6 ± 7.7, P < 0.01). OT concentrations correlated with diameters of the follicles measured 5 d after weaning and when the follicles reached their maximum size after weaning (r = 0.61, P < 0.01 and r = 0.57, P < 0.01, respectively). The results show that sows with prolonged farrowing can show subsequent ubfertility, which may be linked with compromised uterine health. Sows with prolonged farrowing are at higher risk of impaired placental expulsion, puerperal metritis and impaired uterine involution. Using exogenous oxytocin after parturition may be useful in sows with prolonged farrowing in order to lower the risk for retained placentae and subsequent metritis. Obstetrical intervention should be careful and done under antiseptic conditions. Proper parturition management should be used in order to prevent piglets being stillborn. Against expectations, sows with prolonged farrowing duration had higher oxytocin concentrations and larger follicles at subsequent estrus. This merits further investigation. In addition, we adopted and tested a transvaginal ultrasound-guided biopsy method for luteal tissue, which can be used to study corpus luteum function. No effects were observed on the cyclicity and morphology of the reproductive organs of the sows: not on corpus luteum function, pregnancy rate, gestation length or subsequent litter size. A biopsy was obtained for 50% of all attempts. Sows from which at least one biopsy was obtained were older (parity 5.0 ± 2.8 vs. 2.2 ± 0.4; mean ± SD), heavier (290 ± 26 vs. 244 ± 27 kg) and had more backfat (11.4 ± 2.7 vs. 6.4 ± 2.5 mm; P < 0.05 for all) compared with sows from which no samples could be obtained.