Endocrinological aspects of a spontaneous lactational ovulation in partially weaned and grouped sows

In an endocrine study on the characterization of metabolic and reproductive hormones in the periparturient and postweaning sow, 19 sows were chronical ly canulated for blood collections. Blood samples were taken once daily fro m 2 weeks before parturition until 2 weeks after weaning and additionally a t 20 min intervals for one 10-h period during each week. During the 4 weeks of lactation, sows were kept in farrowing crates together with their pigle ts all day, except about 1.5 h access to a grouped exodus without piglets. Sows had no boar contact before weaning and were not treated for oestrus in duction. All sows were offered 5.0 kg lactational feed per day (13.6 MJ ME and 165 g crude protein per kg) and were divided into two groups, suckling seven (B; balanced energy intake; n = 7) or 10-12 piglets (D; deficient ene rgy intake; n = 12). Two of the B sows and two of the D sows showed a spont aneous lactational ovulation (LO), detected by hormonal parameters. On days 15-20 after parturition, oestradiol plasma levels increased from 1.4 pg/ml to 18-36 pg/ml. A luteinizing hormone (LH) peak was detected in one of the se sows and observed over 10 h, the others had increased LH levels on the d ay of the highest oestradiol concentration. Two or 3 days later, progestero ne began to rise from 0.4-1.1 ng/ml up to 11-32 ng/ml, remained high for 16 -20 days and prohibited an onset of oestrus after weaning for 9-13 days. Th e appearance of a LO is thought to be likely at least partially a result of the daily interruption of nursing and the reduced piglet contact, as well as of the grouping during the exodus. The appearance of a LO was related to further endocrine and metabolic alterations during lactation to get inform ation on possible reasons or mechanisms of a spontaneous LO. Around parturi tion, the LO sows had a better consumption of the feed offered, and through out lactation, they tended to higher preprandial glucose plasma levels than the control sows (C) that did not have an LO. The nonesterified fatty acid s levels were lower at the end of the suckling period, and urea was higher during early and mid lactation in sows with LO. The number of piglets suckl ed did not differ and the litter performance was only 5.3% less in LO sows, but the C sows lost 7.1 kg more body weight during lactation. Plasma level s of somatotropin and prolactin tended lower, whereas insulin-like growth f actor (IGF)-1 was significantly higher than in C sows. The follicle stimula ting hormone (FSH) and LH concentrations were higher at the middle of lacta tion, the time of the developing LO. There is evidence that the higher leve ls of anabolic hormones and the lower prolactin concentrations may activate the FSH- and LH-releasing hormone (GnRH) pulse generator and elevate the g onadotropin secretion already during lactation in some sows. But the earlie r cyclicity leads to a longer weaning-to-oestrus interval of approximately 1 week due to the progesterone-mediated inhibition of oestrus immediately a fter weaning, if the lactational oestrus is not detected and the sow is not mated.