Ovulation frequency among sows group-housed during late lactation

Ovulation frequency during late lactation was determined among 114 sows from four commercial farms that group-housed the sows from about 3 weeks of lactation until weaning (G-farms), and among 21 sows from one farm that kept the sows individually penned throughout lactation (C-farm). Ovulation frequency was determined by applying a progesterone assay on faecal samples collected at weekly intervals from time of grouping until 3 weeks after weaning. The groups consisted of 11–22 sows and boar contact was not allowed during the 5–6 week lactation period. G-farm sows were fed ad libitum while C-farm sows were provided with a restricted food ration. During the group-housing period, 28% of the G-farm sows ovulated, whereas none of the singly housed sows ovulated during the corresponding period (P = 0.005). Ovulation frequency varied considerably between sow groups (0–54%) (P = 0.004), owing partly to differences in age. Not a single primiparous sow ovulated, whereas ovulation frequency among second to fourth parity sows and older sows (fifth parity and over) was 6% and 48%, respectively (P < 0.001). At the time of grouping and weaning, neither backfat thickness nor litter size differed between the sows that ovulated and those that were anoestrous. Preweaning mammary gland atrophy, indicating that milk production had ceased, was noted in 16% of the G-farm sows that ovulated but in only one (1%) of the anoestrus sows. Only 65% of the sows showing lactational ovulation were mated within 10 days after weaning. By contrast, 87% of the G-farms sows that were anoestrus during lactation and 100% of the C-farm sows were mated within this period.