OVULATION FREQUENCY AMONG SOWS GROUP-HOUSED DURING LATE LACTATION

Ovulation frequency during late lactation was determined among 114 sow s from four commercial farms that group-housed the sows from about 3 w eeks of lactation until weaning (G-farms), and among 21 sows from one farm that kept the sows individually penned throughout lactation (C-fa rm), Ovulation frequency was determined by applying a progesterone ass ay on faecal samples collected at weekly intervals from time of groupi ng 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 r estricted food ration. During the group-housing period, 28% of the G-f arm sows ovulated, whereas none of the singly housed sows ovulated dur ing the corresponding period (P = 0.005). Ovulation frequency varied c onsiderably between sow groups (0-54%) (P = 0.004), owing partly to di fferences in age, Not a single primiparous sow ovulated, whereas ovula tion frequency among second to fourth parity sows and older sows (fift h parity and over) was 6% and 48%, respectively (P < 0,001). At the ti me 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 ha d ceased, was noted in 16% of the G-farm sows that ovulated but in onl y one (1%) of the anoestrus sows. Only 65% of the sows showing lactati onal ovulation were mated within 10 days after weaning. By contrast, 8 7% of the G-farms sows that were anoestrus during lactation and 100% o f the C-farm sows were mated within this period.