| Summary: | Abstract Production systems with group housing of sows during a part of the lactation are used in certified organic production and can increase the occurrence of lactational estrus thus making batch-wise breeding difficult. The aim of this study was to investigate the occurrence of lactational estrus and time at return to estrus after weaning by following the performance of the sow (change in body weight, back fat and litter size) in three different management routines. The sows and their litters were moved from individual to multi-suckling pen at one (W1; n = 14), two (W2; n = 13), or 3 weeks (W3; n = 16) post farrowing. All sows had a total lactation of 6 weeks. Ovulation was monitored by analysis of fecal progesterone metabolites. Only one sow (W3) ovulated during lactation. Sows in the W2 and W3 groups had a shorter weaning-to-standing estrus interval than W1-sows (2.6 ± 0.3; 2.7 ± 0.2 and 4.0 ± 0.3 days respectively, P < 0.001). The W1-sows and piglets might have kept their nursing bond more intact all through the group housing since the piglets were completely dependent on the nursing at the time of their move to the group pen, thereby staying in lactational anestrus and retaining standard weaning-estrous interval. There was no difference in litter size at grouping or at weaning between management routines and parities. Third and later parity sows had significantly thicker back fat at farrowing and at weaning than 1st and 2nd parity sows (P < 0.05). In conclusion, the occurrence of lactational estrus can be low in a multi-suckling pen and the interval between farrowing and move to a multi-suckling pen can affect the weaning to estrus interval. The short weaning-to-standing estrus interval seen in W2 and W3 suggests that estrus detection should start immediately post weaning for sows kept in multi-suckling pens.
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