Does social competition affect the reproductive performance of sows moved to group housing after weaning?

This study was undertaken to examine the effects of grouping sows immediately after weaning or 4 weeks after insemination on i) the occurrence of skin injuries and ii) reproductive parameters such as weaning-to-service interval and pregnancy and culling rates. At weaning (T0), 106 sows were allocated to multiple group housing (MG, n = 41) or to individual stalls (CG, n = 65). Sows from CG remained in individual stalls until 28 days after service, in compliance with the Council Directive 2008/120/EC, and were then mixed into static groups. The occurrence, localization and severity of skin injuries and lameness were recorded 24 h after allocation ( T1) and 7 days later (T2). Sows were artificially inseminated on natural estrus, between T1 and T2. At T1, 20 of 41 (49%) sows in MG displayed cutaneous lesions. Skin injuries were localized in the regions of the head (20%) and rear legs (2%), while 24% of sows showed multiple localization; 3% of the MG sows were lame. Any lesion was recognized in CG at T1. At T2, the percentage of injured sows in MG decreased to 27%, while 1 sow from CG displayed a superficial skin lesion on the rear leg. Most MG sows showed multiple injuries (10%) and lameness (7%). Overall, 15 sows were culled for replacement, but Group had no effect on the culling rate. Among the remaining sows, 87 were inseminated with an overall 74.7% pregnancy rate (72.9% and 75.9% in MG and CG, respectively, difference p>0.05). According to the multivariable logistic regression, any factor significantly affected the pregnancy rate in MG and CG sows. These results suggested that housing sows after weaning in the multiple group with a reduced number of herd mates induced a stress due to competition for establishing hierarchy, even if this condition was quickly overcome and no negative effects of group housing were observed on the weaning-to service interval and pregnancy rate.