Effects of group housing after weaning on sow welfare and sexual behavior.

This project compared the effects of grouping sows after weaning or within 2 d after insemination on sexual behavior, aggression, injuries, stress, and mating success. At weaning (d 0), 360 sows were housed in groups of 10 sows at 4.4 m2 per sow (group weaned [GpW]) or individual stalls (stall weaned [StW]), with 18 groups per treatment. Six days after weaning (d 6), 7 inseminated GpW sows were moved to pens at 2.1 m2 per sow and keeping acquainted sows, and simultaneously, groups of 7 inseminated StW sows were mixed at 2.1 m2 per sow. Group-weaned sows showed greater variation in the onset of estrus (P=0.02) but not in the length of estrus compared to StW sows (P=0.21), with 7% fewer GpW sows inseminated within 5 d of weaning (P=0.05). Group-weaned sows showed lower sexual receptivity scores, showing less spontaneous standing during boar exposure and partly compensating by a greater response to the back-pressure test (both P<0.01). The GpW treatment also showed greater variability in sows inseminated twice within 6 d of weaning, with 3 out of 18 pens having only 5 mated sows out of 10. Mixing after weaning resulted in higher levels of stress than mixing after insemination, with GpW sows having higher plasma cortisol concentration than StW sows on d 1 (P<0.001) but no treatment differences on d 7 in cortisol concentration or aggression at feeding (P=0.48). Group-weaned sows experienced greater weight loss during the first week postweaning (P=0.05). Anogenital sniffing in GpW sows was frequently observed from d 2 to 5, but mounting and flank nosing increased on d 4 and 5. Frequency of sexual behavior initiated by GpW sows tended to correlate with weight loss (P=0.08), and sexual behavior received correlated positively with cortisol concentration at d 1 (P=0.005). In conclusion, sows housed in groups at weaning and regrouped after insemination experienced higher stress than sows housed in individual stalls at weaning and mixed in groups after insemination. This resulted in lower mating success within 5 d of weaning, which in turn increased between-week variability. The lower sexual receptivity in sows grouped at weaning may be due to suppressed estrus-related behaviour, with ovulation occurring, or delayed ovulation beyond d 6. Further research is needed to identify underlying mechanisms to reduce variability, manage aggression and sexual behavior, and optimize estrus detection in group-housed weaning systems.