The protective role of wallowing against heat stress in gestating and lactating sows housed outdoors

•Lactating sows had higher and more variable body temperatures than gestating sows.•Lactating sows may be at a greater risk of heat stress than gestating sows.•Time in the farrowing hut may be a risk factor for heat stress in outdoor sows.•Wallowing is likely protective against heat stress in outdoor sows.•Sows may be motivated to wallow for reasons beyond thermoregulation. The objective of this study was to describe the physiological response of gestating and lactating sows to naturally-occurring environmental conditions, and to identify factors that may contribute to or prevent heat stress, while being kept outdoors in Québec, Canada during the summer. Six groups of 4 Yorkshire-Landrace sows lived in outdoor pens equipped with a wallow, shade structure, farrowing huts and access to a pasture from July to September 2018. Between week 15 of gestation and week 3 of lactation (inclusive), we recorded the location of each sow 5 days/week during 5 daily 15-min observation periods, and additionally measured the sow's respiratory rate and mud cover at the end of each observation period. Simultaneously, we collected sow body temperature data with vaginal temperature loggers 24 h/d on week 15 of gestation and week 2 of lactation, and monitored environmental conditions with temperature and humidity loggers to calculate the temperature humidity index (THI). Sows had significantly higher and more variable body temperatures during lactation compared to gestation (P ≤ 0.0001), and when THI was analysed as a continuous variable, it was positively associated with sow body temperature during the night in lactation. During gestation, neither respiratory rate nor body temperature were associated with high or low levels of THI (P = 0.15 and 0.79, respectively) or mud cover (P = 0.29 and 0.94, respectively). However, in lactation, respiratory rate was higher when, simultaneously, THI exceeded 74 and mud cover was low (P = 0.006), while a THI higher than 74 and a low mud cover had independent effects on body temperature (P = 0.012 and 0.004, respectively). In lactation, sows that spent an entire observation period in the farrowing hut also had a higher respiratory rate than sows that left the hut at least once (P = 0.009). In summary, lactating sows were more likely to show increases in respiratory rate and body temperature in warmer conditions than gestating sows, and our findings also suggest that time in the farrowing hut may be a risk factor for heat stress. However, mud c