A preliminary assessment of the effects of summer transport on behavioral and physiological stress biomarkers in cull sows selected for heat stress tolerance or sensitivity in the F1 generation1

Cull sows transport in hot summer months can cause morbidity and mortality due to heat stress (HS), and there is a need to reduce these risks. Therefore, the study objective was to evaluate whether genomic selection for HS tolerance (TOL) or sensitivity (SEN) in the F1 generation would alter behavioral and physiological indicators of HS in cull sows transported during the summer. We hypothesized that TOL sows would have a reduction in behavioral and physiological stress responses relative to SEN sows during summer transport. One day post-weaning, 14 TOL and 14 SEN primiparous sows (Large White x Landrace) were loaded onto a trailer (0.65 m2/sow) and transported for 4 h 53 min on 50% two-lane and 50% four-lane roads in central Indiana, USA on July 26, 2023. Data loggers were placed within the trailer to record dry bulb temperature and relative humidity in 5 min intervals. Prior to transport, 9 TOL and 7 SEN sows were fitted with data loggers to record vaginal temperatures (TV) in 5 min intervals. In addition, a thermal imaging camera was used to record temperatures of the periocular area and lacrimal caruncle and salivary cortisol was assessed in all sows pre- and post-transport. Finally, sows were imaged throughout the transport process using mounted cameras to evaluate standing, lying, sitting, and mounting behaviors. All data were analyzed using R software V4.1.1. and the "lmea4", "lmertest", and "nlme" R packages with the sow considered as the experimental unit. Dry bulb temperature, relative humidity, and Enthalpy Comfort Index (ECI) during the trip were 29.7 ± 0.2 °C, 70.33 ± 1.74%, and 76.45 ± 2.76 kJ/kg dry air, respectively. Lacrimal caruncle temperature was greater (P < 0.05) for SEN (34.1 ± 0.40 °C) versus TOL (32.7 ± 0.40 °C) sows. Sitting behavior was positively associated with transport time (β = 0.10; P < 0.01). Likewise, lying behavior was positively associated with transport time (β = 0.03; P < 0.05) and with ECI (β = 1.05; P < 0.01). Salivary cortisol levels were increased overall (P < 0.01) during loading (27.1 ± 2.64 ng/mL) versus post-transport (6.9 ± 3.07 ng/mL), regardless of the genetic line. No other differences were detected with any comparison (P > 0.10). In summary, cull sow transportation during the summer increased behavioral and physiological signs of stress in sows. However, only lachrymal caruncle temperature was influenced by genomic selection in the F1 generation.