Characterizing the effects of gestational heat stress on reproductive efficiency and thermoregulation in replacement gilts divergently selected for thermotolerance in the F1 generation.

This study evaluated whether genomic selection for heat stress tolerance (TOL) in gilts improved thermoregulatory responses and reproductive and litter outcomes when compared to heat stress sensitive (SEN) gilts. An F0 sow population was previously identified based on genomic breeding values for TOL and SEN. In this study, F1 TOL (n = 15) and SEN (n = 13) gilts (Large White × Landrace) were bred to TOL or SEN Landrace boars. Pregnant gilts were housed individually and exposed to thermoneutral (TN; 21.46 ± 2.92 °C; n = 7 TOL and 6 SEN) or cyclic HS (26-36 °C; n = 8 TOL and 7 SEN) from gestation d 6.3 ± 0.9 to 65.3 ± 0.9 (ETP), then TN (21.40 ± 3.13 °C) until farrowing (LGP). Gilts were limit-fed 1.82 kg/d with ad libitum access to water. Vaginal temperature (TV) was recorded every 10 min, respiration rate (RR) was measured at 0800 and 1500 h, and body weight was measured weekly. Hair cortisol was measured on d 3.3 ± 0.9 and 67.3 ± 0.9 of gestation. The number of liveborn, stillborn, and mummified piglets was recorded. Placenta samples were collected (n = 1 male and 1 female per litter) to assess placental 11β-hydroxysteroid dehydrogenase-2 (11β-HSD2) and cortisol concentrations. Data were analyzed as a 2 x 2 factorial with experimental units as either gilt or individual piglet. Vaginal temperature was greater (P < 0.01; 0.31 °C) in HS versus TN gilts and tended to be reduced (P = 0.06; 0.10 °C) in HS+TOL versus HS+SEN gilts. Respiration rate increased (P < 0.01; 20 bpm) in HS versus TN gilts. Average daily gain was greater (P = 0.02; 17.0%) in HS versus TN gilts in the ETP but reduced (P < 0.01; 69.4%) in HS versus TN gilts during the LGP. Hair cortisol was greater (P = 0.02; 81.4%) in HS versus TN gilts at the end of the ETP, and salivary cortisol increased (P = 0.04; 57.8%) for HS versus TN gilts during the LGP. Mummified conceptuses increased (P = 0.04; 105.35%) in HS versus TN gilts. Total number of piglets born tended to be greater (P = 0.09; +1.9 piglets) in TOL versus SEN gilts. Placental 11β-HSD2 tended to be greater (P = 0.09; 19.05%) in TN+TOL piglets versus TN+SEN, and placental cortisol tended to be greater (P = 0.09; 21.54%) in TOL versus SEN piglets. Genomic selection for TOL tended to reduce TV under HS and increase total born in the F1 generation and HS increased chronic stress in gestating gilts.