Characterization and mitigation of pre- and postnatal heat stress through genomic selection

Heat stress (HS) negatively impacts gestating and lactating sows and results in decreased reproductive performance, lactogenesis, and welfare. In addition, offspring exposed to elevated maternal body temperature during gestation experience in utero heat stress (IUHS) which results in negative postnatal phenotypes ranging from compromised health to reduced growth. Genetic selection for improved growth, litter size, and lactogenesis has resulted in pigs with increased total metabolic heat production and decreased the thermal gradient between pigs and their environment. In addition, environmental temperatures continue to increase and add to the challenge of HS mitigation. To address the impact of modern swine genetics and rising environmental temperatures, it is crucial to develop strategies to improve heat tolerance. The first study explored whether increased maternal cortisol across the placenta could be responsible for altered programming of the conspectuses hypothalamic-pituitary-adrenal (HPA) axis in IUHS conceptuses versus in utero thermoneutral (IUTN) conceptuses. In IUHS amniotic fluid, cortisone:cortisol was decreased, cortisol tended to be increased, and cortisone tended to be decreased compared to IUTN amniotic fluid. Cortisone:cortisol tended to decrease in IUHS fetal tissue compared to IUTN conceptuses tissue. This supports the hypothesis IUHS fetuses are exposed to increased cortisol which may play a role in altered programming of the HPA axis. For the second and third studies, pigs were genomically identified as heat stress tolerant (TOL) or heat stress sensitive (SEN). This was done utilizing a genomic selection model based on the rate of internal body change in response to environmental conditions during lactation. The second study biologically characterized total metabolic heat production in F0 TOL and SEN lactating sows. The TOL sows had greater latent heat loss and more efficient use of behavioral thermoregulation (i.e. Utilization of waterer) in comparison to SEN sows. This likely explains how TOL sows were able to maintain increased total metabolic heat production compared to SEN sows while both genetic lines maintained the same internal body temperature. While an increase in total metabolic heat production is correlated with increased lactogenesis, no differences in litter body weight were observed, however we hypothesize that phenotypic differences will become more apparent with greater selection pressure in future generations. In the third study, the physiological stress response, reproductive performance, and litter characteristics were evaluated in F1 TOL and SEN gilts exposed to early gestational HS or thermoneutral conditions. In the F1 population, TOL gilts exposed to HS had decreased internal body temperature when compared to SEN gilts housed under HS conditions during early gestation. Heat stress tolerant gilts were also able to maintain larger litter sizes than SEN gilts regardless of environmental treatment. The ability to maintain a lower internal body temperature under HS conditions and increased litter size compared to SEN gilts supports that it may be possible to genomically select for gilts that are more effective at heat loss. Overall, the work described in this dissertation was to illustrate that the challenges of HS may be addressed by genomic selection for improved heat loss without decreased productivity, and to develop further understanding of the mechanisms behind IUHS phenotypes.