Phenotypic profile of in utero heat stressed grow-finish pigs divergently selected for thermotolerance or thermosensitivity in the F2 generation

Introduction – This study evaluated the interaction between genomic selection for heat stress tolerance (TOL) or sensitivity (SEN) and exposure to in utero heat stress (IUHS) or in utero thermoneutral (IUTN) conditions in grow-finish pigs under postnatal cyclical heat stress (HS). Materials and methods – Twenty-four pigs (n = 6 per treatment; 75.8 ± 10.3 kg) were implanted with intra-abdominal core body temperature (TC) sensors and exposed to alternating thermoneutral (TN: 22-23 °C) and cyclical HS (26-36 °C) periods. The TC was continuously recorded at 5 min intervals. Skin temperature and respiration rate were recorded four times daily. Blood gases, average daily feed intake (ADFI), and average daily gain (ADG) were assessed within each period. Behavior was assessed on d 1 of TN1 and the first 3 d of each HS period. Pigs were euthanized and hypothalamus, pituitary, and adrenal glands were assessed forweight, mRNA abundance, and third ventricle cell counts. Results – Overall, TC was greater in IUHS versus IUTN pigs, and greater in IUHS+TOL and IUTN+SEN versus IUTN+TOL pigs. Respiration rate was reduced in IUHS versus IUTN pigs. Also, ADFI was greater in TOL versus SEN pigs. During TN, ADG was greater in IUHS+TOL versus IUHS+SEN and IUTN+TOL pigs. In HS1, ADG was reduced in IUHS+TOL and IUHS+SEN versus IUTN+TOL pigs. The IUHS pigs had greater CO2 partial pressure, reduced O2 partial pressure, reduced brain weight, and tended tohave greater sternal laying versus IUTN pigs. The TOL pigs had greater pituitaryweight and tended to sit less than SEN pigs. Third ventricle cell counts weregreater in IUHS+TOL and IUTN+SEN versus IUHS+SEN and IUTN+TOL pigs. No other differences were detected. Discussion – Reduced brain and pituitary mass may indicate altered neuroendocrine development. Altered blood gases may suggest respiratory changes, while hypothalamic third ventricle cellularity may have implications for HPA-axis regulation. In summary, IUHS altered physiological, morphological, and growth responses of grow-finish pigs, while genomic selection for TOL supported greater ADFI, ADG, and altered hypothalamic and pituitary development compared with SEN pigs. Genomic selection for thermotolerance, indexed by core body temperature variability relative to ambient temperature, shows promise for mitigating IUHS effects in commercial swine under future climaticconditions.