Effect of early to mid-gestation heat stress exposure on mammary development and milk traits in F1 gilts divergently selected for thermotolerance

Heat stress directly impairs the sow’s ability to produce enough milk to feed her piglets. While several heat stress mitigation strategies have been developed and utilized to improve milk production in sows, none have leveraged genomic selection as a complementary method. Therefore, the current study aimed to assess the effect of early to mid-gestation heat stress exposure on biomarkers of mammary gland development in late gestation and milk quality during lactation and to determine whether genomic selection for improved heat stress tolerance could mitigate the effects of heat stress on mammary development and milk quality in replacement gilts. We determined that early to mid-gestation heat stress exposure had carryover effects, reducing mammary epithelial cell proliferation in late gestation when animals were in thermoneutral conditions. Additionally, genomic selection for heat stress tolerance had no negative effects on mammary development and milk productivity biomarkers studied. Overall, it improved mammary epithelial integrity in F1 replacement gilts. These results suggest that while early to mid-gestation heat stress impairs mammary development, genomic selection for heat stress tolerance may enhance baseline mammary epithelial integrity, providing a resilience mechanism that supports lactation capacity regardless of environmental conditions.