Abstract Heat stress continues to compromise swine reproductive efficiency despite decades of recognition, with seasonal infertility described as early as the 1920s and persisting in modern production systems. Advances in genetic selection have increased productivity but also metabolic heat production, thereby increasing heat stress sensitivity, particularly in gestating and lactating sows. This work focuses on the often “invisible” biological effects of heat stress on sow reproduction, emphasizing critical windows of vulnerability, underlying mechanisms, and practical mitigation strategies. Evidence demonstrates that heat stress during pre-breeding, early gestation, and late gestation reduces fertility, litter size, and piglet viability, with early gestation emerging as a particularly sensitive period. Elevated maternal body temperature during these stages disrupts oocyte quality, placental development, mammary epithelial cell proliferation, and fetal muscle fiber formation, resulting in persistent postnatal consequences. In utero heat stress induces long-term alterations in offspring growth, bioenergetics, thermoregulation, immune function, behavior, and reproductive capacity that occur regardless of postnatal environment and are mediated, in part, through epigenetic modifications. Finally, integrated applied solutions are presented, including targeted cooling strategies, updated decision-support tools, nutritional interventions to reduce physiological and psychological stress, and genomic selection approaches that improve thermotolerance without sacrificing reproductive performance. Collectively, these data underscore the need for stage-specific heat stress mitigation strategies that account for reproductive biology, long-term productivity, and animal welfare in environmentally challenging conditions.
Jay S Johnson (Wed,) studied this question.