15. Gestational Heat Stress and the Impacts on Mammary Gland Physiology in Lactating Ewes.

Abstract Heat stress during gestation has been shown to impair fetal development and to have long-term consequences on offspring physiology and productivity in livestock species. Although previous studies have demonstrated that prenatal heat stress alters metabolic regulation and postnatal performance, less is known about how gestational heat stress influences mammary gland development in sheep. Therefore, the objective of this study was to determine the effects of gestational heat stress on mammary gland structure and developmental programming in ewes and their offspring. Pregnant ewes were exposed to either thermoneutral conditions or controlled heat stress during gestation for approximately 39 d. Lambs were separated from the dams immediately after parturition, and ewes were manually milked twice daily for 7 d. Pair-fed ewes exposed to gestational heat stress exhibited a 44% reduction in milk production compared with thermoneutral controls (P = 0.01). Mammary tissue samples were collected and evaluated using histological and immunohistochemical techniques to assess mammary gland architecture. Hematoxylin and eosin staining was used to evaluate mammary gland microstructure, whereas Masson's trichrome staining was used to assess connective tissue deposition. Exposure to gestational heat stress altered mammary gland morphology, including qualitative changes in alveolar architecture and increased stromal tissue deposition. These structural differences indicate that prenatal environmental heat disrupts normal mammary gland development and will potentially influence future lactational capacity. Collectively, these findings provide insight into how maternal heat stress may program mammary gland development and highlight the importance of managing thermal stress during gestation to improve long-term productivity and animal health in small ruminant production systems.