GLOBAL CLIMATE CHANGE HAS A MASSIVE IMPACT ON AGRICULTURAL OPERATIONS. IN THE UNITED STATES ALONE, MILK PRODUCTION LOSSES ASSOCIATED WITH HEAT STRESS DURING LACTATION COST THE DAIRY INDUSTRY UP TO $1.5 BILLION ANNUALLY. WHILE MOST DAIRY PRODUCERS AROUND THE GLOBE UNDERSTAND THE VALUE OF COOLING LACTATING COWS TO LIMIT THE VERY VISIBLE DROP IN MILK PRODUCTION, MANY STILL DO NOT COOL DRY COWS. THE DRY PERIOD IS A NON-LACTATING PERIOD BETWEEN LACTATIONS AND IS CONCURRENT WITH THE LAST TRIMESTER OF GESTATION IN DAIRY CATTLE. EXPOSURE OF THE PREGNANT DAM TO HEAT STRESS DURING THE DRY PERIOD SIGNIFICANTLY REDUCES HER MILK PRODUCTION BY ABOUT 5 KG/D IN THE FOLLOWING LACTATION, BUT ALSO LEADS TO TRANSGENERATIONAL EFFECTS ON HER OFFSPRING. FOR EXAMPLE, FEMALE CALVES BORN TO HEAT STRESSED DAMS WILL PRODUCE ~3.5 KG/D LESS MILK DURING THEIR FIRST AND SECOND LACTATIONS AND ARE MORE LIKELY TO BE CULLED FROM THE HERD RELATIVE TO THEIR UNEXPOSED HEARD MATES. OUR HYPOTHESIS IS THAT IF AN UNBORN DAIRYCALF EXPERIENCES SUBOPTIMAL THERMAL CONDITIONS IN UTERO, THEY WILL EXPERIENCE ABNORMAL GROWTH AND DEVELOPMENT OF THEIR MAMMARY GLANDS LEADING TO IMPAIRED MILK PRODUCTION IN ADULTHOOD. SPECIFIC OBJECTIVES ARE TO: 1) DETERMINE HOW IN UTERO EXPOSURE TO HEAT STRESS IMPACTS MAMMARY GLAND GROWTH AND DEVELOPMENT AT BIRTH, WEANING AND AROUND PUBERTY, 2) DETERMINE HOW INTRAUTERINE HEAT STRESS IMPACTS GROWTH OF THE MAMMARY GLAND IN RESPONSE TO HORMONES DURING GESTATION, AND 3) TO ASSESS THE RESPONSE OF MAMMARY TISSUE LATER IN LIFE TO THERMAL CONDITIONS THAT ARE SIMILAR TO OR DIFFERENT FROM THOSE EXPERIENCED DURING FETAL DEVELOPMENT. WE EXPECT TO IDENTIFY THE MAMMARY HISTOLOGICAL AND MOLECULAR CHANGES INITIATED BY EXPOSURE TO HEAT STRESS IN UTERO THAT DEVIATE FROM THE NORMAL DEVELOPMENTAL TRAJECTORY OF THE MAMMARY GLAND. THESE CHANGES MIGHT EXPLAIN THE IMPAIRED MAMMARY GLAND FUNCTION AND MILK YIELD OF THOSE ANIMALS LATER IN LIFE WHEN THEY BEGIN LACTATING. THIS PROPOSAL WILL CONTRIBUTE TO FUNDAMENTAL CONCEPTS OF FETAL PROGRAMMING IN MAMMARY GLAND DEVELOPMENT AND FUNCTION IN DAIRY RUMINANTS, ENHANCE THE USE OF DRY COW COOLING AS A MANAGEMENT STRATEGY TO INCREASE MILK YIELD ACROSS MULTIPLE GENERATIONS, AND CHARACTERIZE CRITICAL WINDOWS OF MAMMARY DEVELOPMENT THAT CAN BE TARGETED FOR INTERVENTIONS TO ALLEVIATE THE NEGATIVE EFFECTS OF IN UTERO HEAT STRESS ON FUTURE LACTATION PERFORMANCE.