** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** REPRODUCTIVE FAILURE DUE TO EARLY PREGNANCY LOSS IS ONE OF THE GREATEST CAUSES OF ECONOMIC LOSS TO THE U.S. DAIRY INDUSTRY. HEAT STRESS FURTHER EXACERBATES THESE ECONOMIC IMPACTS AND PRODUCTION LOSSES ARE PROJECTED TO INCREASE WITHIN THE NEXT 5 YEARS. AS THE FOURTH NATIONALLY RANKED STATE FOR MILK PRODUCTION, TEXAS DAIRY PRODUCERS ARE HIGHLY IMPACTED BY BOTH EMBRYONIC LOSS AND HEAT STRESS. WHILE MANY FACTORS ARE KNOWN TO IMPACT REPRODUCTIVE SUCCESS IN CATTLE,THE PRESENCE AND ACTIVITY OF BACTERIA IN THE UTERUS AND ITS EFFECTS ON EMBRYONIC LOSS IN HEAT STRESSED CATTLE IS RELATIVELY UNKNOWN. CHANGES IN REPRODUCTIVE HORMONE SECRETION AND/OR CONCEPTUS-BASED PREGNANCY MARKERS MAY BE ALTERED DURING HEAT STRESS CONDITIONS DUE TO THE UTERINE MICROBIOME PRESENT AT EMBRYO TRANSFER. THEREFORE, THE CENTRAL HYPOTHESIS OF THIS PROPOSAL IS THAT COMPOSITION OF THE FEMALE REPRODUCTIVE TRACT MICROBIOME AT EMBRYO TRANSFER WILL SHIFT UNDER HEAT STRESS CONDITIONS AND IMPACT PREGNANCY OUTCOMES IN DAIRY HEIFERS.THIS RESEARCH PROPOSAL WILL UTILIZE HOLSTEIN DAIRY HEIFERS TO SWAB THE UTERUS DURING HEAT STRESSED CONDITIONS, PRIOR TO EMBRYO TRANSFER TO DETERMINE THE BACTERIAL POPULATIONS PRESENT AND ANALYZE THIS DATA ALONGSIDE PREGNANCY STATUS DATA. HEIFER HORMONE AND PROTEIN PROFILES (E.G., PROGESTERONE, PREGNANCY ASSOCIATED GLYCOPROTEIN, IFNT) WILL BE MEASURED AND ASSESSED TO EVALUATE THE ESTABLISHMENT AND MAINTENANCE OF PREGNANCY AGAINST THE MICROBIOTA DATA. A SUBSET OF HEIFERS WILL BE UTILIZED TO EVALUATE THE MATERNAL-FETAL INTERFACE TO DETERMINE CONCEPTUS DEVELOPMENT UNDER HEAT STRESSED CONDITIONS FOLLOWING EMBRYO TRANSFER AND THEN ANALYZED WITH THE UTERINE BACTERIA DATA. ULTIMATELY, GAINING KNOWLEDGE IN THIS AREA WILL LEAD TO A BETTER UNDERSTANDING OF THE RELATIONSHIPS BETWEEN THE REPRODUCTIVE MICROBIOME, EMBRYONIC LOSS, AND HEAT STRESS IN CATTLE TO RESULT IN REDUCING ECONOMIC LOSSES EXPERIENCED BY CATTLE PRODUCERS. ENSUING EFFORTS WILL FOCUS ON IMPROVING REPRODUCTIVE EFFICIENCY IN DAIRY CATTLE, DEVELOPING SOLUTIONS AND ENHANCING MANAGEMENT PRACTICES TO REDUCE EMBRYONIC LOSS, AND INCREASING THE SUSTAINABILITY OF THE ANIMAL PRODUCTION INDUSTRY.