REPRODUCTIVE FAILURE IS ONE OF THE GREATEST CAUSES OF ECONOMIC LOSS TO THE U.S. BEEF INDUSTRY. WHILE MANY FACTORS ARE KNOWN TO IMPACT REPRODUCTIVE SUCCESS IN CATTLE, THE PRESENCE AND ACTIVITY OF SYMBIOTIC BACTERIA IN THE REPRODUCTIVE TRACT AND ITS EFFECTS ON FERTILITY IS RELATIVELY UNKNOWN. THEREFORE, THE CENTRAL HYPOTHESIS OF THIS PROPOSAL IS THAT VARIATION IN REPRODUCTIVE HORMONE SECRETION AND/OR IMMUNE FUNCTION MODULATE BACTERIAL SPECIES DIVERSITY OF THE UTERUS, THUS AFFECTING SUBSEQUENT PREGNANCY ESTABLISHMENT AND MAINTENANCE IN BEEF CATTLE. THIS RESEARCH PROPOSAL HAS TWO SPECIFIC AIMS THAT WILL EXAMINE THE EFFECTS OF THE UTERINE MICROBIOME ON FERTILITY IN CATTLE, 1) DETERMINE THE IMPACT OF REPRODUCTIVE HORMONE LEVELS (ESTRADIOL AND PROGESTERONE) BEFORE BREEDING ON THE UTERINE MICROBIOME AND PREGNANCY ESTABLISHMENT, AND 2) CHARACTERIZE CYTOKINE PROFILES BEFORE BREEDING AND DETERMINE THE IMPACT OF THESE CYTOKINES ON THE UTERINE MICROBIOME AND PREGNANCY ESTABLISHMENT. GAINING KNOWLEDGE IN THIS AREA WILL LEAD TO A BETTER UNDERSTANDING OF FACTORS THAT INFLUENCE THE REPRODUCTIVE TRACT MICROBIOME IN CATTLE. ENSUING EFFORTS WILL FOCUS ON IMPROVING FERTILITY IN BEEF CATTLE, ENHANCE MANAGEMENT PRACTICES, AND INCREASE THE SUSTAINABILITY OF THE ANIMAL PRODUCTION INDUSTRY.