** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** HEAT STRESS POSES A SIGNIFICANT GLOBAL CHALLENGE TO SUSTAINABLE LIVESTOCK PRODUCTION, LEADING TO DETRIMENTAL IMPACTS ON ANIMAL PRODUCTION AND WELFARE. DAIRY COWS ARE PARTICULARLY SENSITIVE TO HEAT STRESS DUE TO ELEVATED INTERNAL HEAT LOADS CAUSED BY HIGH MILK PRODUCTION AND SUFFER FROM REDUCED APPETITE, IMPAIRED RUMEN FERMENTATION FUNCTIONS, AND GUT MICROBIOTA DYSBIOSIS. THE CENTRAL HYPOTHESIS FOR THE PROPOSED RESEARCH IS THAT THE RUMINAL MICROBIAL COMMUNITIES OF DAIRY CATTLE POSSESS IMMENSE NEUROACTIVE POTENTIAL IN MODULATING HEAT STRESS RESPONSES THROUGH THE BRAIN-RUMEN AXIS. OUR RESEARCH OBJECTIVES ARE TO GAIN A HIGH-RESOLUTION RUMEN MICROBIOME CHARACTERIZATION OF LACTATING COWS UNDER HEAT-STRESSED AND HEAT-STRESS-RELIEVED CONDITIONS, UNCOVER HEAT STRESS-RELATED NEUROACTIVE SUBSTANCES DERIVED BY RUMEN MICROBIOTA AND HOST, AND EXPLORE THE UNIQUENESS IN THE BRAIN-RUMEN-MICROBIOME CROSSTALK BETWEEN HEAT-TOLERANT JERSEY COWS AND HEAT-SUSCEPTIBLE HOLSTEIN COWS. WE EXPECT TO IDENTIFY HEAT-STRESS-ASSOCIATED RUMEN MICROBES IN DAIRY CATTLE AS WELL AS UNDERSTAND THEIR NEUROACTIVE POTENTIAL AND RELATIONSHIPS WITH HOSTS' HEAT-TOLERANCE PROPERTIES AND HEAT-STRESS RESPONSES. THE FINDINGS FROM THIS PROJECT ARE EXPECTED TO ILLUSTRATE: THE CONNECTIONS BETWEEN RUMEN AND BRAIN OF RUMINANTS, HOW THIS SYSTEMATIC COMMUNICATION RESPONDS TO ENVIRONMENTAL STRESS, AND DIFFERENCES IN THIS CROSSTALK BETWEEN STRESS-SUSCEPTIBLE AND STRESS-TOLERANT ANIMALS. THESE OUTCOMES WILL DIRECT FUTURE DEVELOPMENT OF RUMEN-MICROBIOTA-BRAIN AXIS-GUIDED STRATEGIES INCLUDING TARGETED BREEDING AND PRECISE DIET MANAGEMENT TO OPTIMIZE MICROBIAL COMMUNITIES, AND THE DEVELOPMENT OF PROBIOTICS SUCH AS MICROBE-DERIVED NEUROACTIVE ADDITIVES TO ALLEVIATE HEAT STRESS RESPONSES AND IMPROVE ANIMAL WELFARE.