** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** THE EMERGENCE OF CLIMATE CHANGE THREATENS THE SUSTAINABILITY OF ANIMAL AGRICULTURE AND CREATES A CRITICAL NEED TO IDENTIFY LIVESTOCK AT THE HIGHEST RISK FOR HEAT STRESS SUSCEPTIBILITY. HOWEVER, NO SUCH ADVANCEMENTS HAVE BEEN MADE, AND PRODUCERS CANNOT PROACTIVELY MANAGE THESE HIGH-RISK ANIMALS DURING CRITICAL TIMES OF PRODUCTION. STUDIES IN ZOO WILDLIFE INDICATE THAT PREDICTIVE INDEXES, OFTEN REFERRED TO AS ALLOSTATIC LOAD INDEXES, DETERMINED FROM A PANEL OF BIOMARKERS CAN EFFECTIVELY PREDICT POOR OUTCOMES OF SUBSEQUENT STRESS. THE PROPOSED PROJECT IS DESIGNED TO DETERMINE OPTIMAL ALLOSTATIC LOAD INDEXES FOR FEEDLOT SHEEP AND CATTLE TO BETTER UNDERSTAND INDIVIDUAL DIFFERENCES IN HEAT STRESS RESILIENCY. IT WILL TEST THE HYPOTHESIS THAT THESE INDEXES WILL PREDICT POOR HEALTH AND PERFORMANCE IN ANIMALS EXPERIENCING HEAT STRESS. TO TEST THIS HYPOTHESIS, THIS STUDY WILL AIM TO CORRELATE CANDIDATE INDEX BIOMARKERS WITH PERFORMANCE OUTCOMES IN FINISHING LAMBS PRIOR TO AND THROUGHOUT CHRONIC HEAT STRESS AND IN FINISHING BEEF STEERS IN A MIDWEST FEEDLOT PRIOR TO, DURING, AND AFTER SUDDEN CHANGES IN ENVIRONMENTAL CONDITIONS. SUCCESSFUL COMPLETION OF THIS PROJECT WILL PROVIDE A MUCH-NEEDED STEP TOWARD STRATEGIES THAT IMPROVE ON-FARM DECISION-MAKING DURING HEAT STRESS EVENTS TO ALLOCATE AVAILABLE RESOURCES MORE APPROPRIATELY.