**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** INCREASING PRODUCTIVITY (GROWTH RATE, FEED EFFICIENCY, FERTILITY) WHILE DIMINISHING THE NEGATIVE EFFECT OF HEAT STRESS IS THE BEST STRATEGY TO SUSTAINABLY MEET GLOBAL DIETARY NEEDS AND FOOD SECURITY IN A HOTTER WORLD. UNDERSTANDING THE GENETIC ARCHITECTURE OF TRAITS DEFINING HEAT TOLERANCE AND PRODUCTIVITY IN CATTLE WILL ALLOW US TO DEVELOP THE KNOWLEDGE AND TOOLS THE CATTLE INDUSTRY NEEDS TO INCREASE TOLERANCE TO HEAT STRESS, WHILE SIMULTANEOUSLY ALLOWING FOR INCREASED EFFICIENCY OF PRODUCTION AND REPRODUCTION.WE WILL COLLECT HEAT TOLERANCE MEASURES (OBJECTIVE 1) TO IDENTIFY 400 HEIFERS WITH HIGH AND LOW HEAT TOLERANCE. WE WILL DETERMINE THE NATURE AND MAGNITUDE OF THE GENETIC RELATIONSHIPS BETWEEN HEAT TOLERANCE, FEED EFFICIENCY AND REPRODUCTIVE PERFORMANCE (OBJECTIVE 2). WE WILL IDENTIFY KEY-REGULATOR VARIANTS FOR HEAT TOLERANCE WITH POSITIVE OR NO EFFECT ON PRODUCTION USING FUNCTIONAL GENOMICS IN A SYSTEMS BIOLOGY APPROACH (OBJECTIVE 3).THE IN-DEPTH KNOWLEDGE OF HEAT TOLERANCE GENETIC ARCHITECTURE GENERATED WILL ALLOW US TO DEVELOP GENOMIC TOOLS FOR SIMULTANEOUS IMPROVEMENT OF TOLERANCE TO HEAT STRESS AND PRODUCTION AND REPRODUCTION PERFORMANCE. THIS WILL HAVE A SIGNIFICANT IMPACT ON ENHANCING THE PRODUCTIVITY OF THE US LIVESTOCK INDUSTRY AND SECURE GLOBAL FOOD SUPPLIES THROUGH EFFECTIVE STRATEGIES TO IMPROVE THE ABILITY TO COPE WITH HEAT STRESS.