**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** IMPROVING FEED EFFICIENCY (FE) OF LIVESTOCK PRODUCTION IS ESSENTIAL TO SUSTAINABLY FEED THE GROWING WORLD POPULATION. FEEDING 9M US DAIRY COWS REQUIRES MILLIONS OF ACRES OF CROPS AND ACCOUNTS FOR HALF THE COST OF DAIRY PRODUCTION. OUR LONG-TERM GOAL IS TO UNDERSTAND METABOLIC EFFICIENCY SO THAT WE CAN IMPROVE FE IN DAIRY COWS AND THUS IMPROVE OUR ABILITY TO MORE SUSTAINABLY FEED PEOPLE. FE IN DAIRY CATTLE IS A FUNCTION OF MILK PRODUCTION RELATIVE TO BODY WEIGHT AND ENERGY REQUIRED FOR GROWTH AND MAINTENANCE.TO ACHIEVE THIS, WE WILL 1) DETERMINE IF HEPATIC AND MUSCULAR MITOCHONDRIAL EFFICIENCY AND PROTEIN TURNOVER DIFFERS BETWEEN HIGH AND LOW RFI COWS; 2) ELUCIDATE DIFFERENCES IN MAMMARY NUTRIENT EXTRACTION BETWEEN HIGH AND LOW RFI COWS; AND 3) DETERMINE IF BIOMARKERS OF WHOLE-BODY METABOLISM CAN BE USED TO IDENTIFY HIGH AND LOW RFI COWS ACROSS A BROAD POPULATION. WE WILL EXECUTE THESE OBJECTIVES IN CONJUNCTION WITH AN ONGOING FFAR GRANT (2019-2024), ENABLING US TOLEVERAGE THE PHENOTYPES NEEDED TO PROVIDE DIVERGENT POPULATIONS. THE PROPOSED PROJECT WILL FOCUS ON COLLECTION OF LIVER, MUSCLE, AND BLOOD SAMPLES FOR ANALYSES OF MITOCHONDRIAL OXIDATIVE EFFICIENCY, GENE EXPRESSION OF KEY METABOLIC PATHWAYS, UPTAKE OF ENERGY METABOLITES AND AMINO ACIDS BY MAMMARY GLAND, AND GENERATION OF MODELS TO PREDICT FE THAT CAN BE APPLIED TO POPULATIONS WHERE INDIVIDUAL FEED INTAKES CANNOT BE MEASURED.WITHIN THIS PROJECT, WE EXPECT TO IDENTIFY SOURCES OF VARIATION IN FEED EFFICIENCY BETWEEN INDIVIDUAL COWS THAT ARE FED AND MANAGED SIMILARLY. IDENTIFYING THE SOURCES OF VARIANCE WILL ALLOW FOR FURTHER IMPROVING SELECTION OF FEED EFFICIENT COWS AND TO UNDERSTAND HOW TO BETTER FEED AND MANAGE COWS TO IMPROVE FEED EFFICIENCY THROUGH LEVERAGING THESE DIFFERENCES IN NUTRIENT USE.FOCUS ON THE CONTRIBUTION OF NUTRIENT USE EFFICIENCY WILL SERVE TO FURTHER IMPROVE EFFICIENCY AND SUSTAINABILITY OF MILK PRODUCTION WHICH IS KEY TO ENSURING EFFICIENT PRODUCTION OF HIGH PROTEIN FOOD SOURCES, AND FOOD SECURITY, FOR THE GROWING WORLD POPULATION. THIS NEW KNOWLEDGE WILL INFORM GENETIC SELECTION FOR THE MOST FEED EFFICIENT COWS AND WILL ALLOW FOR NUTRITIONAND MANAGEMENT RECOMMENDATIONS TO CONTINUE OPTIMIZING ANIMAL EFFICIENCY. TOGETHER, THESE OUTCOMES WILL POSITIVELYIMPACTENVIRONMENTAL, SOCIAL, AND ECONOMICAL SUSTAINABILITY OF THE ANIMAL AGRICULTURE INDUSTRY.