MILK FROM DAIRY COWS IS AN IMPORTANT SOURCE OF NUTRIENTS FOR HUMAN DIETS.FEED MAKES UP NEARLY 2/3 OF THE TOTAL COST OF GROSS DAIRY FARM EXPENSES IN THE U.S., AND AS A RESULT, IMPROVING WHOLE FARM-FEED EFFICIENCY ISPRIMARY DRIVER OF FARM PROFITABILITY. FURTHERMORE, IMPROVING FEEDEFFICIENCY IS THE MOST IMPACTFUL WAY TO IMPROVE ENVIRONMENTAL SUSTAINABILITY OF DAIRY FARMS. IMPROVING OUR UNDERSTANDING OF HOW SYSTEMIC METABOLISM OF DAIRY COWS ARE REGULATEDBY CIRCADIAN RHYTHMS, WHICH ARE ENDOGENOUS BIOLOGICAL CYCLES OF 24 H, COULD BE A POTENTIAL WAY TOINCREASE FEED EFFICIENCY OF DAIRY COWS. ONE OF THE MAJOR ROLES OF CIRCADIAN RHYTHMS IS TO COORDINATE NUTRIENT AVAILABILITY WITH METABOLISM TO MAXIMIZE ENERGY UTILIZATION.WE PROPOSE THAT THE LIVER CIRCADIAN CLOCK IN DAIRY COWS IS IMPORTANT FOR REGULATING GLUCOSE AND FATTY ACID METABOLISM IN DAIRY COWS, LEADING TO DIURNAL DIFFERENCES IN METABOLISM OF ENERGY SUBSTRATES. THEREFORE, THE OBJECTIVE OF THIS PROJECT TO CHARACTERIZE THE CIRCADIAN CLOCK OF THE LIVER, IDENTIFY HOW IT IS ENTRAINED BY FEED INTAKE AND LIGHTING, AND DETERMINE IF DAILY TIMING OF ENERGY SUBSTRATES INFLUENCE ENERGY EFFICIENCY, LIVER METABOLISM AND OXIDATIVE STRESS.THIS PROJECT WILL LEAD TO NOVEL FUNDAMENTAL INFORMATION THAT WILL GREATLY ENHANCE OUR UNDERSTANDING OF THE REGULATION OF CIRCADIAN RHYTHMS IN DAIRY COWS. FURTHERMORE, THIS PROPOSAL WILL GENERATE APPLIED KNOWLEDGE ABOUT HOW FEEDING DIFFERING AMOUNT OF ENERGY SUBSTRATES ACROSS THE DAY AFFECTS EFFICIENCY AND PRODUCTIVITY OF COWS.