**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** WEANING PIGS BETWEEN THREE AND FOUR WEEKS OF AGE IMPAIRS NORMAL DEVELOPMENT OF THE SMALL INTESTINE, REDUCES FEED INTAKE, AND LOWERS THE EFFICIENCY OF USING DIETARY ENERGY AND PROTEIN FOR GROWTH. TOGETHER, THESE CONTRIBUTE TO POST-WEANING GROWTH LAG, A PERIOD IMMEDIATELY AFTER WEANING CHARACTERIZED BY POOR GROWTH PERFORMANCE. THE SULFUR AMINO ACIDS METHIONINE AND CYSTEINE HAVE KEY ROLES IN GUT HEALTH AND GUT FUNCTION, BUT HOW THEIR METABOLISM IS AFFECTED BY WEANING IN PIGS IS NOT FULLY UNDERSTOOD. WE HYPOTHESIZE THAT LOW FEED INTAKE AT WEANING LOWERS CYSTEINE UTILIZATION FOR GUT GLUTATHIONE PRODUCTION, A MAJOR ANTIOXIDANT, DESPITE ADEQUATE AVAILABILITY OF CYSTEINE FROM ENDOGENOUS SOURCES, SUCH AS THE LIVER AND BLOOD. THE PROJECT OBJECTIVES ARE TO 1) IDENTIFY THE EXTENT TO WHICH FEED INTAKE AFTER WEANING IMPACTS THE ABUNDANCE OF KEY PROTEINS AND METABOLITES RELATED TO SULFUR AMINO ACID METABOLISM AND 2) DETERMINE HOW REDUCTIONS IN FEED INTAKE AFTER WEANING AFFECT CYSTEINE PARTITIONING FOR GLUTATHIONE PRODUCTION VERSUS PROTEIN SYNTHESIS. WE EXPECT THAT LOW FEED INTAKE DECREASES CYSTEINE USE FOR GUT GLUTATHIONE PRODUCTION AND THAT, ALTHOUGH LIVER AND BLOOD CYSTEINE LEVELS INCREASE AFTER WEANING, LIVER AND BLOOD CYSTEINE IS NOT EFFECTIVELY USED FOR GUT GLUTATHIONE PRODUCTION IN NEWLY WEANED PIGS. UNDERSTANDING THE INTERACTION BETWEEN LOW FEED INTAKE AT WEANING AND SULFUR AMINO ACID METABOLISM WILL HELP DEVELOP NEW STRATEGIES THAT SUPPORT GUT HEALTH AND GUT FUNCTION AND REDUCE POST-WEANING GROWTH LAG IN PIGS. A CUT IN THIS GROWTH LAG OF ONE DAY ALONE CAN AMOUNT TO SAVINGS OF $0.60 PER PIG IN FEED COSTS, SCALING TO MILLIONS OF DOLLARS PER YEAR, TO PORK PRODUCERS ACROSS THE U.S.