<p>  </p> <p>In commercial settings, piglet weaning and transportation occur concurrently due to the expansive application of multi-site production systems across the United States and the combination of these events can be defined as an early life stressor. Early life stress is known to reduce pig performance, efficiency, and immune resilience contributing to reduced welfare and increased production losses. To combat the deleterious effects of stress on pigs, the essential amino acid tryptophan (Trp) supplemented above current National Research Council (NRC 2012) recommended levels, improves neuroendocrine responses to stress, lowers plasma cortisol and norepinephrine concentrations, and improves hypothalamic-pituitary-adrenal axis recovery time following stress. However, there are discrepancies concerning the Trp requirement for nursery pigs, suggesting the 2012 recommendations for Trp may be inaccurate for optimizing growth performance and health in modern pigs. Three experiments were conducted to determine the effects of supplemented standard ileal digestible Trp above NRC (2012) recommended levels on performance, feed efficiency, immune vigor, and stress tolerance. The objective of experiment 1 was to eliminate or reduce short- and long-term, transport-induced reductions in piglet feed efficiency and growth by supplementing Trp above NRC (2012) recommendations pre-weaning and/or during the nursery phase. An oral gavage of Trp or a control milk carrier was provided to pre-allotted piglets beginning at day 5 of lactation and continuing to weaning. At weaning all pigs were blocked by sex, weaning weight and pre-wean treatment and randomly assigned to transport and post-wean treatments. Pigs were fed in four nursery phases with diets containing 1X or 2X NRC recommended concentrations of SID Trp and a common grower diet fed in 6 phases during the grow-finish period. At market, loin characteristics were measured via ultrasound and carcass data was collected from the packing plant. Pre-weaning Trp supplementation had no effect on pre-weaning growth performance; however, post-wean Trp increased overall body weight and average daily gain in nursery pigs when Trp was supplemented pre-wean. The objective for experiment 2 was to mitigate the adverse effects of early life transport stress on subsequent immune challenges by providing supplemental Trp during the nursery period. At weaning, pigs were transported for 8 hours and assigned to treatments of vaccine-induced immune challenge and dietary treatment. Pigs were fed standard nursery diets, in four phases, over 35 days with pigs receiving 1X or 2X the NRC (2012) recommended Trp concentration. Half the pigs on each dietary treatment were subjected to a 3-wk vaccine challenge consisting of circovirus, mycoplasma, and influenza vaccines administered in wk 2, 3, and 4 post-wean, respectively. At market, loin characteristics were measured via ultrasound and carcass data was collected.  At the culmination of the nursery period, unchallenged pigs supplemented with Trp were heavier compared to control pigs. This resulted from an overall improvement in average daily gain for 2X Trp fed pigs. However, increasing Trp did not affect market weight, loin eye area, or lean percent in market pigs, although 2X Trp increased back fat. The objective of experiment 3 was to determine what ratio of SID tryptophan, relative to lysine, maximizes growth performance and feed efficiency in weaned pigs during the nursery period. Pigs were blocked by sex and weaning weight and randomly allotted to one of five dietary treatments with Trp increasing stepwise by 0.33%-units, respectively. Pigs were fed standard nursery diets, in four phases, over 35 days with pigs receiving 1X, 1.33X, 1.66X, 2X, or 2.33X the NRC (2012) recommended Trp concentration.  Overall, there were no differences in performance or efficiency across dietary treatments during a 35-day nursery period in unstressed healthy pigs.  In conclusion, increasing Trp in swine diets prior to and/or following stress events can mitigate stress-related perturbations in performance and efficiency.      </p>