Functional Role and Application of Tryptophan and Methionine in Animals

The objective of this research is to evaluate the functional role and potential application of tryptophan (Trp) and methionine (Met) in improving growth, development, health status, and welfare of animals. Experiment one examined the functional role of increasing dietary supplemental L-Trp of pigs under social stress. Seventy two individually housed barrows were randomly allotted to 6 treatments representing supplementation of 0, 2, 4, 6, 8, or 10 g L-Trp/kg for 15 d. On d 5 and 10, all pigs were paired to create mixing stress. This experiment found supplementation of L-Trp improved growth performance of 6 wk old nursery pigs in association with increasing hypothalamic serotonin (5-hydroxytryptamine, 5- HT) production, reducing stress hormone concentrations, decreasing lipid peroxidation, increasing drinking, and reducing sitting and lying. Increase in body weight gain was maximal when daily total Trp intake was 10.8 g. Experiment two examined the effect of reducing large neutral amino acid (LNAA) on facilitating the functional effects of L-Trp supplementation on nursery pigs experiencing social-mixing stress. Forty eight individually housed barrows were randomly allotted to 4 dietary treatments based on a 2 x 2 factorial arrangement, with L-Trp supplementation (0 or 6g/kg) and LNAA concentrations (4.5 or 3.8%) as the two main factors. This experiment found lowering LNAA further enhanced the improvement of feed efficiency by L-Trp supplementation of nursery pigs under social-mixing stress, indicating reducing LNAA in the diet can facilitate the effect of L-Trp on stress response of pigs. Experiment three and four evaluated the application of the functional role of Trp in a commercial setting. Results showed that supplementation with 8 g/kg L-Trp improved growth performance of pigs with social-mixing and relocation stress in association with reduced stress hormone secretion in a commercial setting. Dietary supplementation of 8 g/kg L-Trp had similar effects on feed efficiency as a reduced LNAA diet supplemented with 7 g/kg L-Trp, suggesting lowering LNAA is a valid method of facilitating the functional effects of L-Trp supplementation. Experiment five and six evaluated the role of cerebral Trp metabolism in the detrimental effect of low level dietary exposure of naturally contaminated deoxynivalenol (DON) on nursery pigs. Results showed that the detrimental effects of DON on feed intake were related with L-Trp metabolism through increasing 5-HT turnover index in the hypothalamus but not other regions of the brain. Experiment seven and eight evaluated functional and utilization differences between L-Met and DL-Met on intestinal glutathione (GSH) level, redox status, gut development and consequently growth of young nursery pigs. Results showed that compared with DL-Met, use of L-Met serves better functions for enhancing villus development in association with reducing oxidative stress and improved GSH production of mucosa cells of nursery pigs. LMet was better utilized by young nursery pigs showing enhanced growth performance. Experiment nine evaluated the functional role and utilization differences between LMet and DL-Met on broiler chicks. Results showed that supplementations of either L-Met or DL-Met had beneficial effects on villus development in association with increased GSH production, levels of total antioxidant capacity, and reduced protein oxidation in duodenum. Supplementation of L-Met served a better function on redox status and development of the gut of young chicks compared with DL-Met. Chicks fed diets supplementation of L-Met had better growth response than chicks fed diets with DL-Met.