Effects of short term supplementation of l-tryptophan and reducing large neutral amino acid along with l-tryptophan supplementation on growth and stress response in pigs

• Short term supply of 0.8% l-Trp improved growth of pigs under social-mixing stress.
• Improved growth by 0.8% l-Trp may be related to a reduced salivary cortisol level.
• Benefits of l-Trp are enhanced with a reduced dietary LNAA level.

Two experiments were conducted to evaluate the effects of short term dietary supplementation of l-Trp and reducing large neutral amino acid (LNAA; valine, leucine, isoleucine, tyrosine, and phenylalanine) on growth and stress response in nursery and growing pigs. In Exp. 1, 674 crossbred pigs in 40 pens at 9-week of age were randomly allotted to 2 dietary treatments with supplementation of 0.0% and 0.8% l-Trp to a corn and soybean meal basal diet. Experimental period was composed of 5 days in a nursery and 7 days in a finisher. After 12 days feeding of experimental diets, pigs were provided a common diet for an additional 7 days. In Exp. 2, 108 crossbred barrows at 6-week of age were randomly allotted to 3 dietary treatments: (1) a corn soybean meal basal diet (4.5% LNAA) supplemented with 0.0% l-Trp; (2) a corn soybean meal basal diet (4.5% LNAA) supplemented with 0.8% l-Trp; (3) a reduced LNAA (3.8%) diet supplemented with 0.7% l-Trp, which had the same Trp:LNAA ratio as treatment 2. The experimental period lasted 16 days. In Exp. 1, during the entire period, pigs fed the diet supplemented with 0.8% l-Trp had increased ADG and gain:feed (P < 0.05 and P < 0.01, respectively) compared with pigs fed the diet without l-Trp supplementation. On day 6, one day after mixing, pigs fed the diet supplemented with 0.8% l-Trp had a lower (P < 0.05) concentration of salivary cortisol compared with pigs fed the diet without l-Trp supplementation. In Exp. 2, during the entire period, pigs fed the diet supplemented with 0.8% l-Trp or a reduced LNAA diet supplemented with 0.7% l-Trp had increased gain:feed (P < 0.01) compared with pigs fed the diet without l-Trp supplementation. Weight gain and gain:feed were similar (P > 0.05) between pigs fed the diet supplemented with 0.8% l-Trp and pigs fed a reduced LNAA diet supplemented with 0.7% l-Trp. In conclusion, short term supplementation of 0.8% l-Trp improved growth performance of pigs during period of social-mixing and was associated with reduced stress hormone concentrations. Dietary supplementation of 0.8% l-Trp had similar effects on feed efficiency as a reduced LNAA diet supplemented with 0.7% l-Trp, suggesting lowering LNAA is a valid method of reducing levels of l-Trp supplementation required for mitigating stress response.