Abstract
This study explores the effects of dietary tryptophan (Trp) supplementation on stress mitigation and production parameters in aging laying hens housed under high-density conditions. A total of 700 Hy-line laying hens, aged 70 weeks, were used in the experiment. The hens were divided into four groups, receiving diets supplemented with 0%, 0.25%, 1%, and 2% Trp over a four-week period. The study aimed to evaluate the impact of Trp on Hen-Day Egg Production (HDEP), egg mass, feed conversion ratio (FCR), and a range of physiological and biochemical stress indicators. The results indicated a quadratic response in HDEP and egg mass, with optimal production achieved at 1% Trp supplementation. Egg weight was linearly decreased by Trp supplementation. The FCR was quadratically affected, with lower FCR achieved at 0.25% and 1% Trp supplementation. The content of white blood cells, heterophiles, lymphocytes, and monocytes in blood was linearly reduced by supplementation of Trp. A linear decrease in the content of red blood cells, hemoglobin, and hematocrit was observed with the supplementation of Trp. The concentration of triglyceride was linearly decreased, and an increasing quadratic response was observed up to the level of 1% Trp inclusion and decreased thereafter. The content of glucose in blood was linearly increased by supplementation of Trp. the concentration of immunoprecipitation and lactate dehydrogenase was linearly decreased with supplementation of Trp. The concentration of blood corticosterone was higher in laying hens fed 0 and 0.25% of Trp compared with 1 and 2% supplementation. The concentration of blood serotonin was higher in laying hens fed 0.25 and 2% of Trp compared with 0% supplementation. In week 4, an increasing linear response was observed by Trp inclusion for yolk color, shell strength, and shell thickness. The study concludes that 1% Trp supplementation not only enhances productivity and egg quality but also contributes to reduced stress for laying hens.