Abstract
BACKGROUND AND AIM: Heat stress (HS) negatively impacts poultry production by reducing growth performance and compromising physiological health. Nutritional strategies, particularly amino acid supplementation, are explored to mitigate these adverse effects. This study evaluates the impact of high dietary threonine supplementation on growth performance, health biomarkers, oxidative status, meat quality, and intestinal histology in cyclic HS broilers. MATERIALS AND METHODS: A total of 288 1-day-old Hubbard broilers were randomly allocated to six treatment groups: Thermoneutral, HS control, and HS supplemented with 125% (HS-125), 150% (HS-150), 175% (HS-175), and 200% (HS-200) of NRC-recommended threonine. Birds in the HS groups were exposed to cyclic HS (35°C, 75% relative humidity) from day 22 to day 42. Growth performance was recorded weekly, while physiological parameters, oxidative stress markers, and jejunal histology were analyzed post-exsanguination. RESULTS: HS significantly reduced body weight gain and feed intake, while threonine supplementation did not improve these parameters. However, liver weight, serum albumin, and cholesterol levels improved at higher threonine doses (175%-200%). Threonine also reduced serum corticosterone and malondialdehyde levels, suggesting enhanced stress resilience. Superoxide dismutase activity, an indicator of oxidative defense, improved in threonine-supplemented groups. In jejunal histology, acidic goblet cells increased, and intraepithelial lymphocyte infiltration decreased in birds supplemented with 175%-200% threonine, indicating enhanced gut integrity. Meat quality attributes, including crude protein and oxidative stability, showed minor but inconsistent variations across treatments. CONCLUSION: Although high dietary threonine supplementation (175%-200%) improved stress resilience by enhancing oxidative status, intestinal health, and selected physiological biomarkers in HS broilers, however, it failed to enhance growth performance. These findings suggest that while threonine supports physiological adaptations under HS, its use as a growth promoter under HS conditions may not be economically viable. Further studies are warranted to optimize amino acid balance in HS broilers for improved productivity.