Abstract
Background:
L-methionine (L-Met) is an essential amino acid crucial for growth, metabolic regulation, and endocrine activity in poultry.

Aim:
This study aimed to evaluate the effects of graded dietary L-Met supplementation on growth performance, carcass traits, serum biochemical parameters, and somatotropic axis activity in local quails of white and black genetic lines.

Methods:
A total of 384 1-day-old quail chicks (white and black lines; 144 females and 48 males per line) were randomly assigned to four dietary treatments containing 0.0, 2.5, 3.0, or 3.5 g/kg L-Met. Each treatment included four replicates of 12 birds (male: female ratio, 1:3). We assessed growth performance, feed intake (FI), feed conversion ratio, carcass characteristics, serum biochemical indices, and hepatic expression of growth hormone receptor (GHR) and insulin-like growth factor-I (IGF-I). The optimal dietary L-Met level for maximum growth and feed utilization was estimated using second-order polynomial regression.

Results:
Supplementation with 3.0 and 3.5 g/kg L-Met significantly increased body weight, carcass yield, and the relative weights of the breast, thigh, wing, and back muscles (p ≤ 0.05), with black quails outperforming white quails. FI decreased, whereas feed conversion efficiency improved at higher L-Met levels. Serum biochemical analyses indicated enhanced liver and kidney function at 3.0 g/kg, along with favorable protein and metabolic profiles. IGF-I expression peaked at 3.0 g/kg, whereas GHR expression was highest at 3.5 g/kg, with sex- and line-specific differences observed. Polynomial regression analysis confirmed that the optimal dietary L-Met requirement for maximum growth and feed efficiency was between 3.0 and 3.5 g/kg.

Conclusion:
Dietary L-Met supplementation enhances local quail growth performance, feed efficiency, carcass quality, and somatotropic axis activity. Levels of 3.0–3.5 g/kg are optimal, supporting its practical application as a nutritional strategy to improve quail production efficiency.

Key words: Gene expression; Growth performance; Quail; L-Methionine; Serum biochemistry.