Abstract
In this study, we evaluated genetic parameters and progress in growth traits and heat tolerance across four Thai native synthetic chicken lines-Kaimook e-san, Soi Pet, Soi Nin, and Kaen Thong-over seven generations. Growth traits, including body weight (BW), average daily gain (ADG), and breast circumference (BrC), were recorded and analyzed from hatching to 14 weeks of age, with heat stress assessed based on the temperature-humidity index (THI). Genetic parameters were estimated using a multi-trait animal model incorporating THI thresholds. Significant differences in growth traits were observed among the lines (p < 0.05). Kaimook e-san consistently exhibited superior performance, with males outperforming females, while Kaen Thong showed the lowest performance across all traits. Heritability estimates declined with age, with higher values observed in early growth stages, indicating stronger genetic influence during early development. Strong negative genetic correlations between heat tolerance and growth traits were observed in Kaimook e-san, while Kaen Thong showed weaker correlations, suggesting greater potential for simultaneous improvement in growth and heat tolerance. All lines demonstrated genetic progress in growth traits across generations, with Kaimook e-san achieving the highest gains, although these were accompanied by a decline in heat tolerance. The findings highlight the trade-offs between growth and thermal resilience and underscore the importance of multi-trait selection strategies for sustainable poultry breeding in tropical environments.