As climate change intensifies, improving heat tolerance has become an important objective for sustaining poultry productivity in tropical and subtropical environments. This study evaluated the effects of the temperature–humidity index (THI) on monthly egg production and genetic parameters in black-boned and Thai native chickens, with an emphasis on identifying heat stress thresholds and genotype-specific responses. A total of 136,816 monthly egg production records from 3,680 black-boned and 7,850 Thai native chickens were analyzed using a repeatability test-day model. Heat stress onset was observed at THI 72 in black-boned chickens and THI 74 in Thai native chickens. Thai native chickens maintained higher egg production rates and sustained positive responses over a wider THI range. Heritability declined with increasing THI (0.162–0.107 in black-boned; 0.175–0.132 in Thai native chickens), and genetic correlations between egg production and heat stress shifted from positive to negative in both chicken genotypes (from 0.350 to −0.723 in black-boned chickens and from 0.500 to −0.524 in Thai native chickens). The rate of decline in egg production increased with THI, and was greater in black-boned chickens than in Thai native chickens (from 1.01 to −2.03 eggs/bird/THI level in black-boned chickens and from 1.77 to −1.22 eggs/bird/THI level in Thai native chickens across THI 70–80). The proportion of chickens with heat-tolerant breeding values decreased across THI levels, but remained higher in Thai native chickens. The results of this study demonstrate a nonlinear, genotype-dependent response to heat stress and provide a framework for integrating THI into genetic evaluations to enhance resilience and sustain egg production under tropical conditions.
Promket et al. (2026) studied this question.