Genetic Parameter Estimates and Associations Between Clutch Length and Hen-Day Egg Production Traits in Thai Native Chickens Under Heat Stress

Improving egg production under heat stress remains a major challenge in tropical poultry breeding. This study demonstrates that clutch length and hen-day egg production are robust indicators of genetic variation in heat tolerance and directly applicable to climate-resilient breeding programs in Thai native chickens. Records from 2400 Pradu Hang Dum hens across five generations were analyzed using a reaction-norm repeatability model with the temperature-humidity index (THI) as an environmental covariate. A THI threshold of 74 was identified, beyond which heat stress altered the genetic expression of both traits. Heritability estimates declined with an increasing THI, from 0.49 to 0.32 for clutch length and from 0.37 to 0.26 for hen-day egg production, indicating reduced additive genetic control under heat stress. Genetic correlations between baseline performance and heat-stress sensitivity were moderately to strongly negative (-0.46 to -0.54), revealing antagonism between productivity under thermoneutral conditions and heat tolerance. Reaction-norm breeding values showed substantial genotype-by-environment interactions, highlighting heterogeneity in heat-stress responses among genotypes. Despite thermal challenges, positive genetic trends were observed, with an average genetic trend of 1.34 eggs per generation for clutch length and 8.8 percent per generation for hen-day egg production. These results demonstrate that genetic improvement can be sustained under heat stress and support the integration of THI-based reaction-norm evaluations to identify genotypes combining reproductive efficiency with enhanced heat tolerance for climate-resilient breeding programs.