Comparative Analysis of Genetic Parameters for Test-Day Egg Production in Four Thai Native Synthetic Chicken Lines Under Heat Stress

This study evaluated genetic parameters for test-day egg production in four Thai native synthetic chicken lines&mdash:Soi Nin, Soi Pet, Kaen Thong, and Kaimook e-san&mdash:under heat stress in Thailand. A total of 11,887 monthly test-day egg records from 1134 hens, collected between January 2023 and July 2025, were analyzed using a repeatability test-day model with the temperature&ndash:humidity index (THI) as an environmental covariate. THI thresholds from 70 to 80 were evaluated, and the THI1 equation provided the best model fit with the highest coefficient of determination (R2) and the lowest mean squared error (MSE). With increasing THI, heritability estimates declined from 0.255&ndash:0.323 at THI 70 to 0.173&ndash:0.236 at THI 80, a 26.9&ndash:32.2% decrease reflecting reduced additive genetic variance and consequent lower genetic expression under heat stress. Genetic correlations between egg production and heat stress were positive at low THI (0.250&ndash:0.600) but became negative at THI &ge: 73, suggesting antagonism between productivity and thermotolerance under severe stress. The rate of decline in egg production increased with increasing THI, from &minus:0.35 to &minus:0.45 eggs/bird/THI at THI 73, &minus:0.80 to &minus:1.22 at THI 76, and &minus:1.76 to &minus:2.35 at THI 80. The ranges of heritability and decline rates reflect the variation observed among the four Thai native synthetic chicken lines examined in this study. Kaimook e-san consistently showed the steepest decline in egg production, whereas Soi Nin exhibited the smallest, indicating greater resilience. These findings reveal significant genetic variation in heat tolerance among Thai native synthetic lines and underscore the need to consider both productivity and environmental sensitivity in breeding programs to sustain egg production under future climate change.