Impact of heat stress on phenotypic and genetic parameters of egg fertility traits in four Thai native synthetic chicken lines

Heat stress affects chickens as well as their eggs. Therefore, the objective of this study was to identify the temperature threshold at which heat stress began to negatively affect fertility traits and to estimate the genetic parameters of egg fertility (FER) in Thai native synthetic chicken lines under different thermal conditions. A total of 392,995 egg records from 5,919 individuals of four Thai synthetic chicken lines-Soi Nin, Soi Pet, Kaen Thong, and Kaimook e-san-were used in this study. FER, hatchability of fertile eggs (HOF), and hatchability of egg sets (HOS) were measured from 7 to 52 weeks of production. A multiple-trait random regression test-day model based on the temperature-humidity index (THI), which is calculated from ambient temperature and relative humidity, was used to estimate the onset of heat stress, genetic parameters, and losses in FER, HOF, and HOS. The results identified a THI value of 76 as the threshold at which fertility heat stress began to negatively affect egg fertility traits in all chicken lines, based on the highest R² and lowest MSE across all traits. The estimated direct heritability values for FER, HOF, and HOS ranged from 0.060 to 0.074, 0.040 to 0.049, and 0.037 to 0.046, respectively. The mate sires heritability values were higher, ranging from 0.112 to 0.132 for FER, 0.121 to 0.149 for HOF, and 0.118 to 0.149 for HOS. Genetic correlations among egg fertility traits were high (> 0.80), while their correlations with heat stress were strongly negative in all lines. Permanent environmental correlations were positive among the fertility traits (0.59 to 0.85) but negative in relation to heat stress (-0.80 to -0.92). Among the four lines, Kaimook e-san showed the greatest reduction in fertility under heat stress, whereas Soi Nin exhibited the least decline. These findings demonstrate that heat stress has a strong effect on egg fertility traits and genetic parameters of Thai synthetic chicken lines, supporting the use of THI-based models in breeding for heat tolerance.