Investigation of the effect of heat stress on egg production traits in Thai native chickens (Lueng Hang Kao Kabin) as determined by the temperature-humidity index

Heat stress is a considerable challenge for the poultry industry, impacting even native breeds well-known for their adaptability. This study aimed to examine the effect of heat stress, defined as a temperature-humidity index (THI), on egg production traits in Thai native chickens (Lueng Hang Kao Kabin breed, LHKK). The THI values were calculated using three different equations based on the weather data on the same day (day 0) as the observation and up to seven days (day -7) prior to the observation. The models were preliminarily tested to identify the significant effects on egg production traits based on adjusted R2, residual error variance, and Akaike Information Criterion (AIC) values. The most impactful day of each THI was determined based on the absolute value of the greatest regression coefficient. These days were then analyzed to identify the THI breakpoint associated with a decline in egg production traits. Heat stress had a significantly negative effect on egg production traits in LHKK chickens. THI values effectively captured the impact of heat stress on these traits, with day -3 and day -1 prior egg laying exhibiting the strongest influence on egg weight at first egg and egg weight, respectively. The THI breakpoints for significant declines in egg weight at first egg were found to be higher than those for egg weight. The current findings suggest that heat stress should be considered as a significant variable in models of egg production traits in native chickens.