2.3. Climate Conditions

Climate data were obtained from the meteorological center closest to the chicken farm (3 km distance). The weather information included daily temperature and relative humidity recorded every 3 h, which were used to calculate the temperature-humidity index (THI) based on the formula [21]:

where T is the temperature in degrees Celsius and RH is the relative humidity as a percentage. The mean daily THI 4 weeks before each chicken weighing date was used to determine the heat stress threshold and assess genetic parameters.

The monthly average THI in Khon Kaen province, Thailand, over the past 5 years (between 2015 and 2020) has ranged from 72.60 to 81.36. The month with the highest THI is May, while the month with the lowest THI is December. These months correspond to the summer and winter in Thailand, respectively (Figure 3).

Average temperature-humidity index in Khon Kean province, Thailand, during 2015–2020.