Genetic Evaluation of Growth Traits in Black-Boned and Thai Native Synthetic Chickens Under Heat Stress

Abstract

Heat stress is a critical constraint to poultry production in tropical regions, where the temperature-humidity index (THI) frequently exceeds thermoneutral thresholds. Despite growing interest in climate-resilient livestock, limited research has explored the genetic sensitivity of local chicken breeds to increasing THI levels. This study aimed to evaluate the genetic effects of increasing THI on growth performance traits in two tropical chicken breeds. The data included body weight (BW), average daily gain (ADG), and absolute growth rate (AGR) from 4,745 black-boned and 3,001 Thai native synthetic chickens across five generations. Growth data were collected from hatching to 12 weeks of age, whereas temperature and humidity were continuously recorded to calculate daily THI values. A reaction norm model was used to estimate genetic parameters and rate of decline of BW, ADG, and AGR traits under varying THI thresholds (THI70 to THI80). Results indicated that the onset of heat stress occurred at THI72 for black-boned chickens and at THI76 for Thai native synthetic chickens. Heritability estimates for BW, ADG, and AGR decreased as the THI increased in both chicken breeds. However, the Thai native synthetic chickens consistently exhibited higher genetic potential across all THI levels (average heritability: BW = 0.28, ADG = 0.25, AGR = 0.36) compared to the black-boned chickens (average heritability: BW = 0.21, ADG = 0.15, AGR = 0.23). Under mild heat stress (THI72), black-boned chickens showed sharp declines in all traits (average reduction in BW = -10.9 g, ADG = -0.87 g/day, AGR = -3.20 g/week), whereas Thai native synthetic chickens maintained stable performance. At THI76, both breeds experienced significant reductions, particularly in males. Estimated breeding values (EBVs) for AGR decreased linearly with THI, though Thai native synthetic chickens showed greater individual variability, with some birds maintaining stable or positive EBVs up to THI80-suggesting the presence of heat-resilient genotypes. In conclusion, Thai native synthetic chickens demonstrated superior thermotolerance and genetic robustness under increasing THI conditions. The identification of breed-specific THI thresholds and resilient individuals provides novel insights for climate-smart poultry breeding. These findings offer valuable tools for genetic selection, environmental management, and long-term adaptation strategies in response to global climate change.

Keywords: absolute growth rate; average daily gain; body weight; heat tolerance; synthetic chicken line.

Figure 1

Breed characteristics of (A) black-boned and (B) Thai native synthetic chickens.

Figure 2

Least-squares means of (A) body weight (BW), (B) average daily gain (ADG), and (C) absolute growth rate (AGR) traits separated by sex and chicken breeds in black-boned and Thai native synthetic chickens; a, b, c, and d: Means for the trait with different letters differ significantly at p < 0.05. BW0, BW4, BW8, and BW12: hatching weight, body weight at 4, 8, and 12 weeks of age, respectively; ADG0–4, ADG4–8, and ADG8–12: average daily gain during hatching to 4, 4 to 8, and 8 to 12 weeks of age, respectively; AGR0, AGR4, AGR8, and AGR12: absolute growth rate at hatching, 4, 8, and 12 weeks of age, respectively.

Figure 3

Growth patterns of (A) body weight in black-boned (32,451 male and 18,955 female records), and (B) Thai native synthetic chickens (18,772 male and 14,227 female records), along with the absolute growth rate (AGR) estimated using the Gompertz function in (C) black-boned and (D) Thai native synthetic chickens. Obs. means observed.

Figure 4

Estimated heritability of (A) body weight, (B) average daily gain, and (C) absolute growth rate in black-boned chickens (orange line) and Thai native synthetic chickens (blue line).

Figure 5

Rate of decline in (A) body weight, (B) average daily gain, and (C) absolute growth rate in black-boned and Thai native synthetic chickens at THI72 and THI76.

Figure 6

Integrated reaction norm-based on estimated breeding values (EBVs) for AGR under increasing THI conditions in (A) 10 randomly selected hens, (B) 10 randomly selected cocks in black-boned chickens, (C) 10 randomly selected hens, and (D) 10 randomly selected cocks in Thai native synthetic chickens.