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

## TL;DR

Thai native synthetic chickens show better growth and heat tolerance than black-boned chickens under rising temperatures, offering a solution for climate-resilient poultry farming.

## Contribution

The study identifies breed-specific heat tolerance thresholds and genetic resilience in chickens, offering new insights for climate-smart poultry breeding.

## Key findings

- Thai native synthetic chickens exhibit higher genetic potential and stable performance under heat stress compared to black-boned chickens.
- Black-boned chickens show sharp declines in growth traits even under mild heat stress.
- Thai native synthetic chickens maintain positive estimated breeding values up to higher temperature-humidity index thresholds.

## Abstract

Rising temperatures and humidity due to climate change pose significant challenges to poultry farming, particularly in tropical regions. This study evaluated and compared the growth performance and heat tolerance of two chicken types—black-boned and Thai native synthetic chickens. Over several years, researchers monitored their growth under varying environmental conditions. The findings revealed that Thai native synthetic chickens exhibited superior growth rates and greater resilience to heat stress compared to black-boned chickens. In contrast, black-boned chickens showed reduced growth even under mild heat conditions, whereas the synthetic chickens maintained stable performance until exposed to more severe heat. These insights can guide farmers in selecting more heat-tolerant chicken breeds, thereby improving food production efficiency, minimizing losses, and promoting sustainable poultry farming in the face of global warming.

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.