Analysis of genotype by environment (temperature-humidity index) interaction for production traits in Taiwan Holstein using reaction norm models

Climate change has increased the frequency of extreme heat events, extending heat stress beyond tropical and subtropical regions and posing a growing challenge to dairy cattle production and health. Using DHI records from Taiwan, we evaluated genotype by environment interactions (GEI) with reaction norm models (RNMs) using temperature-humidity index (THI) as the environmental gradient. The analyzed traits included milk yield (MY), milk fat yield (MFY), milk protein yield (MPY), and somatic cell score (SCS). Conventional animal models were initially fitted to estimate genetic parameters. Subsequently, stage-specific THI effects were evaluated using modified conventional models to identify the lactation stage most responsive to heat stress. The selected stage-specific THI was then incorporated into RNMs with either homogeneous or heterogeneous residual variance to assess changes in genetic parameters along the THI gradient. The results showed models incorporating heterogeneous residual variance provided a better fit for production traits, whereas SCS was adequately described using a homogeneous residual variance. Based on these selected models, heritability across the THI gradient ranged from 0.175 to 0.291 for MY, 0.239 to 0.380 for MFY, 0.212 to 0.318 for MPY, and 0.101 to 0.129 for SCS. Genetic correlations between the lowest and highest THI values were 0.857 for MY, 0.869 for MFY, 0.778 for MPY, and 0.793 for SCS. Conventional breeding values were strongly correlated with RNM intercepts but negatively correlated with RNM slopes. Sire groups differed mainly in slope, reflecting distinct responses to increasing THI, and daughter-derived breeding values confirmed consistent patterns of environmental sensitivity. These findings demonstrate that genetic merit expressed under thermoneutral conditions may not be fully maintained under heat stress and highlight the importance of incorporating GEI into genetic evaluation for dairy populations exposed to prolonged heat load.