GxE

Genotype-by-Environment Interaction for Heat Tolerance in Korean Dairy Cattle: A Reaction-Norm Analysis of Milk Yield Across THI Gradients

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Abstract

Dairying in Korea occurs under increasingly variable thermal conditions, with elevated temperature and humidity during summer months posing a major challenge to milk production. Such environmental heterogeneity can lead to differential expression of genetic potential for productivity, indicating the presence of genotype-by-environment interaction (Gà—E) associated with heat tolerance. The objective of this study was to quantify Gà—E for milk yield in Korean Holsteins using an animal-level reaction-norm (RN) random regression model across a gradient of the temperature-humidity index (THI).The initial dataset contained approximately 27.7 million test-day records collected through the national dairy recording system from 2015 to 2024. After stringent quality control, including removal of outliers, pedigree and parity validation, and exclusion of missing or implausible data, 4.72 million test-day records from 297,403 cows (parities 1-3) were retained for analysis. Records from the first three parities were linked to daily THI values derived from KMA (Korea Meteorological Administration) meteorological stations, representing an environmental gradient from 72 (thermoneutral) to 95 (severe heat stress). During the same period, the average THI across regions increased slightly from 64.8 in 2015 to 66.8 in 2024, reflecting gradual warming of the production environment. Phenotypic trends revealed a distinct nonlinear relationship between THI and milk yield. Mean milk yield increased moderately with THI up to approximately 75-80, where it remained stable around 33.8 kg, before declining sharply as THI rose further, reaching about 31.5 kg under high heat-load conditions (THI > 90). This pattern is consistent with the expected thermal threshold beyond which physiological stress reduces lactational performance.The model was implemented using the BLUPF90 software, with additive genetic and permanent environmental effects expressed as linear functions of standardized THI. At THI = 72, representing thermoneutral conditions, the additive genetic variance for milk yield was 6.6 kg², the permanent environmental variance 12.8 kg², and the residual variance 18.6 kg², yielding a heritability (h²) of 0.17 ± 0.003. As THI increased toward 95, additive genetic variance decreased to 4.5, and heritability declined to 0.12 ± 0.004, indicating reduced genetic expression and increased environmental influence under heat stress. The genetic correlation between baseline yield (intercept) and heat sensitivity (slope) was -0.56, demonstrating that cows genetically superior for milk production under cool conditions tend to experience greater yield losses as thermal load rises. The heterogeneity of variance components along the THI trajectory provides clear evidence of genotype-by-environment interaction in the Korean dairy population. These patterns reflect scale and sensitivity differences rather than simple re-ranking, highlighting the importance of incorporating heat resilience into selection objectives. Incorporating THI-dependent reaction norms and developing a heat-resilient milk index will enable the identification of animals that maintain productivity under heat stress, thereby strengthening the sustainability of dairy breeding programs under ongoing climate change.

Keywords: 2026

How to Cite:

Jeong, T., Lee, S., Kim, S., Ha, S., Seo, S., Shin, D. & Lim, D., (2026) “Genotype-by-Environment Interaction for Heat Tolerance in Korean Dairy Cattle: A Reaction-Norm Analysis of Milk Yield Across THI Gradients”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285355. doi: https://doi.org/10.31274/wcgalp.23653

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