GxE

Modelling Heat-Stress Genotype-by-Environment Interactions for Milk Yield in Texas and Wisconsin Holsteins

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Abstract

Genotype-by-environment (Gà—E) interactions associated with heat stress pose major challenges in dairy cattle breeding, contributing to production losses, impaired fertility, welfare concerns, and increased complexity in genetic evaluations. Milk test-day records collected from different environments provide an opportunity to model heat-stress effects and characterize how genetics respond to environmental variation through reaction norm models. The objective of this study was to evaluate the extent of heat stress using test-day milk yields of US Holstein cows in the states of Texas (TX) and Wisconsin (WI). The dataset for this study was provided by the Council on Dairy Cattle Breeding (CDCB) and by the Cooperative Dairy DNA Repository (CDDR). Data from first-lactation cows born between 2000 and 2019, with more than five test-day records from the largest herds within TX and WI were included in this study. A total of 854,650 milk test days for 95,730 animals across 13 herds in Texas and 784748 test days for 87275 individuals across 31 herds in Wisconsin were included in a single trait repeatability test-day model. Seasonality associated with heat stress was modeled by dividing test-day records into summer and non-summer groups. Two definitions of summer months were used: May to September and June to August. Variance components for milk yield were estimated using an Average Information Restricted Maximum Likelihood algorithm. For the single-trait analysis, heritability (SE) estimates for milk yield were 0.22 (0.007) in TX and 0.24 (0.006) in WI. In the two-trait model separating summer (May-September) and non-summer records, heritabilities in TX and WI were 0.22 (0.008) and 0.24 (0.008), respectively, with a genetic correlation (SE) of 0.91 (0.008). Heritability estimates for WI were slightly higher, at 0.23 (0.007) for summer and 0.25 (0.006) for non-summer, with a genetic correlation of (SE) 0.95 (0.005). Results from the restricted summer model (June-August) were consistent with those from the broader May-September model. Overall, the two-trait seasonal models indicated moderate GxE effects mostly in TX, with heritability estimates remaining stable across both seasonal definitions and states. Future work will involve developing a heat-load function using weather-station data to classify records into heat-stress and thermoneutral conditions. Genomic information will then be used to estimate genomic breeding values and perform genome-wide association analyses.

Keywords: 2026

How to Cite:

Dutta, G., Schober, H., McWhorter, T., Tiezzi, F., Miles, A. & Fragomeni, B., (2026) “Modelling Heat-Stress Genotype-by-Environment Interactions for Milk Yield in Texas and Wisconsin Holsteins”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286286. doi: https://doi.org/10.31274/wcgalp.23906

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