Disease & heat resistance

Lactation Curve Responses to Heat Stress in Holstein Cows Stratified by Genomic Breeding Values for Heat Tolerance

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Abstract

Heat stress (HS) is a critical constraint to dairy productivity, particularly during early lactation when cows experience elevated metabolic demands. The objective of this study was to evaluate the impact of early lactation HS on lactation curve dynamics in high‑yielding Holstein cows with divergent genomic merit for heat tolerance. A total of 960 cows exposed to HS primarily during the first 150 days in milk (DIM) were stratified into thermotolerant and thermosensitive groups based on their ranking within the top or bottom 25% of a larger population. Cows were located in U.S. states with prolonged HS conditions (Arizona, California, Florida, Georgia, New Mexico, Texas). Genomically estimated breeding values (gEBV) were calculated by Zoetis Genetics using a reaction norm linear model implemented with single‑step GBLUP. Test‑day milk yield records paired with Temperature-Humidity Index (THI) units served as phenotypes, from which the model estimated an intercept (thermoneutral yield) and a slope describing yield decline above THI 70. None of the cows included in this study contributed phenotypes to the evaluation; their gEBV were derived solely from genomic relationships captured by the H matrix. Individual lactation curves were fitted using Wood's model for 1,092 cow‑lactation combinations, and daily milk yield estimates were analyzed using a mixed‑effects model. Fixed effects included genetic group, DIM, their interaction, herd‑year‑month of calving, lactation number (1-5), and the degree of early‑lactation HS exposure, categorized according to the proportion of days with THI > 70 during the first 150 DIM. To control for potential differences in baseline genetic merit for production, the reaction‑norm intercept (gEBV for thermoneutral milk yield) was included as a linear covariate, ensuring that group differences reflect variation in heat‑stress response rather than underlying production potential. Animal nested within lactation number was modeled as a random effect, and an autoregressive covariance structure accounted for repeated measures. Milk yield trajectories for both groups followed a typical lactation pattern, peaking around 50-60 DIM. Statistically significant differences (P < 0.05) were observed primarily during the HS period (DIM 5-115), with thermotolerant cows maintaining higher yields. From DIM 120 to 140, differences were not statistically significant but showed trends (P < 0.10) favoring the thermotolerant group. Beyond DIM 145, no differences were detected, coinciding with the post-peak phase and the end of HS exposure. These results highlight that performance divergence between genetic groups is closely aligned with periods of elevated THI, while under thermoneutral conditions (THI ≤ 70), differences in heat tolerance do not manifest. These findings underscore the value of genomic selection for heat tolerance in mitigating productivity losses during early lactation under thermal stress. Incorporating gEBV into breeding and management strategies may enhance herd resilience and support sustained milk production in increasingly variable climates.

Keywords: 2026

How to Cite:

Sánchez-Castro, M., Vukasinovic, N., Oliveira, G., Kulkarni, A., Gonzalez-Peña, D., Vargas, G. & Pacheco, H., (2026) “Lactation Curve Responses to Heat Stress in Holstein Cows Stratified by Genomic Breeding Values for Heat Tolerance”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285706. doi: https://doi.org/10.31274/wcgalp.23779

Rights: 1