Disease & heat resistance

Genetic Parameters and QTL Mapping of Within-Lactation Heat Stress Responses in Milk Traits of Sarda Dairy Ewes

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Abstract

Heat stress (HS) is a major constraint for dairy sheep in Mediterranean environments. This study aimed to estimate genetic parameters and identify genomic regions associated with within-lactation responses to HS in Sarda ewes, using milk traits as indicators. The dataset included 179,943 milk yield (MY), 153,835 fat (FP), and protein (PP) contents test-day records (TD) from 4,940 ewes, collected between 2000 and 2022 at an experimental farm in Sardinia. Animals were managed under the traditional Sardinian system with a December-July milking period. The temperature-humidity index (THI) was used as the climatic variable. HS effects were assessed through a random regression animal model fitting a reaction norm to the average THI over the 4 days preceding each TD (THI4d). HS was modeled as a linear function (fH) of THI4d exceeding thresholds of 20 (MY), 22 (FP), and 15 (PP). The model estimated a population-wide effect of fH, and individual intercepts (baseline levels) and slopes (HS responses) as genetic and non-genetic random effects. Genetic (co)variances were estimated using a genomic relationship matrix. Phenotypes for QTL mapping were obtained by combining individual random effects for intercept and slope. Mapping was performed through linkage disequilibrium and linkage analysis using 43,724 OvineSNP50 loci. Population-wide slopes indicated changes per fH unit of -0.20 (MY), +0.14 (FP), and -0.03 (PP) phenotypic SD units. These patterns show that MY and PP decrease under HS, whereas FP appears to increase. This rise likely reflects milk dilution, suggesting the FP slope does not represent a direct physiological response to HS. Variance components revealed substantial genetic variability in slopes, with heritabilities of 0.29 (MY), 0.16 (FP), and 0.36 (PP). Genetic correlations between intercept and slope were trait-specific: strongly negative for MY (-0.72), slightly negative for PP (-0.08), and positive for FP (+0.44). QTL mapping detected multiple significant regions (p ⁻⁶). The 84 intercept-QTL largely matched those previously identified in the same population for production traits. Eighteen regions were significant for at least one slope: 2 for MY, 8 for FP, 5 for PP, 1 for MY and PP, and 2 for FP and PP, with five overlapping intercept-QTL. Major slope-specific QTL were mapped on OAR2 and OAR3 for PP, and on OAR13 for FP. Key slope-intercept QTL were identified on OAR3 near the LALBA gene (FP slope; FP/PP intercepts), on OAR12 (FP and PP slopes and intercepts), and on OAR13 (PP slope; MY, FP, PP intercepts). Several major intercept-QTL were not associated with slopes. The limited overlap between intercept- and slope-QTL indicates a complex genetic architecture, in which few loci influence both baseline production and HS sensitivity, while most act specifically. This highlights partly independent genetic control of milk traits and heat-stress response.

Keywords: 2026

How to Cite:

Usai, M., Salaris, S., Miari, S. & Carta, A., (2026) “Genetic Parameters and QTL Mapping of Within-Lactation Heat Stress Responses in Milk Traits of Sarda Dairy Ewes”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285413. doi: https://doi.org/10.31274/wcgalp.23691

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