Climate sensitivity of milk production traits and milk fatty acids in genotyped Holstein dairy cows

摘要

The aim of this study was the evaluation of climatesensitivity via genomic reaction norm models [i.e., toinfer cow milk production and milk fatty acid (FA)responses on temperature-humidity index (THI) alterations].Test-day milk traits were recorded between2010 and 2016 from 5,257 first-lactation genotypedHolstein dairy cows. The cows were kept in 16 largescalecooperator herds, being daughters of 344 genotypedsires. The longitudinal data consisted of 47,789test-day records for the production traits milk yield(MY), fat yield (FY), and protein yield (PY), andof 20,742 test-day records for 6 FA including C16:0,C18:0, saturated fatty acids (SFA), unsaturated fattyacids (UFA), monounsaturated fatty acids (MUFA),and polyunsaturated fatty acids (PUFA). After qualitycontrol of the genotypic data, 41,057 SNP markersremained for genomic analyses. Meteorological datafrom the weather station in closest herd distance wereused for the calculation of maximum hourly dailyTHI. Genomic reaction norm models were applied toestimate genetic parameters in a single-step approachfor production traits and FA in dependency of THI atdifferent lactation stages, and to evaluate the modelstability. In a first evaluation strategy (New_sire), allphenotypic records from daughters of genotyped siresborn after 2010 were masked, to mimic a validationpopulation. In the second strategy (New_env), onlydaughter records of the new sires recorded in the mostextreme THI classes were masked, aiming at predictingsire genomic estimated breeding values (GEBV) underheat stress conditions. Model stability was the correlationbetween GEBV of the new sires in the reduceddata set with respective GEBV estimated from allphenotypic data. Among all test-day production traits,PY responded as the most sensitive to heat stress. Asobserved for the remaining production traits, geneticvariances were quite stable across THI, but geneticcorrelations between PY from temperate climates withPY from extreme THI classes dropped to 0.68. Geneticvariances in dependency of THI were very similar forC16:0 and SFA, indicating marginal climatic sensitivity.In the early lactation stage, genetic variances for C18:0,MUFA, PUFA, and UFA were significantly larger inthe extreme THI classes compared with the estimatesunder thermoneutral conditions. For C18:0 and MUFA,PUFA, and UFA in the middle THI classes, geneticcorrelations in same traits from the early and the laterlactation stages were lower than 0.50, indicating strongdays in milk influence. Interestingly, within lactationstages, genetic correlations for C18:0 and UFA recordedat low and high THI were quite large, indicating similargenetic mechanisms under stress conditions. The modelstability was improved when applying the New_env insteadof New_sire strategy, especially for FA in the firststage of lactation. Results indicate moderately accurategenomic predictions for milk traits in extreme THIclasses when considering phenotypic data from a broadrange of remaining THI. Phenotypically, thermal stressconditions contributed to an increase of UFA, suggestingvalue as a heat stress biomarker. Furthermore, thequite large genetic variances for UFA at high THI suggestthe consideration of UFA in selection strategies forimproved heat stress resistance.