Estimates of Heritability of Milk Yield in Irish Dairy Cows Using Random Regression Test-day Models

Abstract

ABSTRACTAs exists globally, an accurate genetic evaluation for individual dairy cows in Ireland is aprerequisite for genetic gain in milk yield. This entails using more sophisticated mixedmodels to model the variances and covariances across the lactation trajectory. The studyaimed to model the genetic, permanent environmental and residual variance components formilk yield of Irish research dairy cows using a random regression test-day model. Therefore,routinely recorded milk test-day records of dairy cows milking in the 2019 calendar year wereobtained from Teagasc Animal & Grassland Research & Innovation Centre, Moorepark. Therecords totaled 209,047 test day records. Information on 901 crossbred and purebred cowswith varying proportions of Holstein, Friesian, and Jersey genotypes were available; thepedigree of each animal was traced back to founders which were subsequently assigned togenetic groups. The pedigree included 25,725 individuals. Herd-test-date (the interactionbetween experimental treatments and test dates), heterosis, recombination loss, the fixedeffect of lactation curve nested within parity as well as the effect of age at calving relative toparity were associated with milk yield (p < 0.05). Random regression model involving fourthorder Legendre polynomial was used to model the fixed lactation curve. The models weredeveloped in stages. This was based on the analysis of the eigenvalues of the additive geneticand permanent environment covariance matrices to determine the usefulness of additionalorders to the fitted polynomials. Therefore, various orders of Legendre polynomials weretested to optimize the order of Legendre polynomial for both additive genetic and permanentenvironmental effects. Ten residual error classes associated with 10 different lactation stageswere modelled. Additive genetic variances, permanent environmental variances, residualvariances, heritability, repeatability, genetic and permanent environmental correlations acrossdays in milk were estimated...