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* Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 Ås, Norway
Department of Animal Breeding and Genetics, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
Corresponding author: Jørgen Ødegård; e-mail: jorgen.odegard{at}ihf.nlh.no.
The dataset used in this analysis contained a total of 341,736 test-day observations of somatic cell scores from 77,110 primiparous daughters of 1965 Norwegian Cattle sires. Initial analyses, using simple random regression models without genetic effects, indicated that use of homogeneous residual variance was appropriate. Further analyses were carried out by use of a repeatability model and 12 random regression sire models. Legendre polynomials of varying order were used to model both permanent environmental and sire effects, as did the Wilmink function, the Lidauer-Mäntysaari function, and the Ali-Schaeffer function. For all these models, heritability estimates were lowest at the beginning (0.05 to 0.07) and higher at the end (0.09 to 0.12) of lactation. Genetic correlations between somatic cell scores early and late in lactation were moderate to high (0.38 to 0.71), whereas genetic correlations for adjacent DIM were near unity. Models were compared based on likelihood ratio tests, Bayesian information criterion, Akaike information criterion, residual variance, and predictive ability. Based on prediction of randomly excluded observations, models with 4 coefficients for permanent environmental effect were preferred over simpler models. More highly parameterized models did not substantially increase predictive ability. Evaluation of the different model selection criteria indicated that a reduced order of fit for sire effects was desireable. Models with zeroth- or first-order of fit for sire effects and higher order of fit for permanent environmental effects probably underestimated sire variance. The chosen model had Legendre polynomials with 3 coefficients for sire, and 4 coefficients for permanent environmental effects. For this model, trajectories of sire variance and heritability were similar assuming either homogeneous or heterogeneous residual variance structure.
Key Words: dairy cattle • model comparison • random regression model • somatic cell score
Abbreviation key: 2lnRL = 2 ln restricted likelihood, AIC = Akaike information criterion, AS = Ali-Schaeffer curve, BIC = Bayesian information criterion, CM = clinical mastitis, L = Legendre polynomial, LM = Lidauer-Mäntysaari curve, LSCS = lactation average somatic cell score, MSEP = mean squared error of predictions, NRF = Norwegian Cattle, PE = permanent environmental, REP = repeatability model, RR = random regression, RRM = random regression model, W = Wilmink curve
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