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1 Agricultural University of Poznañ, Department of Genetics and Animal Breeding, Wolyñska 33, 61-627 Poznañ, Poland
2 Agricultural University of Wroclaw, Department of Genetics and Animal Breeding, ul. Ko
uchowska 7, 51-631 Wroclaw, Poland
3 Agricultural University of Kraków, Department of Genetics and Animal Breeding, Al. Mickiewicza 24/28, 30-059 Kraków, Poland
4 Centre for Genetic Improvement of Livestock, Department of Animal and Poultry Science, University of Guelph, ON, N1G 2W1, Canada
Corresponding author: Tomasz Strabel; e-mail: strabel{at}man.poznan.pl.
Test-day milk yields of first-lactation Black and White cows were used to select the model for routine genetic evaluation of dairy cattle in Poland. The population of Polish Black and White cows is characterized by small herd size, low level of production, and relatively early peak of lactation. Several random regression models for first-lactation milk yield were initially compared using the "percentage of squared bias" criterion and the correlations between true and predicted breeding values. Models with random herd-test-date effects, fixed age-season and herd-year curves, and random additive genetic and permanent environmental curves (Legendre polynomials of different orders were used for all regressions) were chosen for further studies. Additional comparisons included analyses of the residuals and shapes of variance curves in days in milk. The low production level and early peak of lactation of the breed required the use of Legendre polynomials of order 5 to describe age-season lactation curves. For the other curves, Legendre polynomials of order 3 satisfactorily described daily milk yield variation. Fitting third-order polynomials for the permanent environmental effect made it possible to adequately account for heterogeneous residual variance at different stages of lactation.
Key Words: dairy cattle • test-day yield • random regression model
Abbreviation key: ACC = accuracy of breeding value, AG = additive genetic, AS = age and season of calving, BIC = Bayesian information criterion, HTD = herd-test-date, HY = herd-year, PE = permanent environmental, PSB = percentage of squared bias, RRM = random regression model, RV = residual variance, TDM = test-day model.
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