Genetic parameters for first and second lactation milk yields of polish black and white cattle with random regression test-day models

Single- and two-trait random regression models were applied to estimate var iance components of test-day records of milk, fat, and protein yields in th e first and second lactation of Polish Black and White cattle. The model in cluded fixed herd test-day effect, three Covariates to describe lactation c urve nested within age-season classes, and random regressions for additive genetic and permanent environmental effects. In two-parity models, each par ity was treated as a separate trait. For milk and the two-parity model, her itabilities were in the range of 0.14 to 0.19 throughout first lactation an d 0.10 to 0.16 throughout second lactation. For fat, heritabilities were wi thin 0.11 to 0.16 and 0.11 to 0.22 throughout first and second lactations, respectively. For protein in the two-parity model, heritabilities were with in 0.10 to 0.15 throughout most of first lactation and within 0.06 to 0.15 throughout the most of second lactation. For milk, genetic correlations bet ween the first and second parities were 0.6 at the beginning of the lactati on, rising to 0.9 in the middle, and 0.8 at the end of the lactation. For f at, the corresponding correlations were 0.6, 0.8, and 0.7, respectively, an d for protein were 0.6, 0.8, and 0.8, respectively. Heritability estimates for all traits were flatter for the two-parity model. Relatively smooth gen etic and permanent environmental variances with the two-parity model indica ted that large swings of heritabilities could be artifacts of single-trait random regression models. High correlations between most of test day record s across lactations suggested that a repeatability model could be considere d as an alternative to a multiple-trait model to analyze multiple parities.