The aim of the study was to analyse the dynamics of changes in economically important traits of dairy cattle: milk fat and protein content, daughter pregnancy rate, productive longevity, residual feed intake, live weight, somatic cell score, when modelling selection for dairy productivity traits such as milk yield, milk fat and protein content. The research material consisted of data from a reference herd of Holstein cows at the “Terezyne” dairy farm (Kyiv region, Ukraine). The total number of cows observed was 14,712 across seven lactations. MRS software was used for modelling. When selecting for milk yield at selection differentials of 100 kg, 300 kg and 500 kg, the genetic response of the next generation for milk yield was 31.59 kg, 94.77 kg and 157.97 kg, respectively. At the same time, there was an increase in such traits as productive longevity, from 0.59 months to 2.94 months, live weight – from 0.36 to 1.79 kg, and the score for somatic cell content in milk – from 0.03 to 0.13. Genetic changes in milk fat and protein content, signs of daughter fertility and residual feed intake had reverse values. Significant phenotypic correlations were obtained between the milk yield of first-calf heifers and milk fat content (-0.2985), milk fat content (+0.9631), milk protein content (-0.2642), milk protein (+0.9924), open days period (+0.0989), productive longevity (+0.0989) and live weight (+0.2199). There were mixed levels of correlation in the daughters of sires between milk yield and milk protein content from -0.1229 to +0.1708, and between milk yield and open days from -0.0726 to +0.1836. Modelling of possible changes in genetic correlations in the range from -0.10 to -0.95 between milk yield and daughter pregnancy rate (DPR) was performed. The rates of correlated response when selecting for milk yield, depending on the values of genetic correlation between these traits, affected the reduction in the level of pregnancy in cows from -0.51 to almost -5.0 points. The impact on the strength and direction of the established relationships can contribute to the stabilisation or deterioration of reproductive traits when selecting for milk yield. The proposed modelling method makes it possible to predict changes in the main traits used for selection, depending on the genetic correlations between them
genetic parameters; phenotypic correlations; dairy productivity; reproduction