Genetic parameters of protein efficiency and its relationships with yield traits in lactating dairy cows

The aim of this study was to estimate genetic parameters for protein efficiency and its correlations with milk yield traits in lactating dairy cows. The data used were 2824 monthly records of 501 lactating Holstein cows located in 2 herds. Three methods were used to measure protein efficiency: Gross efficiency of crude protein (GECP) was calculated as protein yield in milk/CP intake. Crude protein balance (CPB) was measured as the difference of CP intake and protein excreted in milk. Residual protein intake (RPI) was calculated as the difference of actual and predicted CP intakes. Variance and covariance components were estimated using univariate and bivariate analyses of animal mixed models, respectively, using derivative free approach of restricted maximum likelihood algorithm. The estimated heritabilities for GECP, CPB and RPI were 0.07, 0.40 and 0.03, respectively. Repeatability estimates for GECP, CPB and RPI were 0.12, 0.40 and 0.32, respectively. Different measures of protein efficiency had low phenotypic correlations with the yield traits and dry matter intake, while noticeable genetic correlations were observed between different measures of protein efficiency and some of yield traits. It seems that protein efficiency could be improved by direct selection against CPB. Selection for milk fat and protein percentages is probably an effective way for indirect improvement of protein efficiency.