Selection efficiency of first-calf heifers using various index scores in Ayrshire dairy cattle herds

Currently, a linear exterior estimation of dairy breeds is widely used which, under acceptable conditions of feeding and keeping animals, allows for objective selection. The optimal selection method, which takes into account several economically useful characteristics at once, is index breeding. In the course of the work, based on the calculated coefficients of genetic and phenotypic correlations, variances and covariances, heritability coefficients and taking into account the economic component of the studied traits, the authors have developed three variants of the productive-exterior indices: IAYR, IAYR_udder and IAYR_feet which include milk yield for 305 days, fat and protein yield, UDC udder indices (Udder Dairy Composite) and FLC legs (Feet & Legs Composite) with different specific weights of each of them. The purpose of the research is to compare three variants of selection of Ayrshire cows using the proposed breeding indices. The article uses data of 2348 Ayrshire heifers of the leading breeding farms in the Leningrad and Kirov Oblasts, estimated by the exterior in the period from 2006 to 2020. When modeling the selection according to IAYR, Sd breeding differentials for milk yield for 305 days of lactation, fat and protein yield in cows of the Leningrad population were 745.4; 26.7 and 21.8 kg; in the Kirov population, it was 880.8; 38.0, 31.5 kg, respectively, having significant advantages over the selection results according to IAYR_udder and IAYR_feet indices. However, using IAYR_udder and IAYR_feet indices showed improvements in classification score of udder, legs, and overall score, as well as UDC and FLC indices.