POPULATION-GENETIC PARAMETERS OF LINEAR TRAITS OF EXTERIOR TYPE IN ASPECT OF THEIR USE IN THE SELECTION PROCESS OF IMPROVING DAIRY CATTLE FOR MILK PRODUCTIVITY AND LONGEVITY

A literature review was conducted to study the variability of heritability of linear exterior traits and their correlation with milk productivity and height in the breeding process of improving dairy cows of different breeds and breeding countries. Most studies conducted by both domestic and foreign authors have established a reliable positive relationship between group linear traits and cow milk yield in the first lactation with significant variability in correlation coefficients from r = 0.017 to r = 0.700, depending on the assessment of the complex of linear traits, breed, and variants of combinations of genealogical formations. The existence of close and reliable correlation variability between descriptive linear traits of type and milk yield of firstborn cows has been reported by a large number of authors (from r = -0.48 to r = 0.75), which also depends on the breed and country of study. Similarly, for the same reasons, the heritability of linear type traits in dairy cows according to literature sources also varied significantly. The lowest heritability of descriptive type traits was obtained in Holstein cows from Italy (h2 = 0.049), and the highest – from Turkey (h2 = 0.62). Currently, the solution to the problem of longevity of dairy cattle is limited to the possibility of selecting future mothers due to the fact that signs of longevity can be obtained only at the end of the cows' lives. Therefore, linear traits of the exterior type have formed the basis of indirect selection for the longevity of cows due to the fact that some of them have a positive genetic correlation with life expectancy, have moderate, and often sufficient for effective selection heritability. It is especially important that the assessment of linear traits is obtained at the beginning of the cows' lives. Because of this, they can be used as indirect predictors of longevity at the early stages of animal evaluation.