Age and hereditary factors of somatic cells number variability in milk of Ayrshire cows
The results of the study on the influence of cows age and the genotype of Ayrshire sires on the absolute (ANSC) and logarithmic (scoring estimation, SENSC) number of somatic cells in milk (24,800 individual milk samples were tested) are presented. It is established, that cows have a minimum NSC (106.4-103.2 thousand / cm<sup>3</sup>) in the first lactation. As animals mature, the number of somatic cells increased and reached a maximum in the fifth lactation and older (326.5-413.2 thousand / cm<sup>3</sup>), that is 3.1-3.8 times more than that of first-calving cows. The presence of cows of a given age in a herd can be a risk factor for increased udder inflammation. The hereditary effect of 36 bulls on NSC in milk of daughters was evaluated (24362 tests). The absolute value of ANSC for the daughters of the best and worst bull varied from 65 to 547 thousand / cm<sup>3</sup>, logarithmized (SENSC) - from 1,260 to 3,789 points. These differences indicate a significant effect of the fathers' genotype of on somatic cells content in milk of daughters. In the first lactation, the heritability coefficient of the NSC scoring was relatively low, but reliable (h<sup>2</sup> = 0.0369; P <0.001). The value of h<sup>2</sup> for this trait is rather high when taking into account all lactations of daughters (0.1267; P <0.001) and it agrees with the data of other researchers. The heritability coefficient of the absolute number of somatic cells was much lower (0.0497), what confirms the conclusion about the worst suitability of ANSC for use in breeding. With an increase in the multiplicity of testing during lactation, the h<sup>2</sup> value for the indicators of content and milk properties increased significantly. The presented materials show the possibilities of using the NSC scoring in breeding for increasing the resistance of cattle to mastitis.