Determination and use of bulls breeding value indices for somatic milk cells in daughters during selection for mastitis resistance
A method has been developed for determining and using bulls breeding value indices by the number of somatic cells in daughters’ milk during selection for mastitis resistance. The development is based on the investigation of somatic cells number (NSC) in 24800 milk samples of Ayrshire cows of the «Megrega» breeding plant, Republic of Karelia. The average lactational milk yield of cows in the herd was about 8300 kg of milk. For one bull an average 676 assessments of NSC in milk of daughters were made. The calculations used the logarithmic (scoring estimation) NSC. The transformation of absolute NSC (ANSC) in thousand / cm<sup>3</sup> into a scoring estimation (SE NSC) was carried out on the basis of the binary logarithm. Breeding Value Index (BVI) of bulls was calculated by comparing SE NSC of daughters and populations. At the same time, the number of cows-peers and the size of the population were taken into account; the heritability coefficients and the repeatability (regression) coefficients of the indicators for the future daughters of this bull on indicators of existing daughters were determined for specific conditions. Examined bulls were different in BVI. Since selection by health indicators, including mastitis, should be carried out to reduce the value of the indicator, then in this situation the best are the bulls with the minimum values of SE NSC and negative values of the breeding index. The three best sires had a BVI of less than minus one, the three worst ones had more than plus two. The scale has been developed for the distribution of estimated bulls by BVI on breeding categories - effective improvers of the 1st category (BVI is less than the population average by 1σ or more), improvers of the 2nd category (from - 0.4 σ to - 0.9 σ), neutral (+ 0.3 σ), deteriorators (from + 0.4 σ to + 0.9 σ), extreme deteriorators (+1 σ and more). Sires with a high BVI (- 1.012 or less) are selected for breeding use, rejected with a low BVI (+ 1.102 or more). In the studied bulls’ population, the share of 1-st category improvers was 11.1%, 2-nd category - 27.8%, neutral - 22.2%, and extreme deteriorators - 16.7%. Using the method can improve the objectivity and accuracy of selecting bulls for resistance, increase in the concentration of mastitis resistance genes in herds, and decrease the frequency of udder diseases.