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<p>It was estimated the conversion of feed into milk, content of urea, fat, protein, somatic cells in the milk of cows of different breeds in different seasons and months of the year. It was established that the peculiarities of the organization of feeding process significantly influence the productive indicators. Thus, in the conditions of a modern milk complex for 1500 cows with an unattached system of keeping in boxes and feeding with the use of mixed rations, the average value of feed conversion (the measures of digestibility of the feed when it "converted" into dairy products) is 1.38 kg of milk per kilogram of dry matter of<br />feed, and the ratio of fat to protein is 1.10, which is lower than the existing standards. It was established that changes in the conditions by months of the year significantly influenced practically all analyzed qualitative characteristics of milk. And this fact entails the corresponding monthly fluctuations of indicators of safety of products, emissions of by-products of metabolism, and, consequently, fluctuations of dairy cattle breeding impact on the environment and ecology. It was found that milk yield positively and significantly (P≥0.999) correlated with the consumption of feed dry matter (r = 0.454) and feed conversion rate (r<br />= 0.547). At the same time, the milk yield is negatively related to the fat content (r = –0.211) and the protein content (r = –0.192) in milk. The level of urea in milk positively correlates with the content of protein in milk (r = 0.130; P≥0.99). The influence of the factor "breed" on the following important economic-useful traits such as daily milk yield (η2 = 0.131) and feed conversion rate (η2 = 0.130) are proved.</p>

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Evaluation and selection of breeding bulls based on economically important characteristics is the main tool in modern systems of genetic improvement of dairy cattle. The purpose of the study was to analyse the influence of environmental and genetic factors on the traits of milk productivity, reproduction, productive longevity, and the difficulty of calving, to assess the genetic parameters and breeding value of breeding bulls based on these traits. The research material was data on the indicators of productivity, reproduction, and productive longevity of cows of the Ukrainian black speckled milky breed of the agricultural cooperative “Vostok” of the Kharkiv region. A multidimensional linear-threshold model was used to calculate genetic parameters (heritability coefficients and genetic correlations) and breeding values of breeding bulls. A probable phenotypic negative association was identified between calving difficulty and milk yield for 305 days (r= -0.2244±0.0266, P>0.999); an increase in calving difficulty by 1 point leads to a decrease in milk yield for 305 days of lactation by 1281.0 kg. Calculated estimates of genetic correlation indicate the presence of a fairly high relationship between milk yield for 305 days of lactation and the service period (over +0.5), a negative relationship between the service period and productive longevity (about -0.37), and a positive relationship between the service period and calving difficulty (about +0.26). This data indicates the presence of genetic antagonism between milk productivity and the level of reproduction of dairy cows, the negative impact of the extended service period on the length of stay of cows in the dairy herd, and the impact of calving difficulty on the deterioration of cow fertility. Estimates of the breeding value of breeding bulls by milk yield, service period, productive longevity, and calving difficulty were calculated. The results obtained indicate the expediency of including traits of reproduction, productive longevity, and difficulty of calving cows in the breeding index, which is used to evaluate and select bulls-producers of the Ukrainian black speckled milky breed

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