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The purpose of the research was to assess changes in the lactation activity of cows and their impact on the level of reproduction, as well as to study their fluctuations with respect to genetic and environmental factors. A sample of 807 cows of various ages was formed for the study. The cows were kept on a commercial farm in Kharkiv region, Ukraine. Data from DairyPlan C21 software were used in the study. It was revealed that the parameters of the Wood lactation curve model were significantly influenced by the calving season, days open, and cow's origin; lactation persistency of cows was more influenced by environmental factors (parity, calving season, days open) than by the sire. In order to improve the level of reproduction in dairy herds, it is advisable to take into account the lactation persistency of cows, since it has negative relationship with days open (r = -0.074). To improve lactation persistency it is necessary to use for insemination of cows semen of sires with high breeding values for this trait.

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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>