Introduction. Numerous studies of domestic scientists testify to the sometimes significant, but multidirectional influence of the year and season of calving and birth on the formation of milk productivity and other economically useful traits. In our previous studies, it was established that at the population level, among the mentioned environmental factors, the most significant influence on the phenotypic variability of the milk productivity of first-calf heifers is caused by the herd (15–22%), the year of the first calving (4.7–12%) and birth (4.2–12%), the lowest is the season of birth (0.05–0.2%) and calving (1.5–2.1%). A number of other researchers report a higher influence of the herd and the year and a smaller one of the calving season and especially the birth on the milk productivity of first-calf heifers. In our opinion, the phenotypic manifestation of signs of milk productivity in different herds is influenced by different levels of breeding and feeding, technological solutions of cow keeping and milking systems. Research materials and methods. The research was carried out on farms breeding Ukrainian Red dairy cattle of LLC "Nova Niva" of the Donetsk region. The materials of the electronic information database in the DCMS ORSEK format as of October 11, 2021 were used. In controlled animals, the intensity of growth of live weight by growing periods up to one and a half years of age, live weight at 18 months, age of first calving, length of the service period, coefficient of reproductive ability between the first and second carvings, fertility and output of milk fat and protein for the first 305 days three and higher lactation. Research results. On average, during the entire accounting period, the level of rearing of heifers up to a year ensured average daily gains in live weight at the level of almost 600 g, and at the age of 12 to 18 months – more than 550 g. Thus, the live weight of heifers and heifers at the age of one and a half practically met the standard of the Ukrainian Red dairy breed. The age of the first calving reached almost 27.9 months, which corresponded to the optimal recommendations at the time of the research (27 months). In general, the first-calf heifers under control were characterized by a relatively low reproductive ability. The duration of the service period is 72 days or 90% longer than the optimum (80 days). Univariate variance analysis established an almost equal share of the influence of the year of birth and calving factors on the studied characteristics of cows. The year of birth determines from 12.6 to 68.6% of the total phenotypic variability, the year of the first calving – from 12.5 to 70.4% with a high degree of significance. The year factor has a 1.5–2 times greater influence on live weight gains of heifers up to a year old than at the age of 12–18 months. The influence of the year of birth is 10% greater than that of the year of the first calving. The factor of the season of birth (0.1–2.3%) and calving (0.2–2.7) exerts a much smaller influence on the variability of the investigated traits. The influence of seasonal environmental factors on the growth of live weight of heifers and the age of first calving turned out to be somewhat higher. On the signs of milk productivity, seasonal fluctuations in environmental conditions show a low, but reliable influence only during the first lactation. During the second and older lactations, this effect is practically leveled off to an unreliable level of statistical significance. In most cases, intergroup differentiation was higher between years of birth than between years of first calving. In our opinion, this is caused by a higher impact on the productivity of different levels of cultivation in chronologically and naturally climatically different economic years, than a more stable level of feeding dairy cows over the years. Correlation analysis established a direct and reliable relationship between the delivery of first-calf heifer and the average daily increase in live weight of heifers from birth to three months at the level of 39.3 ± 1.51% (P < 0.001), from three to six months – 14.4 ± 1.63% (P < 0.001), from six months to a year – 31.4 ± 1.56% (P < 0.001) and from 12 to 18 months – 22.4 ± 1.60%, which confirms our assumptions. Undoubtedly, the increase in the intensity of growth and milk productivity of cattle cannot be caused only by the improvement of technological and economic conditions. A significant share is accounted for by the genetic improvement of the herd, in particular, the increase in conditional blood of cows introduced into the herd of the improving Holstein breed from 25.3 to 87.5%. In terms of growth intensity, heifers of the autumn birth season had an advantage, which in terms of live weight exceeded the counterparts of the spring season by 22 ± 2.4 kg or by 6.4% (P < 0.001). This resulted in a younger age at first calving (by 49 ± 5.6 days or 5.9%, P < 0.001). According to the season of the first calving, cows that calved in the summer had a slight advantage in terms of live weight at the age of one and a half years, which exceeded the analogues of the spring calving season by 22 ± 2.6 kg or by 6.3% according to P < 0.001. The animals of the winter calving season were characterized by a younger age at first calving (by 42 ± 5.9 days or by 5.0% at P < 0.001 compared to first-calf heifers of spring calving). Cows of the autumn season of the first calving were distinguished by better milk yield and higher lactation, which exceeded spring calving animals by 203 ± 57.5 kg or 3.0% (P < 0.001). Conclusions. The year of birth determines from 12.6 to 68.6% of the total phenotypic variability of the intensity of growth, reproductive ability and milk productivity of cows, the year of the first calving – from 12.5 to 70.4% with a high degree of significance (P < 0.001). The factor of the season of birth (0.1–2.3%) and calving (0.2–2.7%) has a much smaller influence on the variability of the investigated traits. The influence of seasonal environmental factors on the growth of live weight of heifers and the age of first calving turned out to be somewhat higher. The comparison of group averages confirmed the significantly smaller influence of the seasonal factor on the development and productivity of cows compared to the year of birth and first calving, established by variance analysis.

The aim of the research. In breeding practice, it is important to select and take into account genetically and physiologically determined patterns of age recurrence and relative variability (correlation) between different economically useful traits in the process of ontogenetic development of animals. This opens up opportunities to increase the efficiency of selection through the use of early selection (forecasting) on indirect grounds. The aim of our research was to identify and analyze the relative variability of selected traits of dairy cattle in the herd of a leading breeding farm for breeding of the Ukrainian red dairy breed. Materials and methods of research. The study was conducted on the basis of primary breeding records in the herd of one of the leading breeding farm for the breeding of the Ukrainian Red dairy cattle in the LLC "Russia" of Donetsk region. The materials of the electronic information database in the format of the Dairy Management System “ORSEK” were used. To substantiate the accounting period, the average milk yield of the first heifers of the herd was calculated according to the years of the first calving. Significant differences in animal productivity in the chronology from 2005 to 2015 of calving were established. Controlled cows were evaluated by the age of the first calving, the coefficient of reproductive ability and milk productivity and live weight for the first three lactations. A total of 1061 first heifers were evaluated on the exterior during the control period. Regularities of relative variability established in the herd were studied by correlation analysis of milk productivity of first heifers with productivity for subsequent lactations (age repeatability), measurements, linear descriptive features and body structure indices. Research results. Correlation analysis was established a different level and direction of the relationship between the indicators of milk productivity of first heifers with some of the studied traits. First of all, it is worth emphasizing the relatively higher and highly reliable relationship of all studied indicators of milk productivity of first heifers with conditional blood by improving breeds. In the studied breeding herd, the natural antagonism between the main selected signs of milk yield and fat content in milk was overcome. The correlation between these features was quite noticeable, direct and reliable at a higher degree of statistical significance. It is considered positive for the selective improvement of the herd, although the low but highly reliable inverse correlation of the milk yield of the first heifers with the age of the first calving is established. The average age of calving of the studied first heifers was quite late – 965 ± 3,0 days or 31,7 months. The average daily gains of live weight of heifers up to a year was 630 ± 2,5 g, at the age of 12–18 months – 549 ± 2,6 g. The first calving at the age of two (24 months) is currently considered optimal for the Holstein breed. The intensity of growing heifers up to a year is desirable at the level of 750–800 g per day, in 12–18 months – 650 g. Significant direct relationship between milk yield, milk fat content and yield for 305 days of lactation of first heifers with the duration of the period between the first and second calving (0.12… 0.30 at P < 0,001) and the inverse with the coefficient of reproductive ability -0,14… -0,34 for P < 0.001) confirms the presence of natural antagonism between milk productivity and reproductive function of cows. A significant (r = 0.29…0.48 for P < 0.001) level of age recurrence of milk productivity for the first three lactations was established. The milk yield and milk fat of first heifers are positively and reliably correlated (r = 0.12… 0.19 by P < 0.001) with height at withers and rump, length of trunk and rump, width of biiliac, width of biischiatic and perimeter of the thorax. With the height and width of the chest, this relationship was low and unreliable, and with the perimeter of the shin – low, reverse and unreliable. The fat content in milk shows a significant (P < 0.01…0.001) direct relationship (r = 0.09…0.17) with the width of biischiatic, perimeter of the shin and length of rump and reliable (P < 0.001 ) inverse (r = -0,12…-0,17) with the height and width of the chest in the practical absence of correlation with the rest of the measurements. The relationship between the estimated linear evaluation and the milk productivity of the first heifers was low, unreliable, and divergent in five of the ten descriptive features. Conclusions. Milk productivity of first heifers of Red dairy cattle is directly proportional by Holstein conditional blood (r = 0.22…0.29) and inversely proportional (r = -0.15…-0.17) to Angler breeds. Positive for the breeding improvement of the herd is the established inverse correlation (r = -0.10…-0.18, P < 0.001) by milk yield the first heifers with the age of the first calving. The detected antagonism (r = -0.14…-0.34, P < 0.001) between milk productivity and reproductive function makes it impossible to simultaneously select for milk and fertility. The established reliable, statistically significant level (P < 0.05…0.001) of the relative variability of individual features of the exterior by different methods of its evaluation with the milk productivity of first heifers confirms the possibility and feasibility of indirect selection of cows of the desired type of exterior. The established noticeable (r = 0.29…0.48 for p < 0.001) level of age recurrence of milk productivity for the first three lactations gives grounds to expect sufficient efficiency of mass selection of first heifers by their own productivity.