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The objective of this study was to describe the association between the milking frequency and milk production and to determine the effect of milking intervals on the functional properties of the udder of cows in automatic milking systems. Thousand eight milking recordings were enrolled in the study, in total, 106 Holstein cows were observed. The results of studies showed that at a daily milk yield per cow of 22.0 ± 0.6 (means ± SD) kg, the milking frequency was 2 times (13.5 % of the total number of milkings), 32.7 ± 0.4 kg -3 times (57.2 %), 37.7 ± 0.6 kg -4 times (28.0 %), 51.3 ± 4.1 kg -5 times (1.3 %). An increase in the daily milk yield due to a reduction in the milking interval has been established (P<0.001). However, milk yield per milking has the opposite tendency (P<0.001). The average and maximum milk flow rates increased with an increase in the milking interval and reached the highest values in the interval of 7.50-8.99 hours -2.36 and 3.36 kg/min, respectively (P<0.001). Cows with a low indicator of maximum milk flow rate (3.01 ± 0.06 kg/min) had greater milk production (36.6 ± 0.47 kg). With an increase in the milking interval from 4.50-5.99 to 7.50-8.99 hours, it was accompanied by a decrease in the latency period milk flow in the quarters of the udder, when the milking interval reached 9.00-10.49 and 10.50-11.99 hours, the latency period milk flow increased.

The aim of the investigation was to observe the frequency of complex genotypes CSN3/DGAT1 occurrence and their effect on the milk production of different lines cows. It was found that in the herd of cows 4 genotypes CSN3 AA / DGAT1 AK and CSN3 AB / DGAT1 AA, CSN3 AA / DGAT1 AA, CSN3 AB / DGAT1 AK predominate with a frequency of occurrence of more than 7.3%, notably that the greatest number of combinations are found in the lines of M. Chiftein and R. Sowering. The best indices of milk yield (5736 and 5401 kg), milk fat (208 and 201 kg), milk protein (179 and 170 kg) in the lines of V.B. Aydial and R. Sowering were noted in first-calf heifers with the AB/AK genotype. A significant difference (P < 0.05-0.01) in milk yield, fat and protein in the Sowering line was found between the combinations AB/AK and AA/AA, and AB/AK and AB/KK. In M. Chiftein’s line, the highest milk yield (5045 kg), milk fat (182 kg) and protein (163 kg) were detected in the CSN3 BB / DGAT1 AA genotype, when exceeding the combination of AA/AA and AA/AK (P < 0.05-0.01). In high-producing cows V.B. Aydial’s line has the highest milk productivity in cows with the genotype CSN3 AA / DGAT1 AA, when animals with the genotypes AB/AK and AB/KK are exceeded (P < 0.05-0.01).

The research presents the findings of DNA testing of allelic polymorphism by the AC-PCR method of the LEP gene. The research was conducted among 172 Holstein cows at Integrated Agricultural Production Centre “Stud farm named after Lenin” in Atninsky district of the Republic of Tatarstan in 2017–2018. All genotypes of the LEP gene were identified through the research. Associations of the leptin gene polymorphism with dynamics of milk production during three lactations of Holstein cows were established. The best indicators for all three lactations were found in a group of animals with the TT genotype of the LEP gene. These individuals are characterized by increased milk yield, a high yield index, and tend to increase the average daily milk yield during three lactations. These individuals are characterized by increased milk yield, a high milk yield index, and tend to increase the average daily milk yield during three lactations.