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Economically useful traits of cows depending from the various dates of their first insemination

In Sverdlovsk region Holsteinized black-and-white cattle of the Ural type are being bred. This type of cattle is represented by numerous, highly productive animals with a high genetic potential for productivity. The aim of this article is to study the correlation of age of the first insemination of heifers on their economically useful traits. It was found that the age of the first insemination directly affects the milk productivity of Holsteinized black-and-white cattle of Ural type. The difference in age of the first insemination between the groups was 2.7 months, or 18.6%. The difference of productive period longevity between the groups was also observed, which was 0.21 of lactation (6.7%) in favor of the group that got its first insemination at age of 16-18 months. Cows inseminated for the first time at an earlier date featured lower parameters of milk productivity: lower milk yield for life, lower milk yield for the first lactation and for maximum lactation. The difference was significant at P≤0.01 in favor of cows which underwent the first insemination at age of 16-18 months for life-long milk yield. A significant difference in MFP in milk was revealed in favor of the cows in the first group (the period of the first insemination was 14-15 months). It was 0.09% at P≤0.01. The difference in MFP in milk did not have a considerable influence on increase of nutrients yield in milk, in particular milk protein. The duration of the period between calvings and the service period was long in both groups of cows, which allows concluding that, regardless of the age of first insemination, the decrease in reproductive capacity in cows of modern Holsteinized black-and-white cattle is observed. This is proved by the record KVS – 0.87 - 0.88.


Introduction
In the Main directions of Agricultural Development, the peculiar attention is paid to the development of breeding animal husbandry, as this industry is responsible for the sustainable supply of the population with nutritious, high-quality food like milk and dairy products. The dairy products can be consumed by people of any age, health status and income category, which ensures the health of the nation and food security of any country [1][2][3][4][5][6][7].
In this regard the peculiar attention is paid to the development of dairy cattle breeding, since the main share of milk (more than 97% of the total production) as a valuable food product and raw material for the dairy industry, is obtained from cattle. Strict requirements for milk quality are imposed on raw milk. 2 Therefore, along with increasing the productivity of cattle, the other tasks are set to the industry, in particular -the improvement the quality parameters of milk [8][9][10][11][12].
For milk production dairy cattle of both domestic and foreign selection are used. The main livestock is represented by the domestic black-and-white breed. In the recent few decades the genetics of the world best dairy breed, i.e. the Holstein, has been widely used to improve the animals' quality. Long-term introduction of foreign breeding Holstein bulls led to creation of a numerous herd of Holsteinized cattle in various climatic and ecological forage zones of the country, which also differs among themselves in economically useful traits and biological characteristics, inherent to the breed resources of cattle within the breeding zone, used as an improved breed, the country of origin of the breeding bulls involved in crossbreeding and the food supply [13][14][15][16][17][18][19].
In Sverdlovsk region the broodstock of the black-and-white breed of the Ural offspring was inseminated with the servicing bulls of Canadian, Danish and German breeding. As a result, largebodied, highly productive animals with a high genetic potential for productivity and well adapted to industrial milk production were obtained. In 2002 the Ural type of black-and-white cattle was officially registered with percentage of Holstein blood relationship of 75%. In subsequent years the purebred Holstein bulls continued to serve the herds, and the proportion of blood relationship in the broodstock increased to 91% and more for the Holstein breed [19][20][21][22].
Along with the increase in milk productivity, there was a decrease in productive longevity, which raised the acute issue of the herd reproduction and rising of young animals for replacement.
The research of correlation of the age of the heifers first insemination on their economically useful traits is relevant and of practical importance.
The aim of the research is to study the influence of the age of the heifers first insemination on their economically useful traits.

Materials and method
The research was run in one of the breeding farms engaged in breeding the Holsteinized black-and-white cattle of the Ural type. The research included cows that completed their lactation as of 01.10.2020. The animals were separated into 2 groups, depending on the age of their first insemination. Group 1 was inseminated at age of 14-15 months, and group 2 was inseminated at age of 16-18 months.
Data and records of zootechnical and breeding information from Selex database were used for analysis. Milk productivity was taken into account by control milking once a month, as well as by measuring of milk quality parameters: mass fraction of fat (MFF) and mass fraction of protein (MFP) in milk. These parameters per every cow were checked monthly in a dairy laboratory of OJSC "Uralplemcenter" of Sverdlovsk region. The coefficients of milk content, the amount of milk fat and milk protein were calculated. Reproductive functions of young cows were assessed according to the periods of their physiological cycle, the frequency of insemination and the coefficient of reproductivity.

Results and discussion
The economically useful traits of cows include parameters of milk productivity, which is assessed both in terms of quantitative and qualitative parameters. Figure 1 below shows the group ages at their first insemination.
The figure above clearly shows that the difference in age of the first insemination between the groups was 2.7 months, or 18.6%. The difference in the productive period longevity between the groups was also observed, which was 0.21 lactation (6.7%) in favor of the group with the first insemination at age of 16-18 months. Cows that were inseminated for the first time at an earlier age had lower milk productivity: lower milk yield for their life, lower milk yield for the first lactation and for maximum lactation. The difference was significant at P≤0.01 in favor of cows with the first insemination at age of 16-18 months in terms of life-long milk yield (figure 2).  The positive trend was observed in terms of increase in milk yield in the first and maximum lactation among the cows that were inseminated at age of 16-18 months. Along with milk yield, great attention was paid to milk quality parameters, such as MFF and MFP in milk (figure 3).  A significant difference in MFP in milk was recorded in favor of cows of the first group (age of the first insemination was 14-15 months). It was 0.09% at P≤0.01. The difference in MFP in milk did not give a significant influence on increase of milk nutrients yield, in particular milk protein. However, the trend was observed to increase of milk fat from cows with a first insemination period of 16-18 months. Thus it can be concluded that the yield of nutrients per lactation depends on milk yield per lactation ( figure 4).  The longevity of those periods was high in both groups of cows, which allows concluding that, regardless of the age of the first insemination, the decrease in reproductive functions in cows of modern Holsteinized black-and-white cattle was observed. The difference in longevity of these physiological periods, which were shorter in cows with a first insemination at age of 16-18 months, was insignificant. The longevity of the period between the calvings influenced on the coefficient of reproductive capacity, which ranged within 0.87-0.88, respectively per groups (figure 6).

Conclusion
Based on the foregoing, it can be concluded that the age of the first insemination affects the milk productivity of Holsteinized black-and-white cattle of the Ural type. The best yields were shown in cows with the first insemination at age of 16-18 months. There were no significant differences in reproductive qualities in cows with different ages of their first insemination. Age of first insemination, months 14-15