Introduction. In modern animal husbandry an interest in productive horse breeding appears again. This is due to the fact that mare's milk products are in demand among consumers. In this regard, farms are actively increasing the productivity of dairy mares. Most companies have relied on animal breeding. This article proposes another way to increase productivity by changing the technology of growing of Lithuanian heavy horse brood mares. Purpose. The development of the technology of growing mares of Lithuanian heavy horse breed allows to increase the mare's milk production. Materials and methods. Studies were conducted on mares of the Lithuanian heavy horse breed. The stud farm “Semenovsky” was chosen as the location for the experiment. The method was based on determining the optimal timing of the first insemination of mares. This was to increase the term of their economic use. The method includes the characteristics of keeping, feeding and growing young mares of heavy breeds. Results, discussion. The age of first insemination influenced various indicators of the economic use of mares. Thus, milk production was higher in mares which first foaled between the ages of 2 and 3 years. The duration of economic use of mares, foaled for the first time at the age of 2.5 to 4.5 years was the same, therefore, early insemination had no effect on the duration of economic use of mares. Reproductive qualities deteriorated with increasing age of the first insemination. The older the mare was during her first insemination, the more time during her life she was not inseminated. Conclusion. Early insemination of mares – in 2 years instead of 3 – allows you to: reduce by a third the growing time of mares for breeding, reduce the cost of growing one young mare by 60 thousand rubles and increase the proportion of mares in the overall structure of the herd.