One of the ways to increase the profitability of dairy horse breeding is to improve the milk production of mares through breeding. The main characteristics for selecting mares to dairy herd are indicators of their own productivity: lactation duration, productivity for 210 days of lactation and milk composition (mass fraction of fat and protein). An important indicator is the type of constitution of an animal, it must correspond to the milk direction. However, the first lactation in mares is completed only by the age of five years. This leads to the use of little productive animals that do not cover the costs of their rearing. Purpose. In connection with the above, it is economically more profitable to cull mares with low productivity from the herd by early prediction of their potential. Materials and methods. Studies were conducted on mares of Russian Heavy Draft breed. The venue of the experiment was CJSC Breeding Factory “Semenovsky” and the laboratory for clinical diagnostics and biochemical research of the Department of Livestock Production Technology at the Mari State University. The method is based on the dependence of milk production of mares on blood groups of D system. With the help of methodological recommendations of R. M. Dubrovskaya and I. M. Starodumov blood groups were determined. Red blood cell antigens of systems A and D of blood groups were typed by monospecific sera. Milk production of mares was studied using control milking. Daily milk productivity was determined by I. A. Saigin’s formula. Results, discussion. The conducted studies show that there is a relationship between milk production and genotypes of D system blood groups in mares of Russian Heavy Draft breed. The genotypes of D system blood groups, which decrease and increase the milk productivity of mares, were revealed. Conclusion. This fact allows us to recommend the use of genotypes of D system blood groups as a sign of selection of mares to the milking herd of koumiss complexes in the early stages of growth and development of mares.