In the Republic of Mari El, the scientists of the Mari State University more than thirty years engaged in the breeding of Russian heavy draft mares breed for milk production. During this time period the milk yield of mares reached 4007–5235 kg of milk in 210 days of lactation. Mass frac-tion of fat in milk of mares ranged from 1,6 % to 2,0 %, and the milking coefficient was 591,5–696,0 kg. The study investigated morphological characteristics of the udder of mares and correlation of the shape and dimensions of the udder of mares with their milk productivity. The build of dairy mares during this time became less massive and slightly more angular, have little apparent traits of dairy type, on the model of dairy cattle. There was a positive correlation between milk productivity and height at withers and body length mares – from +0,01 to +0,47. There was also a positive correlation between milk yield and live weight of mares +0,24. It was found in the study no corre-lations between body measurements and the measurements of the udder of mares. It is assumed that on the basis of this population of Russian heavy draft horse breed interbreed type of dairy animals is formed.