Changes in the pedigree structure of mares, bred in the years 1986-1999 at the selected stud farms of the north-eastern Poland, as well as changes in their biometric parameters were analysed. Over the 14 years of breeding at the Plękity, Liski, and Nowa Wioska Stud Farms, an increased percentages of mares with medium and high shares of Thoroughbred were observed in the structure of the mare stock investigated. In the Nowa Wioska and Kadyny Stud Farms, a substantially higher share of Hanoverian blood was also observed. In the stud farms under scrutiny, the biometric parameters of the mares examined also changed. Moreover a significant correlation between biometric parameters of the, mares and the increasing share of Thoroughbred and Hanoverian blood in their pedigree was found.

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