Abstract: [Objective] To characterize the differences in milk yield and composition between Yili and Kazak lactating mares throughout various lactation months, so as to provide references for development and utilization of milk characteristics of Yili and Kazak mares. [Method] Twelve healthy Yili mares and twelve healthy Kazak mares of similar age, parity, and foal time were chosen at random. The milk yield of the two breeds of mares at 30, 60, and 90 days of lactation, representing for the first, second and third lactation months, respectively, was recorded under the same dietary nutrition and feeding management levels. Meanwhile, milk samples were collected to determine the milk somatic cell count, milk fat rate, milk protein rate, milk lactose rate, and milk total solid content of the two breeds of mares at different lactation times. [Result] At 30 days of lactation, higher （P>0.05） milk yield, extremely significantly （P<0.01） higher milk fat rate, and significantly （P<0.05） higher milk somatic cell count was observed in the Kazak mares in comparison to the Yili mares, and the other milk composition indexes were higher （P>0.05） in the Kazak mares as well. At 60 days of lactation, the Yili mares had extremely significantly （P<0.01） higher milk yield and milk lactose rate, higher（P>0.05） milk fat rate and milk protein rate, and lower （P>0.05） milk somatic cell count and milk total solid content than those of the Kazak mares. At 90 days of lactation, compared with the Kazak mares, the milk yield of the Yili mares was significantly （P<0.05） higher, the milk lactose rate and milk total solid content were extremely significantly （P<0.01） higher, the milk protein rate was higher （P>0.05）, while the milk fat rate and milk somatic cell count were extremely significantly （P<0.01） lower. [Conclusion] The milk yield and nutritional content of the Kazak mares were higher than those of the Yili mares at the start of lactation （30 days）. The comprehensive lactation performance of the Yili mares was better than that of the Kazak mares at the middle lactation stage （60 and 90 days）. The milk somatic cell count of Kazak mares was higher than that of the Yili mares during the observed lactation periods in this study.

Key words: Yili horse, Kazak horse, milk yield；milk composition