Dynamic Changes in Milk Production, Nutritional Composition, and Bioactive Substances of Milk from Yili Horses Across Different Lactation Stages

Mare milk is rich in nutrients and bioactive compounds, and its composition changes throughout lactation. This study investigated variations in the production, nutritional composition, and bioactive components of Yili mare milk across lactation stages. Twenty-six healthy grazing Yili mares were sampled on days 1, 10, 30, 60, 90, and 120 of lactation. Milk production, nutritional components, amino acids, fatty acids, minerals, vitamins, and immunologically active proteins were analyzed. Milk production peaked on day 30 and then declined. Colostrum contained significantly higher fat, protein, solids-not-fat, total solids, minerals, lactoferrin, lysozyme, and immunoglobulins than mature milk (p < 0.05), whereas lactose increased and stabilized after day 30. Essential amino acids peaked on day 30. As lactation progressed, saturated fatty acids decreased while polyunsaturated fatty acids increased. Vitamin profiles also varied across lactation, with ascorbic acid increasing during late lactation. β-casein content was higher during mid-lactation. In summary, colostrum is enriched in immunoactive proteins and minerals, whereas mature milk exhibits a more balanced amino acid and fatty acid profile. While these observed variations likely reflect the combined effect of lactation stage and seasonal pasture fluctuations under natural grazing, these findings provide practical insights into changes in milk composition in grazing Yili mares and may support the development of mare milk products under similar grazing systems.