Maternal HMB-Ca Supplementation Is Associated with Milk Yield, Foal Growth-Related Outcomes, and Serum and Milk Metabolomic Profiles in Yili Mares: An Exploratory Pilot Study

Lactation imposes substantial metabolic demands on mares, and milk composition is a major determinant of neonatal development. Although β-hydroxy-β-methylbutyrate (HMB) can regulate protein turnover and energy metabolism in several livestock species, its role in the lactation-growth axis of equids remains insufficiently defined. This study evaluated maternal HMB-Ca supplementation in lactating Yili mares, focusing on milk yield measured during a standardized five-session daytime collection schedule, calculated milk-component output during that schedule, foal growth, and day-30 serum and milk metabolomic profiles. Maternal HMB-Ca supplementation was associated with greater milk yield at day 30 and with greater foal average daily gain during early lactation. To reduce over-interpretation of pathway enrichment alone, we performed a structured phenotype-metabolite correlation analysis. First, the day-30 milk-yield endpoint was correlated with serum differential metabolites; second, the serum features showing strong correlations with this endpoint were compared with milk differential metabolites; third, these serum features were related exploratorily to foal body weight and ADG. This analysis identified 26 serum features with |rho| ≥ 0.60 and nominal p < 0.05 for milk yield, 15 of which met FDR q < 0.10. Several of these features showed annotation-level overlap with the milk differential metabolome and exploratory associations with early foal growth traits. These findings support candidate serum-milk-growth metabolic links accompanying improved lactation performance, but they do not establish a causal pathway.