Giving pregnant mares extra branched‑chain amino acids (BCAAs) changes the milk they produce, making it richer in fat and boosting growth hormones like IGF‑1 in their foals. A medium dose (about 76 g per day) helped foals grow faster without stressing the mother’s metabolism, while a high dose gave bigger hormonal spikes but also raised blood urea, a sign of nitrogen overload.

This study evaluated the effects of graded supplementation of branched-chain amino acids (BCAAs) in lactating mares on lactation performance, foal growth, and metabolic responses. Twenty mare-foal pairs were assigned to control, low- (38 g/d), medium- (76 g/d), or high-dose (114 g/d) groups. Milk and blood samples were collected over 60 days for composition, hormone, and metabolomic analyses. Fecal microbiota from the foals was also examined. BCAAs supplementation interacted with lactation stage, enhancing milk fat yield and increasing milk growth hormone and progesterone. The medium dose (76 g/d) was effective, while 114 g/d showed the strongest effects. High-dose BCAAs altered organic acid abundance, influencing lipid, energy, and BCAA metabolism, correlating with milk composition changes. In foals, altered milk reduced serum BCAAs and other amino acids but elevated growth hormones (GH, INS, IGF-1) dose-dependently. Antioxidant and immune parameters were unaffected. The high dose increased blood urea nitrogen, indicating higher nitrogen load, whereas the medium dose supported growth without metabolic stress. Fecal microbiota analysis revealed enriched amino acid degradation pathways, especially for BCAAs. We conclude that BCAAs supplementation regulates milk fat synthesis and promotes foal growth via a milk-microbiota-metabolism axis, providing a basis for improving milk quality and offspring development through maternal nutrition.