Effects of Concentrate Supplementation Under Grazing Conditions on Milk Yield and Milk Nutritional Composition in Yili Mares

Under grazing conditions, it is difficult for lactating Yili mares to meet their nutritional requirements and those of their suckling foals solely through the consumption of natural pasture. Furthermore, seasonal variations and rainfall significantly influence the quality and nutrient content of forage, which severely constrains the healthy breeding of Yili horses and the industrial development of mare milk resources. Therefore, this study aimed to investigate the effects of concentrate supplementation on lactation performance and milk concentrations of amino acids, fatty acids, and mineral elements in Yili horses under grazing conditions. Twenty-two healthy Yili mares in early lactation, with similar ages (3–4 years), foaling dates, and body weights (391.5 ± 13.74 kg), were randomly assigned to either a grazing group (G, n = 11) or a grazing + supplementation group (GS, n = 11). Mares in group G grazed naturally on pasture, while those in group GS received 1 kg of concentrate supplement twice daily (totaling 2 kg/day) in addition to grazing. The experimental period lasted for 100 days, including a 10-day adaptation period and a 90-day formal experimental period. The results showed that: (1) In terms of lactation performance, the GS group exhibited highly significant increases in milk yield and lactose yield (p < 0.01), as well as significant increases in milk protein and milk fat yields (p < 0.05), with an extended duration of the peak lactation period. (2) Regarding the amino acid profile, the concentrations of threonine (Thr), serine (Ser), glycine (Gly), and alanine (Ala) in the milk of the GS group were significantly higher than those in the G group (p < 0.05), whereas the proline (Pro) content was significantly lower (p < 0.01); supplementation improved the uptake of certain functional amino acids by the mammary gland. (3) Concerning the fatty acid profile, the concentrations of polyunsaturated fatty acids (PUFA) and alpha-linolenic acid in the milk of the G group were significantly or highly significantly higher than those in the GS group (p < 0.05 or p < 0.01). (4) For mineral elements, concentrate supplementation highly significantly decreased the potassium (K) content and the K/Na ratio in horse milk (p < 0.01), highly significantly increased the levels of iron (Fe) and cobalt (Co) (p < 0.01), and significantly enhanced the chromium (Cr) content (p < 0.05). In conclusion, concentrate supplementation during grazing improved lactation performance in Yili mares, primarily by increasing milk yield and extending the peak lactation period. However, grazing alone was more favorable for maintaining higher PUFA and α-linolenic acid proportions in milk. Therefore, concentrate supplementation should be regarded as a nutritional strategy that increases milk output and modifies amino acid and mineral element composition, but may involve a trade-off with some beneficial fatty acids.