Effects of Dietary Non-Fibrous Carbohydrate to Neutral Detergent Fiber Ratio on Apparent Digestibility, Fecal Microbiota, and Plasma Metabolomics in Yili Horses

This study aimed to investigate the effects of dietary NFC/NDF ratio on nutrient apparent digestibility, fecal fermentation parameters, microbial diversity, and plasma metabolomics in Yili horses. Twenty-four healthy Yili horses with similar body weights (406 ± 22.73 kg) were divided into four groups, each with six replicates: the Control Group (CG), Low-NFC Group (LG), Medium-NFC Group (MG), and High-NFC Group (HG). The experiment lasted 52 d, comprising a 7-day adaptation period and a 45-day experimental period. Total fecal collection was conducted from days 41 to 45 to calculate nutrient apparent digestibility. On the final day, rectal fecal samples and blood samples were collected for full-length 16S rRNA gene sequencing and plasma metabolomics analysis. The results revealed the following findings: (1) The apparent digestibility of crude protein (CP) in the MG and HG groups was significantly higher than in the CG (<i>p</i> < 0.01), and significantly higher in the LG group compared to the CG (<i>p</i> < 0.05). (2) Significant differences were observed in fecal pH, propionate concentration, and the acetate-to-propionate ratio between the CG and the experimental groups (<i>p</i> < 0.05). (3) At the phylum level, Firmicutes, Bacteroidota, and Verrucomicrobiota were dominant in the fecal microbiota of all groups. PICRUSt2 prediction indicated that the MG and HG groups primarily enhanced energy conversion efficiency through amino acid metabolism and pantothenate and CoA biosynthesis metabolic pathways. (4) A total of 204 differential metabolites were identified between the CG and MG groups, with 98 upregulated and 106 downregulated in the MG group compared to the CG. These metabolites were mainly enriched in pantothenate and CoA biosynthesis, fructose and mannose metabolism, pyruvate metabolism, and starch and sucrose metabolism pathways. In summary, appropriately increasing NFC/NDF content influences the gut microbiota composition and energy metabolism of Yili horses, thereby effectively improving their digestion and absorption of dietary nutrients.