Exploring the Fermentation Products, Microbiology Communities, and Metabolites of Big-Bale Alfalfa Silage Prepared with/without Molasses and Lactobacillus rhamnosus

The influence of molasses (M) and Lactobacillus rhamnosus (LR) on fermentation products, microbial communities, and metabolites in big-bale alfalfa silage was investigated. Alfalfa (Medicago sativa L.) was harvested at the third growth stage during the flowering stage in the experimental field of Linhui Grass Company from Tongliao City, Inner Mongolia. An alfalfa sample without additives was used as a control (C). M (20 g/kg) and LR (106 cfu/g) were added either alone or in combination. Alfalfa was fermented for 7, 14, and 56 d. Lactic acid content in the M, LR, and MLR groups increased, whereas the pH value and butyric acid, 2,3-butanediol, and ethanol contents decreased compared to those of C group after 7, 14, and 56 d of fermentation. A two-way analysis of variance (ANOVA) was performed to estimate the results. The LR group exhibited increased Lactobacillus abundance, whereas the M and MLR groups showed increased Weissella abundance compared to the C group. The relative contents of amino acids (tyrosine, isoleucine, threonine, arginine, valine, and citrulline) in the M and MLR groups were higher than those in the C group. During fermentation, the M, LR, and MLR groups showed decreased phenylalanine, isoleucine, and ferulic acid contents. Amino acids such as isoleucine and L-aspartic acid were positively correlated with Lactobacillus but negatively correlated with Weissella. In conclusion, combining high-throughput sequencing and liquid chromatography–mass spectrometry during anaerobic alfalfa fermentation can reveal new microbial community compositions and metabolite profiles, supporting the application of M, LR, and MLR as feed fermentation agents.