A Novel Additive of Peony Seed Cake to Enhance Alfalfa Silage Quality: Integrating Metabolomic and Microbial Community Perspectives

Background Peony seed cake (PSC) has the potential to be a fermentation supplement in view of abundant bioactive compounds and nutrients, despite a low rate of utilization as an oil production byproduct. In this study, its interaction with lactic acid bacteria (LAB) inoculation and its effects were researched through a factorial design to evaluate the effects of PSC supplementation and LAB inoculation on the fermentation of alfalfa. Results PSC supplementation significantly enhanced the nutritional composition, fermentation characteristics, and sensory evaluation of the alfalfa silage. Specifically, the treatment containing 30% PSC (fresh matter basis) was found to be optimal. Analysis of the microbial community revealed that PSC supplementation effectively reshaped the microbial community and suppressed the growth of undesirable bacteria such as coliforms. The addition of LAB further stabilized the structural changes induced by PSC. GC-MS analysis identified 47 volatile organic compounds. The treatment groups exhibited higher abundances of esters and aromatic compounds, such as methyl benzoate and methyl salicylate. Metabolomic profiling revealed that LAB inoculation accelerated the biotransformation of flavonoid glycosides into their aglycone forms and increased stilbenoid concentrations; structural equation modeling further identified this metabolic shift as the primary driver for the enhanced antioxidant activity of the silage. Conclusions These findings demonstrate that the synergistic effect of PSC and LAB enhances alfalfa silage quality by optimizing its fermentation characteristics, microbiome, metabolome. This work provides a viable strategy for the sustainable valorization of oilseed byproducts, offering a novel pathway for producing functionally enhanced forage.