Background Leymus chinensis is one of most valuable forage on the native grassland for its higher nutrition content. But the local farmer often gained the worse Leymus chinensis hay due to the unfavourable harvest stage, the nutrition of Leymus chinensis was decreasing from jointing stage to solid stage fro its growth characteristics, and the results showed that the crude protein (CP) decreased and the fiber content increased which will caused the nutrition of forage reduced. However, the reason for the nutrient changes in Leymus chinensis has not yet explained at the metabolic level. Results In this research, the Leymus chinensis was harvested at two growth stage(jointing stage and solid stage).Metabolomics was used to analyze the expression changes of metabolites in the whole plant of Leymus chinensis at different harvest stage, and the metabolites were analyzed based on LC-MS/MS untargeted metabolomics. A total of 55 metabolic pathways and 119 metabolites were related to the nutritional change in Leymus chinensis. Most of the metabolites are involved in metabolic processes, including amino acid metabolism, carbohydrate metabolism and acid metabolism biosynthesis. Under the action of EBF1/2 and EIN3, cysteine participates in the ubiquitin-mediated protein degradation pathway. In solid stage, the increasing content of cysteine directly promotes the ubiquitin-mediated protein degradation rate, which is manifested reduce the synthesis of crude protein (CP) in Leymus chinensis, resulting in the decreasing of Leymus chinensis nutrient quality. Carbohydrate metabolism provides raw materials for the synthesis of hemicellulose, leading to the increasing of hemicellulose. In addition, phenylalanine provides the necessary conditions for the synthesis of lignin. These are the internal factors that lead to the decline of quality of Leymus chinensis. Conclusions This study analyzed the elucidate the relationship between the reduction in the nutritional value of Leymus chinensis and complex biological processes by the metabolomic,This study will provides a theoretical basis for producing high-quality Leymus chinensis hay and sets the stage for further research.

Background: Leymus chinensis is one of most valuable forage on the native grassland for its higher nutrition content. But the local farmer often gained the worse Leymus chinensis hay due to the unfavourable harvest stage, the nutrition of Leymus chinensis was decreasing from jointing stage to solid stage for its growth characteristics , and the results showed that the crude protein (CP) decreased and the fiber content increased which will caused the nutrition of forage reduced. However, the reason for the nutrient changes in Leymus chinensis has not yet explained at the metabolic level.Results: In this research, the Leymus chinensis was harvested at two growth stage( jointing stage and solid stage). Metabolomics was used to analyze the expression changes of metabolites in the whole plant of Leymus chinensis at different harvest stage, and the metabolites were analyzed based on LC-MS/MS untargeted metabolomics. A total of 55 metabolic pathways and 119 metabolites were related to the nutritional change in Leymus chinensis. Most of the metabolites are involved in metabolic processes, including amino acid metabolism, carbohydrate metabolism and acid metabolism biosynthesis. Under the action of EBF1/2 and EIN3, cysteine participates in the ubiquitin-mediated protein degradation pathway. In solid stage, the increasing content of cysteine directly promotes the ubiquitin-mediated protein degradation rate, which is manifested reduce the synthesis of crude protein (CP) in Leymus chinensis, resulting in the decreasing of Leymus chinensis nutrient quality. Carbohydrate metabolism provides raw materials for the synthesis of hemicellulose, leading to the increasing of hemicellulose. In addition, phenylalanine provides the necessary conditions for the synthesis of lignin. These are the internal factors that lead to the decline of quality of Leymus chinensis.Conclusions: This study analyzed the elucidate the relationship between the reduction in the nutritional value of Leymus chinensis and complex biological processes by the metabolomic,This study will provides a theoretical basis for producing high-quality Leymus chinensis hay and sets the stage for further research.