Blood biochemical indicators signify the health status and metabolic level of animal body (
Ma et al., 2015). Blood sugar and blood lipids are the main energy substrates of the body and determine the energy supply state and normal metabolic level of the body (
Han et al., 2022;
Pan et al., 2022). The use of PEV and BPS as substitutes for rice straw did not significantly change the blood sugar (glucose) and blood lipid (CHO, TG, HDL-C, and LDL-C) content of fattening cattle, and both remained within the normal reference range. This finding is consistent with previously reported results (
Tao et al., 2020;
Xue et al., 2019). Blood amino acid concentration comprehensively reflects the small intestinal absorption and metabolism of amino acids (
Brown and Cline, 1974;
Eggum, 1970). Consistent with our expectations, dietary changes led to variations in serum amino acid levels in bulls. To obtain useful information on changes in serum amino acid metabolism, we used metabolomics techniques. The results showed that there were differences in metabolites related to serum amino acids among the three treatment groups. L-tryptophan and 3-dehydroquinate are key metabolites involved in phenylalanine, tyrosine, and tryptophan biosynthesis. Upon chemical analysis of TMR, it was observed that the amino acid composition was comparable. The observed upregulation of 3-Dehydroquinate and L-tryptophan may have been influenced by other factors. It is hypothesized that during the silage process, the substantial sugars present in the paper mulberry undergo anaerobic homolactic fermentation by microorganisms, particularly lactic acid bacteria (
Li et al., 2021). This process leads to the generation of numerous intermediate products such as fructose-1,6-phosphate and phosphoenolpyruvate, which serve as precursors for 3-3-Dehydroquinate (
Zhou et al., 2021). Meanwhile, 3-Dehydroquinate is the precursor of quinate, which acts as a precursor of the shikimate pathway. This pathway is integral to the biosynthesis of aromatic amino acids, including tryptophan (
Gallego-Giraldo et al., 2011). In the present study, we also found that tryptophan was upregulated. Tryptophan is an essential amino acid necessary for protein synthesis (
Jiang et al., 2022). Furthermore, tryptophan is hydroxylated to form 5-hydroxytryptamine and further converted into serotonin (
Mora-Villalobos and Zeng, 2018); the serotonin receptor subtype 5-hydroxytryptamine 2C (5-HT2cR) decreases satiety and food intake via the hypothalamic melanocortin system (
Xu et al., 2008). Our analysis of the serotonergic synapse pathway found that 5-hydroxytryptamine led to the downregulation of arachidonic acid through the 5-HT2cR/Gq/PLA2 signaling pathway. Arachidonic acid is the backbone of both endocannabinoids 2-arachidonoylglycerol and anandamide; studies have shown that an increase in arachidonic acid in the diet leads to an increase in 2-arachidonoylglycerol and anandamide, while also increasing animal feed intake (
Alvheim et al., 2012). The upregulation of tryptophan and downregulation of arachidonic acid may be the key reasons for the decrease in DMI in the BPS group. The lysine content in TMR of the three treatment groups is similar, and it is acknowledged that the bulls themselves cannot synthesize lysine (
Liu et al., 2019). However, the BPS group showed significantly increased serum and muscle lysine levels compared to the CON group, suggesting a potential increase in lysine synthesis by rumen microorganisms. The glycolysis process of silage feed involves the conversion of phosphofructose 6-phosphate to pyruvic acid phosphate, and enolase dehydrates 2-phosphoglycerate to form phosphoenolpyruvate (
Narita et al., 2020), providing substrates for the main lysine synthesis pathway of the rumen microbiota, known as the diaminopimelic acid pathway, which promotes the synthesis of rumen amino acids (
Bukhari and Taylor, 1971;
Zabriskie and Jackson, 2000). Furthermore, the rumen also contains the α-aminoadipic acid pathway, another lysine synthesis pathway, allowing certain fungi and bacteria to utilize saccharopine to synthesize lysine through saccharopine dehydrogenase (
Kosuge and Hoshino, 1998). Our results indicated an increase in serum saccharopine in the BPS group, suggesting that rumen microorganisms may synthesize lysine through this pathway. In addition, 5-aminopentamide is the primary metabolite of lysine, which is further degraded into 5-aminopentanoic acid, which is degraded into glutarate and then converted into acetyl-CoA, which participates in the citrate cycle (
Thompson et al., 2019). The upregulation of 5-aminopentamide and the downregulation of 5-aminopentanoic acid in the BPS group seem to be contradictory; therefore, we speculate that the conversion of 5-aminopentamide to 5-aminopentanoic acid was inhibited, which facilitated the participation of lysine, along with other amino acids, in protein synthesis and deposition in the body tissue.