This study evaluated dietary quercetin supplementation to investigate the effects of increasing inclusion levels on growth performance, diarrhea, serum health-related indicators, and whole-blood gene expression in weaned piglets. In a 28-day feeding trial, 128 piglets received diets formulated to contain 0, 100, 200, or 300 mg/kg quercetin (four pens of eight piglets per treatment; pen as the experimental unit). Final body weight and average daily gain were higher in the 200 and 300 mg/kg groups than in the control, and the 300 mg/kg group had the highest gain-to-feed ratio (G:F) (p < 0.05). Diarrhea rate declined linearly (p < 0.001); the rate in the 300 mg/kg group was lower than in the control and 100 mg/kg groups but did not differ from the 200 mg/kg group. Quercetin reduced serum malondialdehyde, pro-inflammatory cytokines, urea nitrogen, and aspartate aminotransferase, while increasing total protein, albumin, total antioxidant capacity, several antioxidant enzyme activities, and interleukin-10 (p < 0.05). Serum diamine oxidase activity and D-lactate concentration also declined linearly (p < 0.001). Whole-blood expression of PTGS2 decreased; HMOX1, NFE2L2, and NQO1 increased; and SOD1 was unchanged. Overall, within the tested range, increasing dietary quercetin improved growth and feed efficiency, reduced diarrhea, and was accompanied by coordinated changes in serum health-related indicators and whole-blood transcripts.

Flower buds of Sophora japonica are flavonoid-rich edible botanical materials. This study developed an enzyme-assisted ultrasonic protocol for producing a quercetin-containing crude extract. A Box–Behnken design evaluated β-glucosidase concentration, digestion time, and the cellulase-to-pectinase mass ratio. The quadratic model was significant (p < 0.0001; R² = 0.9848; adjusted R² = 0.9652). At 14 U/mL β-glucosidase, 72 min of digestion, and a mass ratio of 1.25:1 (w/w), the UV-estimated quercetin-equivalent yield reached 7.34%, compared with 4.17% for ultrasound alone (p < 0.001). HPLC based on retention-time agreement and external calibration estimated quercetin at 71.93 ± 1.35 mg/g initial dry plant material. The dried extract contained 82.93 ± 3.75 mg gallic acid equivalents/g and 154.68 ± 5.56 mg quercetin equivalents/g. It showed concentration-dependent DPPH^•, ^•OH, and ABTS^•+ scavenging, with IC₅₀ values of 1.339 ± 0.061, 3.931 ± 0.108, and 1.004 ± 0.032 mg/mL, respectively, and preliminary antibacterial activity against Salmonella enterica, Escherichia coli, and Staphylococcus aureus. These findings establish a defined extraction route while distinguishing the UV optimization endpoint from HPLC-based quercetin estimation.