## 2 Materials and Methods
### Examination of Antibiotic Concentrations  
The collected samples were filtered through a 0.45 \(\upmu\)m membrane to remove particulate matter. The internal standards were added to the sample (250 mL) at a concentration of 60 ng/L for sulfadimethoxine-d6, 40 ng/L for enrofloxacin-d5, and 120 ng/L for melocycline. Then, 1 mL of Na\({}_{2}\)-EDTA (5%, w/v) was added to the sample, and the pH was adjusted to about 2.5 using 6N hydrochloric acid.  
The Solid-Phase Extraction (SPE) cartridge (Oasis HLB SPE cartridge; 200 mg; 6 cc; Waters, Milford, MA, USA) was conditioned with 6 mL of methanol and DI water, respectively, and the samples were loaded under vacuum at a flow rate of 7-8 mL/min. Afterwards, prepared samples were washed with 6 mL of DI water and eluted with 5 mL of methanol containing 0.1% formic acid. The eluted solutions were dried using nitrogen gas at 40 \({}^{\circ}\)C, re-dissolved in 250 \(\upmu\)L of a 50% methanol/0.1% formic acid solution, and then analyzed by LC-MS/MS.  
Analysis of residual antibiotics was conducted using a Thermo Scientific Vanquish and Quantis mass spectrometer (UPLC-MS/MS, San Jose, CA, USA). Chromatographic separations were performed using an Eclipse Plus C18 column (2.1 \(\times\) 50 mm, 1.8 \(\upmu\)m) under gradient conditions with mobile phases of distilled water containing 0.1% formic acid (mobile phase A) and methanol containing 0.1% formic acid (mobile phase B). The gradient program was as follows: 5% B for 0-0.5 min, 95% B for 7-7.5 min, and 5% B for 7.6-10 min, at a flow rate of 400 \(\upmu\)L/min. The column temperature was maintained at 35 \({}^{\circ}\)C.  
The parameters of the MS were set to a spray voltage of 3.5 kV; vaporizer temperature of 350 \({}^{\circ}\)C; and sheath, aux, and sweep gas of 50, 10, and 1 arb, respectively. Selected antibiotics were ionized as [M+H]+ (precursor ion) in positive ESI mode, and the most abundant product ions were selected for quantitation in selected reaction monitoring (SRM) mode. The SRM transitions were m/z 407 \(\rightarrow\) 126 (collision energy CE, 29 eV) for limomycin, m/z 279 \(\rightarrow\) 186 (CE = 16 eV) for sulfamethazine, m/z 494 \(\rightarrow\) 119 (CE = 37 eV) for tiamulin, m/z 291 \(\rightarrow\) 261 (CE = 25 eV) for trimethoprim, m/z 363 \(\rightarrow\) 72 (CE = 22 eV) for marbolfloxacin, m/z 461 \(\rightarrow\) 426 (CE = 18 eV) for oxytetracycline, m/z 479 \(\rightarrow\) 444 (CE = 21 eV) for chlorotetracycline, m/z 300 \(\rightarrow\) 159 (CE = 34 eV) for fenbendazole, m/z 317 \(\rightarrow\) 162 (CE = 22 eV) for sulfadimethoxine-d6 (internal standard IS), m/z 365 \(\rightarrow\) 321 (CE = 20 eV) for enrofloxacin-d5 (IS), and m/z 477 \(\rightarrow\) 460 (CE = 17 eV) for meclocycline (IS).  
This study selected target analytes based on their high levels of consumption by livestock. Based on the 2022 monitoring data on antibiotic usage and resistance released by the Ministry of Food and Drug Safety, a total of eight veterinary drugs (seven antibiotics and one anthellimtic) were chosen for examination: sulfamethazine, chlorotetracycline, oxytetracycline, marbolfloxacin, linomycin, tiamulin, trimethoprim, and fenbendazole [39]. Further details on the analytical methods and performance parameters can be found in Figure S1, and Tables S1 and S2.