*2.1. Experimental Design, Chickens, and Diets*

The experiment was approved by the Institutional Animal Care and Use Committee, Kangwon National University (KW-170519-1). Three hundred and twelve Korean native chickens (Hanhyup 3, 914.3 ± 26.3 g, 49 days old) were fed for 35 days. A scopoletinrich product extracted from SK was added to the experimental diets from the first day of study. Four treatments included SK supplementation levels (0 and 20 ppm) and stocking densities (14 and 16 bird/m2). Each treatment consisted of 6 replicates with 13 birds per replicate (floor pen, w2350 × d1500 × h850 mm), where the rice hull was used as litter. The temperature and humidity of the broiler house were controlled by an automatic ventilation system to have an average temperature and humidity of 22.1 ◦C and 46%, respectively. The diet contained 18% crude protein, 3100 kcal/kg metabolizable energy, 0.86% calcium, 0.33% available phosphorus, 0.21% sodium, 1.01% available lysine, 3.5% crude fiber, and 6.86% ether extract. White light was provided (25 lux at bird-head level), with a light schedule of 19L:5D (lights off from 2200 to 0300 h), and water and feed were maintained ad libitum. The mash-type diet was prepared to meet the nutrient requirements of Korean native chickens. The SK supplement contained 2090.5 mg scopoletin/kg (Table 1).

#### *2.2. Sample Collection*

At day 35, a total of 72 birds, 18 chickens per treatment (3 chickens/replicate), were used for carcass characteristics analysis, meat quality, relative organ weight, antioxidant status, and plasma metabolites evaluation. Birds in the same range of bodyweight (BW) based on the average BW of a treatment were applied. Blood samples were taken from the wing vein by using a syringe. Collected blood was moved into non-treated vacuum tubes, and was immediately sent for plasma separation, then centrifuged at 2500× *g* for 10 min. Serum was aspirated and located in a 2.5 mL centrifuge tube then stored at −20 ◦C before analysis for malondialdehyde (MDA), catalase, SOD, and total antioxidant capacity (TAC). All selected birds were decapitated at the first cervical vertebrae. After defeathering and

removal of organs and feet, the carcass, breast muscles (both sides), drumsticks (both sides), and abdominal fat were weighed and then stored at −20 ◦C. The meat quality, antioxidant status, and weights of liver, spleen, and bursa of Fabricius were measured to calculate the relative carcass weight and internal organ weight, and then these parts were stored at −80 ◦C.


**Table 1.** Analyzed composition of *Sophora koreensis*.

#### *2.3. Antioxidant Status in Serum and Muscle*

In order to assay the antioxidant factors activity in muscle and serum, the samples were pre-treated and measured as explained by Hosseindoust et al. [5] by using a Cayman kit manual (Enzyme activity assay, Cayman Chemical, Ann Arbor, MI, USA). The harvested samples from the muscle and serum were subjected to the evaluation of the concentration of MDA (Cat #10006438, Cayman Chemical, Ann Arbor, MI, USA), catalase (Cat #707002, Cayman Chemical, Ann Arbor, MI, USA), SOD (Cat #706002, Cayman Chemical, Ann Arbor, MI, USA), and TAC (Cat #709001, Cayman Chemical, Ann Arbor, MI, USA). To evaluate the absorption detection, a microplate reader (Power Wave XS, BIoTeK, Winooski, VT, USA) was applied according to the Cayman kit's manufacturer's manual.

#### *2.4. Radical Scavenging Capacity*

The evaluation of 3-ethylbenzothiazoline-6-sulfonate (ABTS)-reducing activity was performed using the supernatant collected from thigh meat according to Hosseindoust [5] and Blois [21]. In brief, 200 μL of supernatant was placed in a 5 mL centrifuge tube and added to 800 μL deionized distilled water. The determination of ABTS-reducing activity and the preparation of the ABTS solution were conducted following the method described by Erel [22]. The prepared ABTS solution was diluted with ethanol for adjusting absorbance approximately 0.70 at 734 by using a UV spectrophotometer (Optizen 3220UV, MECASYS, Daejeon, Korea). The diluted ABTS solution (3 mL) was mixed with 20 μL of supernatant and the absorbance was measured by a UV spectrophotometer at 734 nm. The ethanol was used as a blank. The percentage inhibition was obtained by the following

equation: ABTS-reducing activity (%) = ((absorbance of the control − the absorbance of the sample)/absorbance of the control) × 100.

#### *2.5. Meat Quality*

Leg muscle (with bone, and without skin, tendon, and fat) meat color was examined using a Chroma Meter CR-400 instrument (Minolta Co., Osaka, Japan) according to International Commission on Illumination (CIE) L \* (lightness), a \* (redness), and b \* (yellowness). Water holding capacity (WHC), cooking loss (%), shear force (%), pH, and MDA were examined using muscle pectoralis major according to Hosseindoust et al. [5]. The WHC was measured by placing a 0.5 g meat sample on a round plastic plate in a tube (Millipore Ultrafree-MC; Millipore, Bedford, MA). The samples were heated for 20 min (80 ◦C) in a water bath, then cooled (23 ± 1 ◦C), and centrifuged (2000× *g*) for 10 min (4 ◦C) to evaluate WHC as follows: WHC = (moisture content−water loss)/moisture content × 100. To measure the cooking loss, 3 g of leg muscle meat was placed in a plastic bag and heated in a water bath (85 ◦C) for 20 min, then cooled at room temperature to calculate cooking loss according to this equation: (sample weight before cooking/sample weight after cooking)/sample weight before cooking × 100 [5]. For shear force determination, a texture analyzer (TA-XT2i, Stable Microsystems, Surrey, UK) equipped with a Warner-Bratzler shear blade, a 25 kg load cell, and a test speed setting at 2.0 mm/s was used with the maximum force (kg) [23]. The pH of meat was evaluated as explained by Hosseindoust et al. [5]. In brief, a 5 g sample of meat was homogenized in distilled water (45 mL) by a homogenizer (DIAX 900, Heidolph, Kelheim, Germany) for 15 s, and then the pH was determined by using a pH meter (Orion 230A Thermo Fisher Scientific, Waltham, MA, USA). After performing the pH process, a Watman no. 2 (Hillsboro, OH, USA) was used to filter homogenized samples.

#### *2.6. Blood Metabolites*

The mixed blood samples with K2 EDTA were used for determination of total cholesterol, total protein, triglyceride, glucose, glutamic pyruvic transaminase (GPT), glutamate oxaloacetate transaminase (GOT), albumin, phosphate, and calcium (Hemavet 950, Drew Scientific, Miami Lakes, FL, USA). The serum corticosterone was analyzed using the ELISA kit (Corticosterone ELISA kit, Enzo life Sciences, Farmingdale, NY, USA).
