**1. Introduction**

Stocking density (SD) exceeding the comfort zone causes stress in farm animals [1]. Presently, the SD ratio is markedly increasing worldwide to minimize costs. The continuous increase in the SD of broiler chickens in the poultry industry to decrease production costs increases health and welfare issues [2]. In addition to low body weight gain, production of broiler chickens in a high-SD situation decreases meat quality including water-holding capacity and meat tenderness by increasing the oxidative reactions [1,3]. As the traditional broiler's meat is costly because of higher meat quality compared with modern broilers, the low meat quality can compromise the marketability. Moreover, the high stress level affects polyunsaturated fatty acids of meat, which increases the vulnerability to oxidative deterioration [3–5]. The control of lipid oxidation associated with SD entails the supplementation of antioxidant factors to block the production of free radicals [6]. Several important

**Citation:** Ha, S.H.; Kang, H.K.; Hosseindoust, A.; Mun, J.Y.; Moturi, J.; Tajudeen, H.; Lee, H.; Cheong, E.J.; Kim, J.S. Effects of Scopoletin Supplementation and Stocking Density on Growth Performance, Antioxidant Activity, and Meat Quality of Korean Native Broiler Chickens. *Foods* **2021**, *10*, 1505. https://doi.org/10.3390/ foods10071505

Academic Editors: Benjamin W. B. Holman and Eric Nanthan Ponnampalam

Received: 8 May 2021 Accepted: 17 June 2021 Published: 29 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

macromolecules or enzymes are under the influence of reactive oxygen species (ROS) and free radicals, which have the potential of increasing lipid peroxidation in organs [7–9]. An oxidative stress condition in animals refers to the progressive loss of anti-oxidative status caused by various internal stressors or environmental factors. Normally, the natural antioxidant defense system is able to prevent cells from oxidative injuries by enzymatic control of free radicals [10]. Superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase are the major enzymes to counteract free radicals and diminish the lipid peroxidation rates [11,12]. In addition, the lower antioxidant defense in the body results in the excessive generation of ROS leading to oxidative injuries [6]. Therefore, improvement of oxidative status may alleviate the detrimental effects of high SD on growth performance and meat quality.

*Sophora koreensis* (SK), from the Fabaceae family, is a perennial herb in the mountainous area of the Korean Peninsula [13]. The root and flower of SK species contain a large quantity of flavonoids [14], isoflavonoids [15], and scopoletin [16,17] having anti-oxidative capacity [18]. Scopoletin is a kind of phenolic coumarin with promising anti-inflammatory effects [19] that can protect the body from microbial attack and environmental stress, including mechanical injury [20]. The antioxidant properties of scopoletin were proved by scavenging superoxide anion, which may prevent stressful conditions related to oxidative damage [16]. In our dose-dependent pre-study, the antioxidant properties of SK in chickens had been proven (unpublished). Therefore, we hypothesized that the effects of SK can be better highlighted when used during a stressful condition. To our knowledge, there is a lack of reports regarding the antioxidant effects of scopoletin-rich diets, not only on the growth performance but also on the meat quality of broiler chickens during high SD. This experiment was thus designed to determine the effects of a supplementary scopoletin-rich feed additive on meat quality and antioxidant status in broilers under high SD stress.

### **2. Materials and Methods**
