## 1 Introduction  
Interest in safe and natural antioxidants is currently increasing the use of plant extracts feed additives to serve as antioxidants in animal nutrition. Consumers are interested in safe and natural foods of animal origin, and in some cases, they are also willing to pay a premium price for them [1]. Moreover, the European Union has banned in-feed antibiotics in animal nutrition to prevent antibiotic-resistant bacteria, which has increased interest in the use of plant extracts feed additives as dietary ingredients in animal nutrition [2]. Moreover, the possibility of using plant extracts feed additives, such as essential oils, plant extracts, and in particular, plant food industry by-products, is of great interest since it reduces industry wastes and is in harmony with the philosophy underlying environmental sustainability [3]. Marigold (_Tagetes__erecta_ L.) is a natural forage plant and an annual herb of the genus Tagetes in the family Asteraceae; it is also known as Hibiscus and is native to Mexico and South America [4]. It is widely cultivated in China for its strong weather resistance, wide adaptability, long flowering period, varied flowers and pure flower colours. It can be used as an ornamental flower, medicinal plant and food spice [5; 6]. Studies have shown that marigold contains a variety of bioactive components, including carotenoids [7], flavonoids [8], phenolic acids [9] and thiophenes [10]. Additionally, marigolds have been shown to have a range of pharmacological activities, including antioxidant [11; 12], vision protection [13], anti-inflammatory [14], diuretic [15] and antibacterial [16] activities. Marigold residue and marigold powder are widely used as feed ingredients. Xanthophyll pigments from marigold flowers, including zeaxanthin and lutein, have been used as food additives and food colouring agents [17]. Intriguingly, lutein, the main pigment in marigold, has been used as an antioxidant, a food colouring agent and a nutraceutical [18], and it also effectively reduces the chance of developing macular degeneration, cataracts, atherosclerosis and certain cancers [19]. In particular, antioxidants such as the macular carotenoids lutein, zeaxanthin and meso-zeaxanthin possess significant antioxidant and anti-inflammatory effects in the retina [20]. In addition to lutein, marigolds contain many other active bioactive components that can be exploited [7]. Therefore, the development of potential antioxidants from marigold into plant feed additives is an economically important goal for the high-value utilization of marigolds. Conventional bio-guided fractionation for the discovery of active ingredients from natural products generally employs a series of isolation and purification techniques for obtaining monomeric components, followed by in vivo or in vitro pharmacological screening, and this method plays an important role in drug development; however, it is a time-consuming, laborious and costly task [21; 22]. Additionally, the activities of some natural ingredients often decrease during isolation and purification due to the breakdown of components [23]. Offline and online DPPH-HPLC-Q-TOF-MS/MS are suitable for achieving efficient screening and identification of bioactive compounds from natural product extracts [24]. Therefore, a rapid and efficient DPPH free radical spiking test was explored to screen antioxidants from marigold. The oil composition in feed is the main energy source of animal nutrient intake. The presence of rancid flavour in the feed means that it has been oxidatively spoiled, which is mainly indicated by oxidative rancidity involving unsaturated bonds of lipids [25]. Soybean oil is the one of the most heavily consumed vegetable oils worldwide and has been thought to aid the absorption of some liposoluble nutrients while also reducing feed dust and increasing feed energy [26].