## 1 Introduction  
The growing consumer desire for safe and natural foods of animal origin and the European ban of in-feed antibiotics in animal nutrition have increased the interest in the use of plant feed additives (PFA) as a dietary ingredient in animal nutrition [1, 2, 3]. PFA are represented by a wide group of plant tissues, plant extracts, essential oils, and plant by-products containing biologically active compounds with potential positive effects on animal productivity [4, 5]. Many PFA have been demonstrated to express significant antioxidant activity, potentially improving animal production and the oxidative status of both the organism [6] and product. The antioxidant components of PFA belong to different chemical classes of plant secondary metabolites which may be included in animal diets as extracts (plant extracts), essential oils, or resins and are characterized by a significant content of bioactive molecules [7, 8].  
From PFA, it is possible to insulate polyphenols--flavonoids being the main group that comprises several molecules of interest for animal feeding--phenolic acids, and stillbenoids [9]. Polyphenols are bioactive compounds capable of interacting with feed matrices, reducing or enhancing the accessibility and availability of nutrients. The metabolized product generated after their absorption is the main bioactive component that enters the systemic circulation and reaches the targeted organs [10]. Moreover, from each PFA, it is also possible to extract individual active components, thus reducing the dose of supplement to be administered, and avoid some issues related to odor and taste [11]. Besides the potential detrimental organoleptic characteristics, several factors may negatively influence the effectiveness of feeding essential oils or plant extracts to animals. Some are related to the plant harvesting phase, while others are linked to the interaction with the feed matrix, intestinal bioavailability, and the mode of action [3]. Moreover, a variety of factors influencing the antioxidant activity of some PFA rich in polyphenols need to be considered. For example, it has been emphasized that the absorption of flavonoid compounds at the intestinal level is poor, which reduces their concentration in the target tissue where the antioxidant effect should be exerted [9]. Despite these controversial aspects, PFA have been widely tested in poultry in an effort to maintain a high level of growth performance and other feed efficiency parameters, as well as increasing the quality of poultry products, ameliorating negative carcass traits, and delaying lipid oxidation processes in breast and thigh meat [1].  
A number of studies have been conducted to evaluate the potential of PFA as vitamin sources, hence the requirement for a summary of the published scientific evidence on the impact of the use of PFA on poultry productivity and poultry food product quality. The influences of PFA on poultry's feed intake, feed efficiency, growth traits, and nutrients digestibility; on metabolic and hematological parameters and antioxidant status; on macro- and microscopic anatomical features; and on the intestinal microbiome profile have already been presented by [PERSON] et al. [6]. Therefore, the present review focuses more detail on the antioxidant capacity of plant extracts (PE), essential oils (EO), and by-products of plant origin (BP) as potential alternatives to synthetic vitamins in poultry-derived food products' yield, quality, and oxidative status.