## 1 Introduction  
Plant feed additives (PFA) constitute a wide group of biologically active compounds with potential positive effects on animal health and productivity [1, 2]. The antioxidant components of PFA belong to different chemical classes that can be recovered and/or obtained as plant extracts, essential oils, or resins [3]. Compounds in PFA are generally plant secondary metabolites resulting from a long adaptation process of plants to face infections and environmental stressors [4]. It is possible to isolate polyphenols, which include flavonoids, phenolic acids, and stillbenoids, from PFA [5]. Within polyphenols, flavonoids are the more abundant group and comprehend several compounds of interest for animal feeding.  
Essential oils are mixtures of different volatile and non-volatile compounds, chemically divided in two main groups: terpenoids (e.g., thymol, carvacrol) and phenylpropanoids (e.g., cinnamaldehyde, eugenol) [6]. Some of these molecules have a synergistic role in the antioxidant system. For instance, the antioxidant properties of roesmary as a whole plant is due to the simultaneous presence of phenolic diterpenes, phenolic acids (polyphenols), and essential oils, showing the capacity of scavenging free radicals as vitamin E (VitE) and C (VitC) [3]. Similarly, by-products from grape industry as well as from citrus contain a wide range of polyphenols [7; 8], such as epicatechin, hesperidin, and quercetin, with the highest antioxidant potential [9].  
Vitamins are nutrient compounds needed in small amounts (mg per day) which are essential for many metabolic processes and for the efficient use of other nutrients. They are naturally present in animals' ration ingredients, but their supplementation is needed to guarantee the fulfillment of the animals' requirements. VitE and VitC are considered exogenous antioxidants. In fact, they play a key role in the animal's oxidative stress balance, as it was widely reviewed in poultry. For example, VitE has an essential role in stabilizing the cellular membranes [5].  
Since use of antibiotics as growth promoters was banned in the EU for poultry production in 2006, several management and nutritional strategies in poultry industry were proposed in order to maintain high standards of productivity, healthiness, and welfare [6; 9; 10; 11; 12]. Among the nutritional strategies, the antioxidant activity of PFA compared with synthetic vitamins was assessed [9]. Under this scenario, several studies evaluated the potential of PFA and plant industry by-products as alternatives to vitamin in poultry nutrition. Nevertheless, the exploitation of nutritional strategies based on plant origin components is still ongoing due to their many challenges: control of zoonotic diseases of public relevance, sustainability, quality of products, perception of consumer, and growing restraints on synthetic antimicrobial use for therapeutic purposes [12; 13].  
Therefore, a systematic review including peer-reviewed papers published between 2000 and 2020 was conducted regarding the antioxidant capacity of plant extract (PE), essential oils (EO), and by-products of plant origin (BP) as potential alternatives to synthetic vitamins in livestock and the effect of PFA on animals' performance and physiological parameters compared to synthetic antioxidants.