## 5 Final and General Remarks  
This review focused on the antioxidant capacity of plant extracts, essential oils, and by-products of plant origin as potential alternatives to synthetic antioxidant vitamins in livestock mammals' feedstuffs. Natural antioxidants, in comparison to synthetic ones, have been described to show several advantages like having a wide range of solubility and of antioxidant activity and being completely metabolized by the organism. Because of this, they are perceived as innocuous substances, and the interest in their use is increasing even if limited by their high cost. On the other side, synthetic antioxidants are generally inexpensive and widely applied in livestock nutrition; they possess low water solubility and medium to high antioxidant activity, but some of them are stored in the adipose tissue, so safety concerns are rising, and the use of some of them is nowadays banned. VitE is the only synthetic antioxidant that has been compared with PFA in livestock mammals' trials, and only a few studies in both monogastric and ruminants deal with this issue. Most studies have tested PFA for its effects on feed intake, growth, slaughter, and milk productive performance parameters. A small number of studies have evaluated the impact of PFA on metabolic and hematological parameters, oxidative status, and HSP gene expression. Both positive and negative effects of PFA on several animal functions were found. Oregono essential oil, alone or in association with quercetin, compared with a basal diet, decreased F:G ratio by increasing ADG and final BW in pigs. Moreover, in rabbits, the oregano plant alone or in combination with aqueous rosemary extracts showed positive effects on the above-mentioned parameters in comparison with both basal and VitE rich diets. Marigold (_Calendula officinalis_), plant extract mixture (carvacrol, capsicum oleoresin, and cinnamaldehyde), and oregano essential oil alone or in combination with quercetin, sweet chestnut, or plant extract (grape skin and oregano), respectively, showed a similar action to VitE in reducing oxidative stress with a trend for higher antioxidant enzymes activity in monogastric animals. The PFA did not affect ruminants' growth performance and feed efficiency. Saffron petal extracts, naringin, _Andrographs paniculata_, turmeric acid, and plants rich in polyphenols in general (rosemary, grape, grapefruit, and marigold) showed similar actions to VitE in reducing lipid peroxidation and, consequently, oxidative stress without affecting biochemical parameters in bovine, ovine, and caprine.  
In some cases, the effect seems to be dose-dependent, such as in the case of the grape seed polyphenol extract that induced a linear increase of SOD and GSH-Px activities with increasing inclusion rate in pigs' diets. The beneficial effects of both PFA and VitE in PUFA rich diets were in general decreased, probably due to the higher susceptibility of the nutrients to oxidation.  
Some variability in the results can also be attributed to the animal species (monogastric vs. ruminants). The rumen may mitigate the impact of PFA on the digestion process through the fermentation activity on the dietary substrates with partial modification of the molecules reaching the gut. Considering the presence of these fundamental variables and the enormous number of molecules and formats of PFA available on the market, it should be highlighted here the need for standardization of the evaluation systems that could allow an optimization of the studies in this field, leading to a more efficient and conclusive validation process for these products.