## 4 Potential Plant Extracts and Plant By-Products as Alternative Sources of Vitamins in Animal Feeds
### Monogastrics
In addition, ADG was greater (by 4.3%) in green tea catechins than in the control groups (fed a basal diet including grass meal at 10% or 20% in the weaning and finishing phase, respectively) [30]. In fact, among several flavonoids (rutin, dihydroquercetin, quercetin, epigallocatechin gallate, and epicatechin gallate), catechins showed the greatest capacity to inhibit microsomal lipid peroxidation [7]. The oxidation processes can, in fact, decrease the palatability and the nutritional value of feeds and the use of antioxidant vitamins (i.e., VitE) is considered the first defensive line against lipid peroxidation in tissues,protecting cell membranes from free radicals damage which ultimately influence ADG especially during the starter period [47]. Similar to vitamins, the flavonoids tested are able to chelate Fe\({}^{2+}\), Fe\({}^{3+}\), and Cu\({}^{2+}\) and are effective O\({}_{2}\)\({}^{\bullet-}\) scavengers to varying degrees [7], which can explain their impact on ADG. In contrast to these findings, the supplementation of diets with OO (at 0.01% or 0.02%) for 42 days did not significantly affect neither the growth nor the feed efficiency parameters in rabbits compared with those receiving a basal (only vitamin premix) or a VitE enriched diet (200 mg/kg) [48]. In young growing pigs, no differences in feed intake (FI) and BW were observed after supplementing the diet for 14 days with a propylene glycol extract of Marigold (_Calendula officinalis_ L.; at 3 mL/day) or with a plant extract mixture (containing: carvacrol 5%, cinnamaldehyde 3%, and capsicum oleoresin 2% wt/wt, at 271.2 mg/kg) compared with those supplemented with VitE (100 mg/kg or 90.4 mg/kg) [35; 36]. No differences among the ADG, final BW, and FI values were reported when pig diets were supplemented with rosmenary extract (RE; at 0.04%), alone or in combination with gallic acid (at 0.02%) or VitE (at 0.04%) [49]. Weaned pigs supplemented with green tea catechins (at 0.2%) did not vary their feed consumption (FC) compared with pigs fed four control diets and with VitE (200 mg/kg). No significant differences were found in pigs' ADG, FI, and feed:gain (F:G) ratio when fed a basal diet enriched with sunflower oil alone, or linseed oil alone, or linseed oil with olive leaves (OL; at 0.5 or 0.10%) or linseed oil with VitE (200 mg/kg) simultaneously for 90 days [50]. In another study, the dietary supplementation with OL extract (OLE1: 0.000384%; OLE2: 0.00384%; OLE3: 30.096%) did not change the final BW of piglets or their WG compared with two control diets containing the same level of VitE but different fat contents (one had a high fat content due to the inclusion of 8.3% of linseed oil) [51]. Nevertheless, ADFI was lower in OLE1 (by 10.5%), OLE2 (by 10.8%), and OLE3 (by 10.5%) in comparison with the low fat diet fed group. However, the OLE3 group had lower F:G ratio (by 11.0%) compared with the control group [51] (Table 1). The inconsistency of some of the results described above can be explained by the generally low and variable bioavailability of PFA. Their absorption at the stomach level is limited due to their glycosylated form because most of them need to be hydrolyzed in the intestine by the microbial population or by enzymes in order to be absorbed; furthermore, they can be extensively metabolized at the hepatic level [42]. Moreover, although aromatic herbs and essential oils can improve taste and palatability, thereby increasing consumption, in some cases, the opposite was found. Some authors have, in fact, also shown that as the concentration of dietary OEO increases, consumption decreases [52]. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**PFA** & **FFA listed Dose** & \begin{tabular}{c} **Animal** \\ **Species** \\ **involved** \\ \end{tabular}  & \begin{tabular}{c} **Vtiminfo-Vtimian** \\ **Computer** \\ \end{tabular}  & \begin{tabular}{c} **Type of** \\ **Parameter** \\ \end{tabular}  & **Taint Evaluated** & \begin{tabular}{c} **Comparison to** \\ **Negative Control** \\ **Effect (PFA Dose)** \\ \end{tabular}  &