## 4 Potential Plant Extracts and Plant By-Products as Alternative Sources of Vitamins in Animal Feeds  
### Monogastrics  
.1.1 Effects on Feed Intake, Growth, and Other Productive Performance Parameters in Pigs and Rabbits  
There is evidence to suggest that herbs, spices, and various plant extracts have appetite- and digestion-stimulating properties and antimicrobial effects [38]. Plant extracts contain different molecules with intrinsic bioactivities on animal physiology and metabolism. Several studies have reported significant effects of PFA on animals' feed intake, growth, and productive performances (Table 1). Plant extract effects may be due to the greater efficiency in the utilization of feed, which enhances growth. In finishing pigs, dietary supplementation with oregano oil (OO; at 0.0025% of the diet) for 28 days improved their average daily gain (ADG) (by 10.2%) and reduced (by 8.6%) their feed conversion rate (FCR) compared with the basal diet. Animals supplemented with VitE (200 mg/kg) scored intermediate values between the OO and the unsupplemented pigs. Moreover, final weight gain (WG) tended to be greater (by 2.81%) in the OO than in the control group [39]. When fattening pigs were fed with OO and quercetin (QU, 98% extract from _Sophora japonica_ L.), during a 28 days trial, growth parameters were significantly affected [40]. More specifically, greater final body weight (BW) was achieved in pigs fed with OO (at 0.025%) or QU (0.025%) diets compared with those consuming a basal diet containing a low level of VitE (by 4.2% and 3.5%, respectively). A similar improvement was observed in ADG (by 18.6% and by 13.4%, respectively). Although average daily feed intake (ADFI) was not affected by feeding experimental diets, Feed:Gain (F:G) ratio was higher in the OO group compared with the control (13.8%) and VitE (12.3%) groups (Table 1) [40]. Dietary supplementation with oregano extract (OE, at 2.0% of the diet) or oregano (_Origanum vulgare_) and rosmears extract (_Rosmarinus officinalis_) (both at a concentration of 0.1%) for 50 days improved the BW of rabbits (by an average of 2.87%) and their ADG (by 5.1%) compared with those consuming a basal diet that included 50 mg/kg of VitE [41]. The results suggest that supplementation with OO can be more effective than VitE in improving growth and some productive performance parameters. The main components of OO are thymol and carvacrol (polyphenols), with their precursors \(\rho\)-cymene and \(\gamma\)-terpinene being the most abundant monoterenes. These results could be explained by the capacity of polyphenols to exert antioxidant effects within the gastrointestinal tract, where they come into direct contact with the cells without being absorbed and metabolized [42], with possible protective action on the gut mucosal cell membranes. Studies comparing the antioxidant properties of Mediterranean food spices and common food additives have shown that extracts from oregano were more effective than butylated hydroxyanisole and butylated hydroxytoluene in inhibiting lipid peroxidation [43]. In vivo, thymol shows the ability to protect the microvilli, which are responsible for the absorption of nutrients [43]. Moreover, unabsorbed polyphenols flow to the hindgut, where they can modulate the gut microbial community acting as prebiotics [44]. Antibacterial effects have, in fact, been reported for thymol and carvacrol from oregano, for example, against _Clostridium perfringens, Pseudomonas aeruginosa_, and _Staphylococcus aureus_[45]. Moreover, the higher average daily gain (ADG) and greater feed conversion rate achieved in the studies described could be attributed to the positive effects of extracts on nutrient digestibility. Some authors speculated that herbs could develop their initial activity by adding flavor and, therefore, influencing the eating pattern, total feed intake, and secretion of digestive fluids [43; 46].