**2. Oregano essential oils composition**

When working with locally grown OEO, there is a wide range of secondary metabolite compositions that determine its wide variation in the biological activity. In our studies, 54 compounds were identified in the volatile fraction of *Origanum* and *Lippia* genus essential oils. Carvacrol and thymol and their precursors p-cymene and ϫ-terpinene were the majority of phenolic monoterpenoid compounds found. *Origanum vulgare* L. ssp. Hirtum, grown in Greece, showed the highest value of carvacrol (90.3%), in contrast to the major volatile compounds found in *O. vulgare* L. Grown under greenhouse conditions was thymol (21.51%). Carvacrol showed the lowest value (4.3%). In contrast, *Lippia origanoides* Kunth, an endemic species of the Patia region [2], presented the highest value of thymol (78.7%). *L. origanoides* is very important for Colombia because it is an endemic species of arid tropical lands [11]. *L. origanoides* is a native species of oregano adapted to arid conditions, dry soil in the Patia region of Colombia's southwest [12]. These conditions allow that *Lippia origanoides* Kunth is a unique species because of its high thymol content (close to 80%), low variation in metabolites, and low percentage of precursor compounds. *Origanum majorana* was another species cultivated under a greenhouse. It was represented by the bicyclic monoterpenoids sabinene and cis- and trans-sabinene hydrates (17.1%), with a high level of precursors ϫ-terpinene and terpinen 4-ol (20.0%), very low thymol (10.0%), and carvacrol (3.7%), with the presence of sesquiterpenoid compounds such as germacrene and bicyclogermacrene, acyclic monoterpenoid b-myrcene, and

### *Oregano Essential Oils as a Nutraceutical Additive in Poultry Diets DOI: http://dx.doi.org/10.5772/intechopen.113313*

sesquiterpenoid trans-β-caryophyllene [2]. In contrast, in other tropical areas, it has been found that carvacrol is the major component, together with ϫ-terpinene, thymol, and p-cymene, in oils from *L. origanoides* leaf hat [13]. In contrast, in our study, carvacrol was present only as a trace constituent (0.9%), but thymol contents ranging from 1.4 to 74.4% are reported [14] due to new chemotypes found. It can be concluded that the crop, the weather, the species, and the seasonal conditions can affect secondary metabolite production [15].

Despite the great variability, European and Greek oreganos have been the most evaluated and processed to be distributed on a commercial scale throughout the world because they have the highest content of carvacrol [13]. However, to tropical ecosystems such as America and Africa ecosystems, where *Lippia* genus native species with high thymol content is found, it might be important. Additionally, carvacrol has been shown to have more negative effects than thymol [16], raising the question of whether a high carvacrol content in the essential oil is a good indicator of its quality.
