**4. Benefits as an additive in poultry feed**

The food-animal industry has used antibiotics in the feed for decades. The use of antibiotics in subtherapeutic doses in animal feed has been questioned due to its possible relationship with resistance in bacteria that has an impact on human and animal health and the antibiotic resistance is one of the top health challenges of this century [24]. In recent years, there has also been a growing interest in antibiotic-free broiler meat production, therefore, it is necessary to generate nonantibiotic alternatives for their use in feed and animal nutrition. Bioactive compounds derived from medicinal plants have been proposed as natural sources of phytochemicals with antibacterial, anti-inflammatory, and antiviral properties, among others, which are ideal for use as feed additives in food animal production [25, 26]. Within these phytochemicals, oregano essential oils have been evaluated and used as additives in poultry feed. Various studies have verified the positive effects of OEOs on feed intake, metabolism, digestive secretions, and growth, among others [4, 27, 28], as well as nonculturable pathogens [29]. Both studies and commercial products with OEO have been based on essential oils from Greek oregano species (*Origanum* spp.) with high carvacrol content. It has been proposed that the effectiveness of these OEOs may be the result of their antimicrobial activity as presented back. Our studies on *Lippia origanoides* Kunth, also commonly named oregano, have shown that AEO rich in thymol can be a viable natural additive to produce chicken meat with a possible anticoccidial effect. We found that the inclusion level of AEO depends on the broiler's health status. This species, native to tropical America, was analyzed by gas chromatography, and 22 compounds were identified in the volatile fraction of the essential oil. We established that thymol was the major secondary metabolite (78.7%) in *L. origanoides* Kunth with a very low level of precursors such as p-cymene (6.6%), ϫ-terpinene (2.7%), and carvacrol (0.9%); however, this type of oregano has been little studied. Our studies included the effect of OEO from *L. Origanoides* Kunth on cecal microbial diversity and composition in broilers challenged with attenuated coccidia oocyte vaccine (2×) and found that supplementation with 100 ppm of AEO reduced the negative impact of the challenge with attenuated oocysts of coccidia. A significant interaction was identified, because 65 ppm was OEO level that maximized the body weight in non-coccidian-challenged chicken groups, but 147 ppm was required for the coccidian-challenged group. The OEO supplementation to coccidia-challenged broilers

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

improved the body weight (9.3%) and the feed conversion ratio (FCR) (-6%). It was shown that AEO rich in thymol can be a viable natural additive to produce chicken meat without antibiotic [30].

Average daily gain (ADG) and feed conversion ratio (FCR) were better in broilers fed with OEO than those fed with a control diet during days 1–21 and days 22–42 of age [3]. Although there is no agreement on the inclusion levels, 150 or 300 mg/kg of OEO has proven to be effective in increasing the average daily feed intake and average daily gain [31]. Other studies raised higher levels of 150–750 ppm of OEO in broiler diets and concluded that during the grower period, the OEO-supplemented groups showed a better conversion of feed, with linear and quadratic performance. A linear increase in body weight, body weight gain, the protein efficiency ratio, the relative growth rate, and better conversion was observed at 750 ppm of OEO dietary level compared to the control groups. With the broken-line regression model, it was found that 580 ppm of OEO optimizes the final body weight and the feed conversion [32].

We compared OEO from four chemotypes against antibiotics in broiler diets and, contrarily, *O. majorana* with slow carvacrol, thymol, and antibiotics presented higher values of energy and fat ileal digestibility and a higher body weight. Interestingly, a negative correlation between the body weight and carvacrol intake (r: −0.55) and a positive correlation with thymol intake (r: 0.46, p 0.05) have been reported. These results found a possible negative effect of high carvacrol intake and different responses of broilers as a function of OEO composition [33]. The impact of OEO on digestive efficiency had been more consistent, with a greater amylase enzyme activity, a quadratic elevation in chymotrypsin, and higher chymotrypsin and lipase activity, which would be expressed in a better efficiency in the use of food in broilers, greater average daily gain (ADG) and a lower fructose-to-glucose (F/G) ratio [8], a linear increase in villus height to crypt depth ratio, a quadratic decrease in feed conversion ratio with a lower feed conversion ratio (200 ppm) than the control during weeks 1–12 of the trial, and higher eggshell thickness at the end of weeks 4, 8, and 12 [8]. With a level of 275 ppm of OEO in the diet of laying hens, it improved the body weight, lowered the triglycerides, and raised the high-density lipoprotein levels [34]. The OEOs, or secondary metabolites, have been very widely used in the poultry industry in mixtures with other types of essential oils with beneficial effects on body weight and feed conversion ratio values when evaluated in coccidia-challenged broiler chickens [35, 36]. Coccidia is an important parasite because it generates a high economic impact and welfare and becomes a challenge for the poultry industry. On the other hand, the use of drugs as feed additives is being withdrawn, thus, these OEO mixtures as well as *L. origanoides* essential oils are a potential option as a natural alternative to antibiotics for improving the poultry productive performance. However, more research is needed to fully understand the mechanisms underlying these effects and determine the optimal dosage and duration of OEO supplementation.
