Essential Oils, Chemical Compounds, and Their Effects on the Gut Microorganisms and Broiler Chicken Production: Review

Abstract

The influence of essential oils (EOs) on gut microorganisms and broiler chicken production was evaluated through the systematic analysis of scientific reports. The present study was focused on the EO antimicrobial activity oriented toward broiler chicken production. There is a great biodiversity of plants, and various compounds with different biological activity have been isolated from them. The EO molecules extracted from plants have been employed recently in livestock feeding. Microbial resistance to antibiotics has led to their reduced use in animal production. To maintain competitive broiler chicken production with reduced antibiotic use, EOs have been explored. In broiler chickens, EOs are supplemented in the diet or drinking water to enhance weight gain and feed efficiency and reduce mortality. EOs are an alternative to antibiotics, and their research is dynamic in poultry production. The present review focused on the antimicrobial activity oriented to broiler chicken production. The search for information in databases used the terms “broiler chicken”, “essential oils” and combined them with the name of the plants. It was detected that the EO of Cinnamon bark or its compound cinnamaldehyde could reduce pathogenic bacteria in the digestive tract and improve intestinal morphology. Essential oils from Cymbopogon spp. and Origanum vulgare had an effect mainly against Gram-negative bacteria, such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella spp., and others, and against some Gram-positive bacteria, such as Staphylococcus spp., or yeasts, such as Candida albicans. Essential oils of Cymbopogon citratus acted against Salmonella. Citrus japonica affected Bacillus subtilis, Escherichia coli, and Salmonella typhimurium. Origanum EO improved the antioxidant status and gut health of chickens, while EO of Eucalyptus with carvacrol, thymol, and Citrus lemon improved the productive performance of broiler chickens; also, Citrus spp. reduced the number of oocysts of Eimeria and showed activity against Listeria monocytogenes, Staphylococcus aureus, and Pseudomonas aeruginosa. It is concluded that EOs are a sustainable alternative to antibiotics in the production of broiler chickens. Future research includes the standardization of EO from different plants and active molecules, as well as the interaction with other feed additives and their impact on the health and production of broiler chickens. In addition, safety for consumers and the environment must be considered.

1. Introduction

Antibiotics have frequently been used to promote growth in poultry production; however, in recent years, the scientific community has evidenced the alarming increase in antibiotic resistance, which has led to failure in infectious disease treatments [1,2]; for this reason, antibiotics have been restricted as supplements in animal production [3]. Because of this bacterial drug resistance that occurs more frequently, the need to intensify the search for alternatives of non-antibiotic growth promoters for poultry has arisen, which could improve the microbiota of birds, avoiding the adverse effects produced using antimicrobials.

As an option for antimicrobials as growth promoters, natural products obtained from plants that have been shown to have marked antimicrobial activity have been proposed [4]. Since ancient times, plants have been used for a wide variety of purposes, from treating diseases to preserving foods [5], and have influenced the microbiota of animals and humans [6,7]. Essential oils are liquid extracts of aromatic plants and have been given various uses in both industry and medicine. In industry, essential oils are employed in cosmetics [8]. In foods, they have been widely used to extend their shelf life [9]. In medicine, they have been used as antimicrobial, antiviral, anti-inflammatory, digestive, carminative, antipyretic, antitussive, antiseptic, healing, and anticancer agents [10,11,12], and antioxidants [8,13]. EOs contain secondary plant molecules as active ingredients and include different extracts such as essential oils and pure compounds that are isolated from them [4]. The EO molecules improve the physiology and metabolism of broiler chickens in various forms; they enhance the digestion of nutrients, decrease gut microorganisms with potential pathogenic, reduce oxidative stress, control free radicals from metabolism, and fortify the immune system and other variables that enhance the nutritional parameters [14,15,16] and the productive performance of broiler chickens [11,13]. The objective of this review was to evaluate the influence of essential oil molecules on health and applications in broiler chicken production through systematic analysis of scientific reports.

2. Methodology

A detailed systematic review was conducted over a 6-month period (from February 2024 to August 2024). The databases of Google Scholar, Web Science, PUBMED, and Scielo were used. The terms “broiler chicken”, “essential oils”, combined with the names of plants, like “Lemongrass”, “Citrus”, “Origanum”, “Rosmarinus”, “Phoebe”, “Curcuma”, “Thyme”, “Eucalyptus”, “Allium”, “Monarda”, “Lavandula”, “Cinnamond”, etc., were used. All research that combined words and provided information relevant to this topic was included, regardless of the year of publication or language. Articles that were not relevant to the topic were excluded. A total of 158 publications were included in the current manuscript, and about 71% were published in the last 10 years (2014 to 2024). The journals included in the Journal Citation Report (JCR) were mainly considered; however, some others that provided important data for the purpose of the review were also considered.

3. Essential Oils: Main Compounds and Applications

Paracelsus, in the sixteenth century, was the first to coin the term “essential oil” when he identified the activity of these compounds [17]. The components of essential oils form a total of around 500 compounds [18,19]. Because there is a great biodiversity of plants throughout the world derived from the different climatological conditions in regional areas, there is a wide biological and structural variety of their compounds [20,21,22]. Various molecules have been reported in leaves, flowers, roots, and other parts of the plants. The main molecules are phenolic compounds (phenols, phenylpropanoids, stilbenes, and their glucosides), isoprenoids (terpenes, carotenoids, storage lipids), alkaloids, and special amino acids (alliin, canavanine) (

Table 1. Main structural components of plants for some essential oils.

Compound	Part Used	Plant	References
Carvacol	Aerialparts	Origanum vulgare	[25]
Phenols, flavonoids	Leaves	Citrus lemon	[26]
Monoterpene	Leaves	Rosemary officinalis	[27]
Aromadendrene	Fruit	Eucalyptus globules	[28]
Terpene Monoterpene Sesquiterpene	Immature flowers, Stem, fruit	Eucalyptus oleosa	[29]
Cinnamaldehyde	Cassia leaves	Cinnamomum cassia	[30]
Sesquiterpenes Monoterpenes	Rhizome	Curcuma spp.	[31]
Trisulfid, Disulfide	Rhizomes	Allium sativum	[32]
Sesquiterpene hydrocarbons Oxygenated sesquiterpenes	Leaves	Eugenia	[22]
Monoterpene citral	Leaves	Cymbopogon citratus	[33]
Carvacol	Dried herb	Thymus sp., Thymbra sp. Satureja sp., Lippia sp. Coridothymus sp.	[34]
α-copaeno, α-muuroleno δ-cadineno, 1s-calameneno	Leaves	Phoebe bournei	[11]
Linalool, estragole	Leaves	Ocimum basilicum	[5]
Linalool acetate, linalool	Leaves	Lavandula angustifolia	[35]

) [23]. This generates a diversity of natural oils that favors the development of new antibacterial, antifungal, antiparasitic, and insecticide compounds [8,13], as well as being used in human and animal feeding [24].

Among this variability of compounds of essential oils obtained from plants, we can mention Carvacol, which is extracted from Origanum vulgare, Lippia sp., Coridothymus sp., and Thymus spp. [25]. Citrus lemon produces phenols and flavonoids [26,34]. EO from Curcuma spp. and Eucalyptus oleosa produce Monoterpene and Sesquiterpene [29,31]. Rosemary officinalis has Monoterpene [27]; another example is Linalool found in EOs from Ocimum basilicum [5] and Lavandula angustifolia [35], all of them with antibacterial effects.

Several essential oils have also been used as antioxidants; in eukaryotic cells, EO can act as pro-oxidants that affect internal cell membranes and organelles such as mitochondria; and depending on the type and concentration of the oil, they show cytotoxic effects in living cells, changes in the intracellular redox potential, and mitochondrial dysfunction and may, therefore, be associated with their capability to exert antigenotoxic effects [8,36,37].

Another use of essential oils is in industry; the biological activity of the EO of different species of Juniperus (Juniperus communis L., J. oxycedrus L., J. pygmaea C. Koch., and J. sibirica Burgsd) has been evaluated and observed that the EO tested had significant repellent and insecticidal activity against the two aphid species Rhopalosiphum padi (bird cherry–oat aphid) and Sitobion avenae (English grain aphid) and moderate activity against selected pathogens such as Fusarium spp., Botrytis cinerea, Colletotrichum spp., Rhizoctonia solani, and Cylindrocarpon pauciseptatum [38].

Currently, EOs have been used to improve food preservation, inhibit bacteriological growth to allow for longer food preservation, and inhibit the proliferation of pathogens that can upset the health of consumers, whether human or animal [24]. Microencapsulation of EOs in the food industry has been evaluated, and it has been observed that this positively affects the action of the active compounds in the essential oils, improving their antioxidant activity [39].

A combination of various EOs has been evaluated as a dietary supplement as an alternative with synergistic effects; this promotes a beneficial and healthy option for the food industry by improving food safety for consumers [24]. Evaluations of emulsions prepared with colloidal oil-in-water from essential oils have also been carried out to assess antimicrobial activity [40].

Some EOs are being used to improve nutritional parameters, such as feed conversion [14,15,16]. In broiler chicken production, natural products obtained from plants that have shown marked antimicrobial activity, and an improving effect on the intestinal mucosa have been proposed [4]. There are several studies that evaluated the effect of essential oils alone [40] or in combination with other feed additives and had synergistic effects [24], such as the antibacterial effect, to increase weight gain and improve feed conversion [16,41,42].

4. Modes of Action of Essential Oil Molecules

The action mode for essential oils to improve the health and productive performance of broiler chickens is shown in Figure 1. Essential oils improve nutrient absorption mainly due to the improvements in gut condition. Essential oils reduce the growth of pathogenic microorganisms and the toxins they produce; essential oils also prevent the formation of reactive oxygen species (ROS) that affect enterocyte function. The better balance for intestinal microorganisms and the reduction in toxins and oxidative agents (ROS) are beneficial effects of EO on the enterocyte condition. Essential oils also increase the absorption area in enterocytes, mainly villi and crypts. Additionally, EO increases the production of digestive enzymes. All these effects improve the nutrient absorption in chickens. On the other hand, EO also reduces oxidative stress and improves the immune system of chickens. In total, the improvements in productive performance with EO are due to improvements in digestion (gut) and health of broiler chickens (Figure 1).

Essential oils and their components have been studied for their antimicrobial activities (antibacterial, antifungal, and antiviral). Several studies show findings indicating only whether EOs have an effect on microorganisms, either by inducing bacterial lysis or inhibiting replication; however, the mechanisms of action of molecules have been little studied. Different forms of antimicrobial action have been proposed, such as cell membrane malfunction, inhibiting DNA/RNA synthesis or expression, or interfering with quorum sensing by interrupting cellular communication [43,44]; it has also been documented that some phytocompounds inhibited the production of bacterial toxins [43]. Carvacrol, one of the most frequently found compounds in phytopharmaceuticals, modifies the permeability of the bacterial membrane to cations such as H+ and K+, generating an imbalance in the essential processes of the cell, leading to the death of the bacteria [45,46,47]. Thymol, like carvacrol, contains a phenolic group and a delocalized electron system; consequently, carvacrol, thymol, cymene, menthol, and methylcarvacrol have shown that the hydroxyl group and the presence of a delocalized electron system are important elements for antibacterial activity [48]. Phytopharmaceuticals can also act on the membrane, inducing the release of autolytic enzymes associated with the cell membrane, which can induce lysis, as in the case of essential oil of Melaleuca alternifolia in Staphylococcus aureus [49,50]. Some others, such as berberine, gallic acid, and capsaicin, can inhibit bacterial efflux pumps [51]. In addition to their antibacterial properties, essential oils and their active components modulate the activity of some essential molecular mechanisms and signaling pathways in eukaryotic cells; for example, a-pinene inhibits respiratory activity in yeast mitochondria, and b-pinene has similar effects on intact yeast cells or isolated mitochondria [48,52]. On the other hand, the EO of clove, thyme, Cinnamon, and eugenol inhibit the production of toxins in some pathogens, such as Listeria monocytogenes, reducing its pathogenicity [43].

5. Types of Essential Oils and Antimicrobial Activity

Essential oils from different species of plants contain volatile and non-volatile compounds [10]. Different EOs have been reported; among the most common are those of Cymbopogon citratus, Origanum spp., Citrus spp., Rosmarinus spp., Origanum spp., Eucalyptus spp., and others (

Table 2. Some plants whose essential oils (EOs) inhibit microbiological replication.

Plants	Activity Against	MIC	References
Cymbopogon citratus	Acinetobacter baumanii Aeromonas veronii biogroup sobria Candida albicans Enterococcus faecalis Escherichia coli Klebsiella pneumoniae	0.03% v/v 0.12% 0.06% 0.12% 0.06% 0.25%	[53]
Origanum vulgare	Pseudomonas aeruginosa Salmonella entericas Serratia marcescens Staphylococcus aureus	2.00% v/v 0.12% 0.25% 0.12%
Cymbopogon sp.	Salmonella Heidelberg	0.5% v/v	[54]
Citrus japonica	Bacillus subtilis E. coli Salmonella typhimurium	1.56 µL/mL 1.56 100	[55]
Citrus sinensis	Listeria monocytogenes Staphylococcus aureus Pseudomonas aeruginosa	10 mg/mL 10 10	[56]
Origanum vulgare	Escherichia coli Salmonella Indiana Listeria innocua	0.9 mg/mL 0.9 0.9	[42]
Origanum spp.	Bacillus subtilis	0.15 mg/g	[46]
Oreganum vulgare ssp. Hirtum	Listeria monocytogenes, Salmonella typhimurium Escherichia coli O157:H7	4.5 mm a 10 10	[36]
Rosmarinus officinalis	Escherichia coli Salmonella indiana Listeria innocua	0.9 mg/mL 0.9 0.9	[42]
Rosmarinus officinalis	Eschericia coli MDR Shigella sonei Salmonella typhi Salmonella enteritidis Staphylococcus aureus S. epidermidis	18.6 b 19.4 18 26.6 17.2 18.2	[27]
	Escherichia coli Salmonella indiana Listeria innocua	5 mL/mL 10 mL/mL 5 mL/mL	[57]
Phoebe bournei	Epidermophyton floccosum Microsporum gypseum	74.2 µg/mL 32	[11]
Beilschmiedia madang	Bacillus subtilis Staphylococcus aureus Aspergillus niger Aspergillus fumigatus	125 µg/mL 125 62.5 62.5	[37]
Curcuma spp.	Bacillus cereus B. coagulans B. subtilis S. aureus E. coli Psudomona aeruginosa	100 ppm c 50 100 100 200 200	[58]
Eucalyptus spp.	E. coli S. aureus P. aeruginosa	28.5% d 67.5 36.8	[40]
Lavandula angustifolia	Salmonella pullorum S. enteritidis S. typhimurium Staphylococcus aureus P. aeruginosa Candida albicans	0.50% v/v 0.625 1.50 0.25 2.0 0.625	[59]

^a Inhibition zone (mm); ^b Inhibition zone (mm)/(20% EO); ^c parts per million; ^d 50 ppm.

).

In a study where C. citratus essential oil was evaluated, LGEO had antimicrobial activity [53]. The action of C. citratus essential oil against multidrug-resistant Salmonella enterica subsp. enterica serotype Heidelberg (S. Heidelberg) was also published [54]. S. Heidelberg included in foodborne illnesses and poultry products promote the spread of this bacteria; this study showed that all tested concentrations of LGEO significantly reduced complete inhibition of motility, biofilm formation, and the presence of S. Heidelberg in skin samples. Considering this, Dewi proposed that LGEO be used for its antibacterial effect in the scalding process to reduce S. Heidelberg in broiler chickens [54].

Mandarin peels are used for the production of essential oils; some of them, such as Citrus reticulata, C. reticulata, C. japonic, and C. sinensis, are often used to produce EO. Lin (1995) evaluated the antibacterial and antioxidant activity of four types of extracted EO and determined that monoterpene hydrocarbons are the main components, and limonene was the predominant compound for all citrus essential oils. C. japonica EO had the best inhibitory effect on B. subtilis, E. coli, and S. typhimurium with MIC (minimum inhibitory concentration) values of 1.56, 1.56, and 6.25 μL/mL, respectively. Citrus EOs showed antioxidant activity; thymol was only found in C. reticulata EO, and the proportion of γ-terpinene (8.19%) was the highest; this might be the reason C. reticulata EO had the best antioxidant activity [55].

Another citrus fruit studied, Citrus sinensis, extracted EO from Citrus sinensis by steam distillation and extraction with hexane. The major component was d-limonene (73.9–97%); however, lower percentages of linalool, geraniol, and nerol were also found. Geraci (2017) reported that Citrus sinensis had antimicrobial activity against Listeria monocytogenes, while the hexane extract inhibited the development of Staphylococcus aureus, L. monocytogens, and Pseudomonas aeruginosa [56].

Origanum, Thymus, Coridothymus, Thymbra, Satureja, and Lippia are rich in carvacrol, an isomer of monoterpene phenol with thymol [34]. Carvacrol is a monoterpenoid phenol [60] and is responsible for the biological activities of oregano. Diverse activities of carvacrol have been demonstrated, such as antimicrobial, antitumor, antimutagenic, antigenotoxic, analgesic, antispasmodic, anti-inflammatory, angiogenic, antiparasitic, antiplatelet, anti-elastase, insecticide, antihepatotoxic, and hepatoprotective, and also as a feed additive [5,34,53,61].

The antimicrobial activity of Origanum spp. oils was reported by several authors [5,62], evidencing activity on bacterial strains such as Escherichia coli, Salmonella indiana, Listeria innocua, Staphylococcus aureus, and Bacillus subtilis [42,46], in addition to antioxidant activity [36]. On the other hand, Origanum vulgare ssp. Hirtum, another variety, showed antioxidant and antimicrobial activities, inhibiting the development of Listeria monocytogenes, Salmonella typhimurium, and Escherichia coli O157:H7 [36].

Rosmarinus officinalis, another plant from which essential oil is extracted, according to what was published by Mathlouthi (2012), reported activity against three pathogenic bacteria, Escherichia coli, Salmonella indiana, and Listeria innocua [42,57]. It was also observed that combining Rosmarinus spp. and Origanum spp. EOs in broiler chickens showed in vitro antimicrobial activities and effects on growth performance [42]. Bozin (2007) found that the distilled oil of R. officinalis was composed of approximately equal amounts of oxygenated monoterpenes (46.9%) and monoterpene hydrocarbons (46.7%) and observed high sensibility for Gram-negative bacteria such as Eschericia coli, even multidrug-resistant, and for Shigella sonei, Salmonella typhi, Salmonella enteritidis, and some Gram-positive bacteria, such as Microccoccus flavus, Staphylococcus aureus, Staphylococcus epidermidis, and Bacillus subtilis. Rosmarinus officinalis EO has antifungal activity in Candida albicans, Epidermophyton floccosum, Microsporum canis, and Trichophyton spp. [27]. These data were consistent with what was published by Stojiljkovic (2018), demonstrating the antibacterial effect of Rosmarinus extract [57].

EO obtained from leaves of Phoebe bournei (Hemsl.) mainly included α-copaene, α-muurolene, δ-cadinene, and 1s-calamenene; these compounds had inhibitory activities against fungi, such as Epidermophyton floccosum and Microsporum gypseum [63]. In another study, moderate growth inhibition was reported for bacteria, such as Bacillus subtilis and Staphylococcus aureus, and filamentous fungi, such as Aspergillus niger and Aspergillus fumigatus, by the essential oils of Beilschmiedia madang Blume (Lauraceae), in addition to their antioxidant activities [37].

Negi (1999) published antibacterial activity of EOs from Curcuma spp. against Bacillus cereus, Bacillus coagulans, Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa [58]. Furthermore, Avanço (2017) reported the action of Curcuma as an antifungal and anti-mycotoxigenic on Fusarium verticillioides [64], and Singh (2010) reported their antioxidant activities [65]. Traditionally, Curcuma is used for the treatment of gastrointestinal, inflammatory, wound, pain, cancer prevention, and anti-aging conditions, among others [31].

Sakkas and Papadopoulou (2017) studied the antimicrobial activity of EO from Thyme [5]. Hammer (1999) published that Thyme EO inhibits the growth of C. albicans and E. coli [53]. Thyme and Oleum menthae piperitae essential oils have been suggested to reduce bacterial contamination of houses in broiler chicken production; the effect was to decrease the bacterial count of Enterobacteriaceae and Sthaphylococcus spp. Thyme oil favored the decrease in coliform bacteria, while O. menthae piperitae oil had a greater inhibitory effect on Staphylococcus proliferation [66].

Essential oils of Eucalyptus are commonly used for treating disorders of the respiratory system such as pharyngitis, bronchitis, and sinusitis [67]; however, emulsions from eucalypt EO have shown antimicrobial activity against Escherichia coli, Sthaphylococcus aureus, and Pseudomona aeruginosa [40]. Moreover, Eucalyptus EO has also shown antifungal action against yeast-like fungi, as well as an antioxidant action [68]. In another research with broiler chickens, the mixture of eucalypt EO with carvacrol, thymol, and lemon added in drinking water considerably decreased S. Heidelberg colonization in birds’ crops, and it was proposed that this pathogen could be reduced in birds treated with these essential oils [69].

Essential oils from Allium sativum have been evaluated based on their potential antibacterial and antioxidant activity on intestinal bacteria of broiler chickens [70]. Encapsulated A. sativum EO was used in this study, which evidenced a significant increase in beneficial effects on in vitro (antibacterial and antioxidant properties) and on in vivo (performance and gut health) conditions of the broiler chickens. Therefore, this supplementation in the feed was considered as an alternative to antibiotics as growth promoters in poultry production [70].

The EO Monarda didyma showed antibacterial, antioxidant, and anti-inflammatory activity. It was suggested that adding essential oils of M. didyma to the diet of broiler chickens offered an alternative substitute to the antibiotic growth promoters [71].

The essential oils of Lavandula angustifolia have antioxidant, antifungal, antibacterial, sedative, antidepressant, anti-inflammatory, hypnotic, analgesic, and anticancer activity [35,72]. In this plant, 26 compounds were identified, of which linalool acetate (46.25%) and linalool (35.17%) were the main components. Essential oils of L. angustifolia had an activity on ATCC bacteria such as Staphylococcus aureus, Salmonella pullorum, Candida albicans, Escherichia coli, Salmolella enteritidis, Psudomona aeruginosa, and Salmonella typhimurium [59]. Additionally, the essential oils of L. angustifolia also increase the antioxidant status in the blood serum of broiler chickens [73].

There are other less-studied essential oils, such as those extracted from the genera Eugenia and Syzygium. These are important for their economic potential due to their pharmacological properties. Their main volatile compounds are sesquiterpene hydrocarbons and oxygenated sesquiterpenes, mainly with skeletons of caryophyllan and germacraine, and mostly pinane-type monoterpenes. The essential oils show some biological activities, especially antimicrobial, anticholinesterase, anticancer, antiprotozoal, antioxidant, acaricide, antinociceptive, and anti-inflammatory [22].

Mohammadhosseini (2017) published that the EO from the rhizome of Hedychium coccineum had significant nematicide activity against Meloidogyne incognita, insecticidal activity against Spodoptera litura, moderate herbicidal activity against R. raphanistrum sub sp. sativus, and good antifungal activity against Fusarium oxysporum and Curvularia lunata. They also had activity against Staphylococcus aureus and Salmonella enterica serotype Typhi [74].

Typical products in food have also been studied as in the case of the EO of Muscadinia rotundifolia, where twenty-three volatile compounds were reported. The EO of the flowers showed very weak antioxidant power and antibacterial activity against Bacillus cereus, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, and antifungal activity against Candida albicans [75].

In other research, the essential oils of Pinus brutia and Pinus pinea were reported to exert antimicrobial activities against Micrococcus luteus and Bacillus subtilis and showed activities on Ephestia kuehniella eggs, phytotoxic activities on Lactuca sativa, Lepidium sativum, and Portulaca oleracea, as well as antioxidant potential [13]. Essential oils obtained from Origanum vulgare and Origanum dictamnus had activity against S. aureus; the strain with reference NCTC 6571 was shown to be extremely sensitive in most and was resistant only to essential oils of Ocimum basilicum [9]. Essential oils from the leaves and bark of Santiria trimera, distilled in water, were also evaluated. The essential oils in the leaves were dominated by sesquiterpenes (76.5%), among which alpha-humulene (34.6%) and beta-caryophyllene (14.9%) were the main components. The EO of the bark was almost exclusively monoterpene, with alpha-pinene (51.5%) and alpha-terpineol (16.8%) as the main structural compounds, and showed effectiveness against Bacillus cereus and Enterococcus faecalis. The EO of the bark was more active and showed activity against Proteus mirabilis [76]. In the EO extracted by hydro distillation of Origanum acutidens, two components dominated: p-cymene and carvacrol, which were attributed with antibacterial activities against Proteus vulgaris, Salmonella typhimurium, Enterobacter cloacae, Klebsiella pneumonia, Escherichia coli, Serratia marcescens, and Pseudomonas aeruginosa [77]. EO extracted from Thymus willdenowii Boiss and Reut may be a potential new source of natural antimicrobials that could be applied in the pharmaceutical and food industries [78].

In other studies, it has been mentioned that Eos, such as β-caryophyllene, γ-gurjunene, τ-cadinol, and calamenene, have been obtained from Psidium guajava. The terpenoids in all the sample oils in this plant were mainly sesquiterpene hydrocarbons, followed by oxygenated sesquiterpenes. The evaluation showed that EO had a higher antimicrobial activity against all Gram-positive bacteria and two fungi [79]. The oils of A. wilhelmsii exhibited higher antibacterial and antifungal activities with high effectiveness against Escherichia coli and Candida albicans [80].

6. Essential Oils on Microbes from the Digestive Tract of Broilers

The effect of EO on poultry may not only be limited to animal metabolism but could also extend to the gut microflora. The bacterial populations in broiler chickens are very varied and depend on the anatomical area in their digestive system, substrate, and age; the establishment of the dominant microbiota depends on the feed and the host. There is a relationship between the microbial community that inhabits the gastrointestinal tract and the health of broiler chickens. The intestinal commensal microbiota plays a primary role in preventing the colonization of pathogenic microorganisms in the digestive system by competitive exclusion [81]. In the first 14 days, microorganisms establish themselves in the small intestine, and mainly fecal Streptococci spp. and Coliform bacteria are found until 40 days [82] later, Labobacilus spp. bacteria establish themselves and dominate and are present in between 80 and 90% of the total microbiota [83]. In the cecum, the microbiota is subsequently established around 40 days, mainly with facultative bacteria such as the Enterobacteriaceae family and anaerobic bacteria such as Bacteroides [84]. In the chicken cecum, the most common families within the order Clostridiales are Lachnospiraceae, Ruminococcaceae, and Veillonellaceae [85]. In production systems, indicator bacteria, such as Escherichia coli, Clostridium perfringens, Staphylococcus spp. [81], and Listeria monocytogenes [86], are monitored due to their importance for animal health and economy, in addition to Enterococcus cecorum and Salmonella due to their importance in monitoring antimicrobial resistance (AMR) [81]. The EOs in chicken feed could be used as alternatives to antibiotics, which generates a modification in the intestinal microbiota; however, it is necessary to study more deeply if the effects of the different types of EOs vary with the microorganism strains.

Essential oils are composed of a wide variety of molecules, as described above; phytotherapy has been isolated in veterinary medicine since ancient times, whose foundations were the Ethnoveterinary Practices [87,88]. EO molecules have digestive enzyme activity and stabilize the intestinal microflora; some of them assist in digestion [89,90,91]; others act against pathogens [42,54,57] or are antioxidant [8,36,37], and together, they optimize intestinal health [87]. In broiler production, to maintain or improve productivity during stress, alternatives such as improving intestinal health have been proposed. Phytopharmaceuticals favor the reduction in pathogens by promoting the replication of microorganisms beneficial to animal health [87].

In one study it was observed that the dietary addition of essential oils showed a decrease in the population of E. coli in the ileocecal digesta. In addition, a high dose of EO resulted in a significant increase in certain activities of digestive enzymes of the pancreas and gut in growing broiler chickens [89]. In another study, it was observed that in the experiment where the group with antibiotics showed the lowest count of Lactobacillus spp. and the highest count of Escherichia coli in the ileal digesta, the group of broiler chickens fed with the diet supplemented with essential oils had reduced E. coli populations [92]. Results of another research suggest that cinnamon bark oil at 0.3 g/kg diet and polyunsaturated fatty acids could reduce pathogenic bacteria in the gut and improve gut morphology along with improving the immune response of broiler chickens [93].

The effect of feeding diets supplemented with essential oils on broiler chickens’ productive performance and gut microbiota has been reported. The evidence shows that supplementation with essential oils exerts a positive effect on the gut microbiota with an improvement in growth performance [94,95,96,97,98]. Modulating the composition and activity of the intestinal microbiota of broiler chickens with essential oils has been suggested to be effective in improving broiler performance [94].

7. Influence of Essential Oil Molecules on Broiler Chicken Production

In broiler chicken production, essential oils from plants are suggested as growth promoters as an alternative to conventional antibiotics; however, the enhancement expected in the productive behavior of broiler chickens receiving essential oils is not always observed. In another study, they found no influence of essential oils (thymol) on production variables in broiler chickens [89]. Other researchers also reported no improvement in the productive performance of broiler chickens supplemented with a compound consisting of carvacrol, cinnamaldehyde and eugenol (extracted from oregano, cinnamon and cloves, respectively). The lack of response was attributed to the good nutritional and sanitary management of birds. In other cases, the productive variables of weight gain or feed efficiency can be absent when essential oils are supplemented to broiler chickens; however, they cause beneficial effects in different ways [99]. Another study, in broiler chicken fed diets with the addition of essential oils containing cinnamaldehyde and thymol, did not find changes in weight gain, feed intake, or feed efficiency; however, the positive influence of essential oils on mortality reduction when compared to animals from the none-treated group [100]. In different results observed greater weight gain and enhanced feed conversion when broiler chickens received similar essential oils in their diet [101]. Different results between different studies are difficult to establish; however, differences are observed in the diets, with lower concentrations of crude protein and energy [100]. This way, the diet composition of broiler chickens must be considered when essential oils are supplemented (

Table 3. Influence of essential oils (EO) molecules on broiler chicken production.

EO or Molecule, Doses, and Duration	Active Compound	Net Effect of EO on Production	References
EO commercial product. (25 and 50 mg/kg) in diet. Trial of 35 d	Thymol	No effect on BWG, FI, or FCR	[89]
EO commercial product (100 and 150 g/ton) in diet. Trial of 42 d	Carvacrol, cinnamaldehyde eugenol	No effect on BWG, FI, or FCR	[99]
EO commercial product (100 g/ton) in diet. Trial of 42 d	Cinnamaldehyde thymol	No effect on BWG FI or FCR; the EO.	[100]
EO commercial product (25, 50, 100, and 150 mg/kg) in diet. Trial of 36 d	Cinnamaldehyde thymol	EO improved BWG (11.5%) and FCR	[101]
EO commercial product (50, 100, 200, and 400 mg/kg) in diet. Trial of 42 d	Thymol, carvacrol, cinnamaldehyde, and carrier is dextrin	EO had quadratic. improvement for ADG (6.4%) at 1 to 21d with EO. N other effects of EO on bird production	[102]
Commercial product of EO plus organic acids (0, 0.05, and 0.075%) in diet. Trial of 35 d	Thymol and vanillin plus organic acids	EO had linear improvement for BWG (3.8%) and FCR (3.4%) with EO plus organic acids	[103]
Commercial product of EO plus organic acids (150, 200, or 250 mg/kg) in diet. Trial of 70 d	Thymol and vanillin plus organic acids	EO had linear improvement for ADG (2.9%) and FCR (2.1%) at 1 to organic acids	[104]
EO commercial product (0.075%) in diet. Trial of 42 d	Thyme extract	EO improved BWG (13%) and FCR (22%)	[105]
EO of thyme (150 or 300 mg/kg); EO of rosemary (100 or 200 mg/kg) in diet. Trial of 42 d	Thyme EO Rosemary EO	No effect on BWG, FI, or FCR	[106]
EO of thyme (0.5 and 1.0 g/kg) in diet. Trial of 42 d	Thyme EO (14.2 and 18.7%)	EO improved ADG and FCR (5.7 and 7.8% at 0.5 and 1.0 g/kg)	[107]
EO commercial product (0 or 100 mg/kg) in diet. From 9 to 35 d of age. Values for low and high ME in diet	Carvacrol, cinnamaldehyde, and capsicum oleoresin	EO improved ADG (2.1 and 2.7%) and FE (1.8% and 2.8%)	[108]
EO commercial product (0, 60, 100, and 200 mg/kg) in diet. Trial of 42 d	Thymol and carvacrol improvement for	EO had linear ADG (4%) and FE (6.3%)	[90]
EO commercial product. (0 or 100 mg/kg) in diet. From 9 to 35 d of age.	Carvacrol cinnamaldehyde capsicum oleoresin	EO improved ADG (14.5%) and FE (9.8%)	[109]
Commercial product of EO plus organic acids (0.30 g/kg) in diet. Trial of 42 d	Thymol plus	EO improved FCR	[92]
Commercial product of EO (300 mg/kg) in diet. Trial of 42 d	Cinnamaldehyde, carvacrol, thymol, eugenol	EO only improved FCR in the starter phase	[110]
Commercial product of EO (100 g/t) in diet. Trial of 42 d	Eucalyptus oil, carvacrol, cinnamyl	No effect on BWG, FI, or FCR aldehyde, capsaicin	[111]

).

The quadratic tendency may indicate that the productive efficiency is improved, and when it is at the maximum peak, no further improvement can be achieved. In a report where they used a mixture of essential oils (thymol, carvacrol, and cinnamaldehyde), they observed a quadratic enhancement in the body weight of broilers during the initial growing phase. Essential oils did not influence the productive variables during the finisher phase or total trial. In this study, it is difficult to explain the low influence of the essential oils on the productive variables of broiler chickens in the overall period, particularly considering that essential oils had significant improvement in the digestibility of dry matter, crude protein, ether extract, and gross energy [102]. Consistently, several authors reported that broiler chickens receiving diets with a blend of essential oils (thymol and vanillin) plus organic acids (citric acid and sorbic acid) had improved weight gain and feed conversion in the starter phase [103,104]. Nevertheless, in the finisher phase, the productive performance was not changed by the essential oils plus organic acids. In these reports [102,103,104], the essential oils probably contributed by supporting the higher nutrient requirements in the starter phase. However, in the finisher phase, there are lower nutrient requirements, and if the diets are rich in nutrients, they provide a lower opportunity to show the positive influences of the essential oils. Moreover, other aspects may also contribute, for example, adequate management practices in all aspects. On the other hand, it has been suggested that greater doses of the essential oils during the finisher phase could be required, although the research reports used the same dose of essential oils during both the starter and finisher phases [105].

In other conditions, the favorable influences of essential oils on the productive performance of broiler chickens can be evidenced with other probiotic complements. Gumus and Gelen (2023) did not find the influence of essential oils from thyme (Thymus vulgaris L.) at 0.15 g/kg and 0.30 g/kg or rosemary (Rosmarinus officinalis) at 0.10 g/kg and 0.20 g/kg on the production variables (weight gain, feed intake, or feed conversion) of broiler chickens. Probably, the essential oil of thyme was not used in sufficient amounts to influence the productive performance of broiler chickens. Conversely, essential oils from thyme may have a synergic influence on the productive behavior of broiler chickens when complemented with a probiotic [106]. In research reports, when compared to animals of control groups, enhanced live weight gain and feed conversion efficiency were observed in broiler chickens receiving a probiotic plus essential oils from thyme (T. vulgaris L.) at 0.75 g/kg [105] and 1.0 g/kg [107].

There might be a synergic influence on the productive behavior of broiler chickens when combining two or more different essential oils or with essential oils plus other feed additives. In a study they mentioned that the use of 100 mg/kg of a blend of carvacrol, cinnamaldehyde, and capsicum oleoresin in diets enhanced the gain efficiency of broiler chickens; the improvements were related to improvement in dietary metabolizable energy value by about 50 kcal/kg [108]. Another researcher report that a mixture of essential oils (thymol + carvacrol) enhanced the live weight and feed efficiency of broiler chickens. The effect was evident at 60 ppm of the mixture of essential oils [90]. In another study, they found improved feed conversion efficiency and weight gain in broiler chickens receiving a supplement formed by carvacrol, cinnamaldehyde, and capsicum in the diet during the starter phase of feeding (1–21 d) [109].

Researchers reported that a blend of essential oils plus organic acids (thymol, sorbic acid, and fumaric acid) and enramycin enhanced feed conversion due to their antimicrobial action [92]. Another group of researchers tested organic acids (benzoic acid, formic acid, and lactic acid) and/or essential oils (cinnamaldehyde, carvacrol, thymol, and eugenol) in broiler chickens; essential oils enhanced feed conversion in the starter phase, but no influence was observed during the finisher phase. The combination of essential oils with organic acids had no additional influence on the production of broiler chicken. However, improved broiler chicken production is not always reported with essential oils [110]. Other researchers have reported finding no influence of plant oils (eucalyptus oil, carvacrol, cinnamyl aldehyde, capsaicin) on the production variables (weight gain, feed intake, or feed conversion) of broiler chickens [111].

Effect of essential oils on production in challenged broiler chicken: there are some research reports that evidence the positive influence of essential oils on the productive performance of broiler chickens that were challenged or exposed to different, potentially pathogenic microbes [62,112,113]. The productive performance enhancement of challenged broiler chickens is consistent with the supplementation of different EOs. In broiler chickens challenged with Eimeria spp. and Clostridium perfringens, tested a compound of organic acids (fumaric, sorbic, malic, and citric acids) plus essential oils (thymol, vanillin, and eugenol). When compared to the non-treated group, both essential oils and antibiotics were effective in improving the productive behavior of broiler chickens; they suggested the use of essential oils as a replacement for antibiotics as growth promoters for broiler chickens for commercial purposes [114]. Other researchers in their reports also observed improved productive performance in broiler chickens infected with Salmonella Enteritidis (S. enteritidis) when feeding coated essential oils and organic acid mixture (thymol, carvacrol, cinnamaldehyde, caprylic acid, benzoic acid, and butyric acid) [115,116]. Similarly, another group of researchers, in broiler chickens challenged with pathogenic E. coli, found that 400 mg/kg of eugenol in a nano-emulsion improved growth efficiency [117]. Also in another study in broiler chickens challenged with necrotic enteritis found that 100 ppm of an encapsulated compound of eugenol and garlic tincture enhanced feed conversion efficiency, reducing feed intake and maintaining body weight at similar levels as challenged birds that did not receive additives in the diet [115]. A group of researchers found that a mixture of EO in drinking water improved weight gain and reduced mortality in broiler chickens exposed to Salmonella Heidelberg [69,118]. In addition, EO reduced SH colonization in the crops of the chickens [118]. Another study on broiler chickens infected with Eimeria tenella (E. tenella) observed that dietary supplementation of 200 mg/kg water-soluble extract of rosemary improved broiler productive behavior and alleviated intestinal damage produced by coccidiosis. The authors suggested that this rosemary extract could contribute to alleviating or controlling coccidiosis in poultry production [119].

Productive performance of broiler chickens: essential oils from oregano. There are various species of oregano in the world; most of them contain thymol and carvacrol as the main active molecules in their essential oils, besides other compounds that are in minor quantities. Most of the research reported improved productive behavior of broiler chickens. Improvements in feed conversion efficiency have been reported in the finishing phase (22 to 42 d) or total feeding period (1 to 42 d) with essential oil from oregano (Origanum vulgare L.). The results were similar to the ones obtained with the coccidiostat (maduramicin ammo-nium) and the ones with the essential oil; however, both treatments had better productive performance than the animals from the non-treated group [120]. Consistently, Mohiti-Asli and Ghanaatparast-Rashti, 2015, also found improved growth performance and coccidia control in broiler chickens receiving essential oils from O. vulgare [61]. In another study, in an experiment with a slow-growing breed of chickens, also found that supplementation with 150 to 300 ppm of essential oils from oregano (extracted from O. vulgare) improved growth performance, with a beneficial effect on the intestine, modulating gut microbes, and on the immune system. The authors proposed essential oils as an option to reduce the antibiotics in broiler chicken production for local breeds [121]. Similarly, it was reported that Mexican oregano essential oils from Lippia berlandieri Schauer and Poliomintha longiflora Gray had beneficial effects on broiler chicken productive behavior and enhanced meat quality [122].

Researchers reported that productive variables were improved more evidently in challenged Eimeria sp. broiler chickens receiving essential oils from Lippia origanoides. Carcass traits were enhanced (main effect) by essential oils [123]. Another study, in broiler chickens under heat stress, tested essential oils from Lippia origanoides alone or in mixtures of Rosmarinus officinalis with red beetroot or natural betaine. Results revealed that all treatments were effective in improving feed conversion efficiency compared to the control group (no additives in diet); however, weights of the carcass were higher in the treatments of blends, including Rosmarinus officinalis with red beetroot or natural betaine, when compared to the control animals or those treated with only Lippia origanoides [124].

Improved growth was reported in broiler chickens fed essential oils from oregano troughs, enhancing the intestinal immune system and alleviating the oxidative stress of broiler chickens. In addition, better responses were observed with natural extracts of essential oils than with synthetic essential oils (a mixture of thymol and carvacrol). The essential oils were at 200 ppm in the diet [125]. Nevertheless, researchers observed better productive performance in broiler chickens fed essential oils from synthetic oregano (thymol and carvacrol). The essential oils were at 200 and 400 ppm in the diet. Probably, in this study, the beneficial effects were more evident by the greater dose of synthetic oregano than those observed by other researchers [63,125].

In a study in broiler chickens reared under heat stress, reported greater final body weight with a single supplementation of vitamin C or oregano (Origanum vulgare L., ssp. Hirtum). However, a synergic effect was observed with the combination of both additives (vitamin C plus the essential oil) where the animals had 200 g more body weight than the control group [126]. Tekce and Gül (2016) reported that essential oils from Origanum syriacum had evident improvement in productive performance in broiler chickens under comfort temperature (22 °C), while under heat stress (36 °C) the improvement in the productive performance was almost nonappreciable [127]. Another investigation, in broiler chicken diets, used a mixture of essential oils formed by extracts from oregano, rosemary, cinnamon, and chili pepper. Related to the control group (with no additives), the animals fed diets with essential oils had similar feed intake; however, accumulated weight gain and feed conversion efficiency of broiler chickens were significantly improved with the essential oils [128].

There is limited information on where essential oils from oregano did not have an effect on the productive behavior of broiler chickens. Other researchers did not report improvement in weight gain or feed conversion in broiler chickens receiving 50 or 100 mg/kg of essential oils from oregano in the feed. There might be different factors that influenced this difference with respect to other reports that found the improved productive performance of the chickens. In particular, we observed that the low dose of oils from oregano in this study could be related to the lack of effect on the production of broiler chickens [129]. In another study neither reported improvement in feed efficiency in broiler chickens receiving 0.01% oregano oil (Origanum vulgare L.) or 0.005% oregano oil and 1% oregano powder. Also, this essential oil dose is low in order to promote the growth increase in the broiler chickens [130].

Productive performance of broiler chickens; essential oils from Cinnamon: the essential oils from cinnamon, or the cinnamaldehyde (extracted from oils from Cinnamon), did not show consistent improvement in the productive performance of broiler chickens. In a preliminary report did not find differences in weight gain and feed conversion in female broiler chickens fed diets supplemented with thymol, cinnamaldehyde, and a commercial mixture of essential oil components [91]. In the same manner, Yang et al. [131] tested Cinnamon essential oil alone or combined with bamboo leaf flavonoid in broiler chickens. The treatments had no effects on the productive variables of weight gain, feed intake, or feed conversion efficiency. Yang et al. (2021) also observed no improved productive performance of broiler chickens with cinnamaldehyde (CIN) at 50 mg/kg or 100 mg/kg in the feed; however, mortality reduction was observed with CIN [132].

In one study it was observed that encapsulated CIN or citral improved the productive performance of broiler chickens, although no additive effect among the two additives was found. Likewise, the combination of cinnamaldehyde with other extracts or medium-chain fatty acids has been explored [133]. Other researchers observed that a compound of microencapsulated carvacrol and cinnamaldehyde improved the productive variables of broiler chickens [134]. In another study reported enhanced growth performance, intestinal morphology, and gut microorganisms of yellow-feathered broiler chickens supplemented with a complex of lauric acid monoglyceride and cinnamaldehyde in the diet [135].

The differences in the productive behavior of broiler chickens shown in the different reports might be due to different factors like sanitary management, environmental conditions, feed management, and other factors like the dose and presentation of the essential oils. In broiler chickens receiving essential oils from Thyme (Thymus vulgaris), cinnamon (Cinnamomum verum), and mint (Mentha piperita) in two forms—free form and encapsulated form—observed that the encapsulated form had superior improvement on the productive variables than free form of essential oils [136]. In relation to the health status of the broiler chickens that were not challenged with E. coli, did not find improvement in productive behavior with using a blend of cinnamaldehyde plus organic acids, but the productive performance was improved when challenged animals received the same additives. It was postulated that productive performance enhancement was related to improvement in the health status of the animals [137]. Similarly, researchers reported improved productive behavior of broiler chickens challenged with coccidia when essential oils (garlic and cinnamon extracts) and betaine were supplemented [138]. In other research also reported that the enhancement in productive behavior was evident when encapsulated essential oils from cinnamon were fed to broiler chickens during the warm season of the year and presented a necrotic enteritis outbreak. In addition, the essential oils also reduced the mortality of birds [139].

Productive performance of broiler chickens: essential oils from Lavender. Consistently, the essential oil of lavender (Lavandula angustifolia) in the drinking water has enhanced the live weight gain and feed conversion efficiency of broiler chickens compared to a non-treated group of animals. These studies suggested the lavender addition to drinking water during the finisher phase was enough to improve the production of broiler chickens [73,140]. Similarly, other researchers also found improvements in weight gain and feed conversion ratio during the finisher phase and for the total feeding cycle of broiler chickens receiving lavender essential oil in the diet. Similar enhancements in the growth performance of broiler chickens supplemented with lavender essential oil and the antibiotic virginiamycin were observed in this research compared to the non-treated group of animals [141].

Only a small improvement in the productive behavior of broiler chickens was observed when lavender essential oil was tested at different levels in broiler chickens fed during three phases: starter (3 to 10 d); grower (11 to 23 d); and finisher (24 to 35 d). When compared to the non-treated group, the only influence they observed was an improved feed conversion ratio in the grower phase. There was no effect of treatment on the productive variables of broiler chickens during the starter and finisher phases [142]. Moreover, in another investigation did not observe a benefit in the productive performance of broiler chickens supplemented with lavender essential oils. This research included lower crude protein than previous reports [143].

Productive performance of broiler chickens: essential oils from Rosmarinus officinalis (Rosemary). In a preliminary study, they reported improved productive performance of broiler chickens supplemented with essential oils from rosemary, which had cineole as the main active compound [42]. In a study, improved productive performance of broiler chickens supplemented with both non-encapsulated and encapsulated rosemary essential oils was also observed; however, the non-encapsulated EO reduced the ileal coliform count and had greater antioxidant activity than the encapsulated form. In this study, non-encapsulated rosemary essential oils at 150 mg/kg were suggested for poultry production [144]. A group of researchers found that nanoencapsulated essential oils from rosemary were more effective than free forms of essential oils from rosemary in enhancing the productive behavior of broiler chickens, especially at 200 mg/kg feed. In this report, the major active compounds in the essential oils from rosemary were camphor, α-Pinene, and 1,8-Cineole [145]. It was also published, in broiler chickens challenged with Eimeria tenella (E. tenella), reported that dietary supplementation of 200 mg/kg water-soluble extract of rosemary improved broiler productive performance and alleviated gut damage caused by coccidiosis. The authors suggested that this rosemary extract could contribute to alleviating or controlling coccidiosis in poultry production [119]. Similarly, in Japanese quails under heat stress, found that rosemary oil (Cineole as the main compound) improved the feed conversion ratio [146].

There are few reports where rosemary essential oils did not improve the productive performance of broiler chickens. Researchers reported that rosemary or oregano essential oils used separately did not change the productive performance of broiler chickens; however, when they were blended (rosemary + oregano), the production was enhanced. In this case, pinene, cineole, and borneol were the main compounds in rosemary oils [147]. in another research argued that under heat stress conditions, rosemary essential oils (ferruginol was the main compound) in the feed did not improve the production of broiler chickens. Although, under these conditions, oregano essential oils were effective in improving the productive performance of the birds. The difference in the productive responses of broiler chickens in the different reports can be attributable to the differences in the active compounds in the oils from rosemary and could be associated with the different extraction methods of the oils from rosemary [148].

Productive performance of broiler chickens: essential oils from garlic. In a preliminary study did not observe the influence of essential oils from garlic on the feed conversion efficiency of broiler chickens [149]. However, in another research found that improved feed efficiency of broiler chickens was evident at 100 ppm of garlic acid. The authors also observed that the encapsulated form had superior improvement on the productive variables than the free form of essential oils from garlic [70]. Similarly, in another research, in broiler chickens raised under high ambient temperatures, found improved feed conversion efficiency by supplementing essential oils from garlic and/or lemon. The enhancement was evident in the finisher phase and the total period [150].

Productive performance of broiler chickens: essential oils from other plants. Different essential oils from different plants have improved the productive performance of broiler chickens; among them are Coriander sativum [151,152,153], Capsicum annum [154,155], Syzygium aromaticum [156,157], Monarda didyma L. [71], a blend of parsley, mint, and carrot seed oils supplemented with black pepper oil or radish seed oil [158].

8. Research Perspectives on Essential Oils in the Production of Broiler Chickens

Since antiquity, traditional medicine and agri-food science have benefited from compounds obtained from plants. In this bibliographic research, it was detected that plant essential oils are presented as a feasible option to reduce the quantity of antibiotics in broiler chicken production. Numerous research has been undertaken to understand the action mechanisms of essential oils and their active compounds from different plants. To promote growth in broiler chickens, the essential oils had antioxidant effects, reduced pathogenic bacteria and coccidia, and improved the digestion and metabolism of poultry. With this scenario, there are still various plants in the world to study. Future research considering the synergic effect of different essential oils from different plants and with other feed additives is warranted.

In the present research, we also detected that there were different procedures for the extraction of essential oils from different plants. There are also different forms of essential oils, such as liquid, encapsulated, or nanoparticles. A challenge is the standardization of essential oils that must contain an adequate dose of active molecules to be used in broiler chicken production. Other themes in essential oils that have received minor attention but need to be attended to are the security for the broiler chicken and humans. Additionally, the impact of the essential oils from plants on the environment also needs to be attended to. In total, broiler chicken production must be seen in a holistic way to maintain sustainability as food for humans.

Conclusion: Several researchers have focused their studies on the search for essential oils, observing the variability of phytochemical compounds and verifying their antimicrobial, antioxidant, and other effects; it has been proposed that together, they improve intestinal health and fight pathogens; so, the supply of EO in broiler chicken diets has begun to be evaluated to help maintain a varied microbiota in the intestine and consequently better health and production in birds. Further research is needed to establish the standardization of EO active molecules, as well as the interaction with other feed additives and their impact on the health and production of broiler chickens. In addition, safety for consumers and the environment must be considered.