An essential component of the existing research on phytobiotic uses in poultry diets is using herbs and spices as feed supplements (Al-Kassie et al., 2012; Puvača et al., 2015; Saeed et al., 2018). These supplements have been effectively incorporated into the diet to enhance the health of chickens and their welfare (Alagawany et al., 2019; Elwan et al., 2019; Salah et al., 2021; Moharreri et al., 2022).

Phytogenic feed supplements are substances derived from plants that are added to animal feed to improve growth performance (Vakili et al., 2022). In addition to increasing antioxidant, antiviral, anthelmintic, and antibacterial properties, active compounds in spices, herbs, and extracted oils can enhance the secretion of digestive enzymes, boost feed intake, and activate the immune system (Dalle Zotte et al., 2016). Because of its seductive smell, typically spicy flavor, and tingly sensation, black pepper, BP (Piper nigrum L.), is a well-known spice (Srinivasan, 2007). Black pepper oil (BPO) is extracted by steam-distilling the dried fruit of the BP plant (Piper nigrum L.). It is frequently utilized as a flavoring substance in food products, cosmetics, and aromatherapy. The possible health advantages of BPO for animals, notably Japanese quail, have gained more attention in recent years. Enhanced digestion, anti-inflammatory properties, antibacterial properties, enhanced respiratory health, and stress reduction are a few advantages of black pepper oil for quail that have been previously highlighted. The majority of the studies have either been carried out in vitro or on other poultry species, therefore it is crucial to keep in mind that there has only been little study on the effects of black pepper oil, especially in quail. Black pepper increases nutrient digestive efficiency.

Black pepper oil contains a compound called piperine, which has important nutritional and medicinal effects on the neuromuscular system, It can also promote and speed up fat thermogenesis, improve digestion and absorption, and improve nutrition (Malini et al., 1999). With regard to black pepper's ability to effectively stimulate the gut and enhance digestive output, it has been used extensively for bronchitis, cancer, and stomach problems. When medicinal herbs were included in poultry diets, the body weight gain and feed conversion ratio improved (Iqbal et al., 2011). Compared to chili, BP has been discovered to have antioxidant benefits and an anticarcinogenic impact (Nalini et al., 2006).

In traditional Indian families, black pepper has been utilized as a common treatment for many of human diseases (Moorthy et al., 2009). It suppresses lipid peroxidation, inhibits induced oxidative stress, and destroyed various free radicals such as superoxide and hydroxyl radicals (Weiner, 1994). In total, 80 % of the world's population still relies on medicinal and herbal plants and natural remedies for the majority of their healthcare requirements.

Black pepper has been used medicinally for a variety of illnesses, including the treatment of piles and worms. It also has antibacterial effects, antifungal, antidepressant, antidiarrheal, and inflammatory properties. It also has antitumor, antioxidant, antispasmodic, and antipyretic properties (Ahmad et al., 2012; Islam et al., 2015). Terpenoids, the main components of black pepper essential oil (BPEO), have been found to have possible antibacterial activity (Menon et al., 2003; Zengin and Baysal, 2014). It is predicted that including BPO in the diet will have positive benefits on quail that are in the growing period. Therefore, the goal of the present study was to investigate the effects of dietary supplementation of various BPO levels on growth performance, antioxidant indices and immunity, lipid profile, cecal microbiota, and carcass traits of the growing quail.

According to some reports, BP increases feed intake, increases nutrient digestibility by stimulating digestive enzymes, and prevents pathogen growth in the gastrointestinal tract (GIT) by acting as an antibiotic (Abd El-Hack et al., 2022). It may improve the villi's capacity for absorption to aid in nutrient uptake and assimilation (Pliego et al., 2020). Additionally, it has been discovered that broiler hens that received dietary BP supplements had thicker mucosa and sub-mucosa linings in their duodenum, jejunum, and ileum (Shahverdi et al., 2013) and improved ileal muscular contraction (Pliego et al., 2020). Our work demonstrates that BPO addition has a positive effect on quail growth performance parameters such as body weight, body weight gain, and feed conversion ratio of quail. It has been noted that feeding BP to chickens significantly improved their zootechnical parameters (Galib et al., 2011; Ndelekwute et al., 2015; Ufele et al., 2020). FCR and BWG were improved (2 %–9 %) in broiler chickens fed BP at 2.5, 5.0, 7.5, and 10.0 gkg-1 (Al-Kassie et al., 2011), which may be attributed to BP's antioxidative action in preventing villi damage from oxidative deterioration (Ashokkumar et al., 2020, 2021). These findings were confirmed by Ufele et al. (2020), who showed that broiler chicken feed intake and BWG were positively impacted by BP supplementation at a rate of 5 gkg-1. These developments in zootechnical features are perhaps attributed to the presence of bioactive components in diets based on BP's capacity to boost the expression of digestive enzyme genes and decrease the growth of harmful bacteria in birds' GIT. This might also be explained by the capacity of BP to promote digestive enzymes' release, boost digestion in GIT and utilization, and shorten feed retention time in the small intestine, all of which result in greater body weight gain (Galib et al., 2011). In contrast, Puvaca et al. (2014) found that BP-supplemented diets in poultry at 5 or 10 gkg-1 for 42 d had no impact on BWG or FCR in comparison to the control. For broiler chickens in the last stage of the fattening period fed 7.5 or 10 g of BP seeds per kilogram of feed, a decrease in BWG and FCR was reported (Ndelekwute et al., 2015), suggesting that chickens' growth is negatively impacted by eating an excess of BP seeds. This result validates the findings of Akbarian et al. (2012), who found that male broilers' diets supplemented with 5 gBPkg-1 exhibited no difference in BWG in comparison to the control. These differences in the performance characteristics of chickens' diets supplemented with BP may be related to the age of the chicken or the level of supplementation. The impact of BP-based diets on broilers' carcass traits and organ weight has been studied by several researchers (Galib et al., 2011; Ghaedi et al., 2014; Al-Kassie et al., 2011; Rahimian et al., 2016). The results of this research, however, have not always been dependable, but they do agree with the findings of Abou-Elkhair et al. (2014), who reported that broilers' diets supplemented with 5 gBPkg-1 showed no differences in the weights of the thymus or the bursa of Fabricius, gizzard, liver, heart, proventriculus, spleen, or carcass yield. Likewise, Galib et al. (2011) found that supplementing the broiler ration with BP at levels of 10 gkg-1 had no appreciable impact on the relative organ weights. Also, our result agrees with Al-Kassie et al. (2012), who stated that the organ weight or carcass of broiler chickens was not significantly affected by a blend of black pepper and red pepper powder. The main place for digestive processes and the absorption of nutrients is the small intestinal tract (Abdelnour et al., 2019). According to the results of the current study, adding BPO to the diet of growing quails decreased caecal content pH. According to Jamroz et al. (2006), phytogenic feed additives may improve gut health because they modify the intestinal contents' pH and promote mucus secretion in birds, which reduces pathogen adherence. Furthermore, numerous in vivo investigations have shown that black pepper essential oil has strong antibacterial properties against dangerous strains of Staphylococcus and Escherichia coli (Zhang et al., 2017; Abdallah and Abdalla, 2018). According to certain research done on Japanese quails (Mehri et al., 2015) and broiler chickens (Chowdhury et al., 2018), various polyunsaturated fatty acids (PFAs; peppermint and cinnamon) have an antibacterial effect in vivo against intestinal tract pathogenic microorganisms including coliform and E. coli. For all examined levels, birds fed BPO-enriched diets had lower levels of Salmonella, coliform, TBC, lactobacilli, and E. coli colonization in comparison to the control. It has been established that BP increases the growth of beneficial gut microorganisms such as Lactobacillus (Ghaedi et al., 2014; Kishawy et al., 2022). Moreover, BP reduced the development of harmful microorganisms in the chicken gut (Naidu et al., 1999).

By supplying birds with herbal products, one can alter the blood's biochemical profile (Moharreri et al., 2022). Higher plasma protein concentration levels correlate to higher dietary protein quality (Ogbuewu et al., 2015). In birds, Akinfola et al. (2007) discovered a favorable link between protein quality intake and the levels of total plasma proteins. Furthermore, the supplementation of 0.5 gBPOkg-1 of broiler chickens boosted plasma protein levels (Kishawy et al., 2022), indicating that BP supplementation did not impact the quality of protein in the diet. The current results of total protein and globulin in plasma showed a significant increase. According to earlier research, Tayeb et al. (2015) reported that quail given red pepper powder (5 gkg-1) at 49 d of age had significantly lower albumin levels than the control group; however, there was no significant difference in globulin levels. Akinfola et al. (2007) reported that globulin is required for the development of immunoglobulin, and so elevated globulin levels in birds supplemented with BP in diets imply an improved resistance to infection. Other investigations discovered that BP had no effect on total protein or albumin levels in plasma in broiler chickens (Abou-Elkhair et al., 2014; Sugiharto et al., 2021). Several studies stated no significant influence of BP on ALT and AST levels in plasma in broiler chickens (Abou-Elkhair et al., 2014; Kishawy et al., 2022; Sugiharto et al., 2021), indicating that BP has a hepatoprotective effect in poultry. Broilers fed BP essential oils may have a non-significant elevation in both ALT and AST levels due to BP's hepatoprotective capacities, as Damanhouri and Ahmad (2014) discovered that supplementing BP in broilers' diets enhanced liver activity. The renal function is evaluated via serum creatinine levels, and a rise in serum creatinine indicates impaired kidney function (Aslam et al., 2010). In the present study, there were significant differences in creatinine and urea levels when BPO was added to the quails' diets (P < 0.05). In addition, the group treated with BPO at a level of 0.4 g had the lowest plasma creatinine level (0.31 mgdL-1) in comparison to the control; additionally, the group supplemented with BPO at a level of 1.2 g had the lowest plasma urea level (1.18 mgdL-1) in comparison to the control and other treated groups, indicating that BPO is safe for renal function.

Serum cholesterol and triglyceride concentrations can represent fat metabolism (Sharifi et al., 2023). The present studies found that the group supplemented with BPO had decreased serum cholesterol and triglyceride concentrations. HDL is required for fat transport and distribution to tissues and cells. Low levels of BP (5 gkg-1) are stated to produce moderate changes in the metabolism of cholesterol by boosting the cholesterol 7 alpha-hydroxylase enzyme activity (Ramesh et al.1996). Our findings revealed that BPO supplementation in the diet improved the concentrations of HDL and decreased the concentrations of LDL and VLDL. Our result is similar to other authors' observations, who reported that supplementation of BP in the diet of chickens reduced lipid and cholesterol levels (Ghaedi et al., 2014; Shahverdi et al., 2013; Kishawy et al., 2022; Akbarian et al., 2012). This result may be attributed to BP's reduction of acetyl-CoA production, which is required for fat metabolism. Studies on animals have demonstrated that the bioactive components of BP, especially piperine, have antiperoxidative effects (Dhuley et al., 1993). Because of this, broiler hens' diets supplemented with BP caused lower levels of plasma cholesterol and LDL cholesterol (Galib et al., 2011; Ghaedi et al., 2014; Shahverdi et al., 2013; Puvaca et al., 2014), which could be attributed to BP antioxidative properties. Ashokkumar et al. (2020) and Lee et al. (2020) also discovered that incorporating BP into the diet significantly elevates blood HDL cholesterol concentrations. Antioxidants are substances, either natural or artificial, that can be utilized to decrease the risk of degenerative and chronic diseases by scavenging free radicals and preventing their formation (Halliwell, 2000). The significant activity of CAT and GSH in groups that were fed BPO, as well as the low levels of MDA, confirms that it is an effective antioxidant that can neutralize free radicals. Piperine possesses potential antioxidant action and anti-lipid peroxidation in rats supplemented with a high-fat diet, which results in oxidative stress in cells (Vijayakumar et al., 2004). Our findings showed that dietary supplementation with BPO elevated the concentrations of SOD, CAT, and GSH and decreased the concentrations of MDA. Our results agree with Jeena et al. (2014), who discovered that BPO removes superoxides and reduced tissue fat peroxidation in vitro. Herbal preparations boost immunological responses by stimulating the creation and release of immunoglobulins, lymphocytes, and interferon-γ (Barnes et al., 1972; Faramarzi et al., 2013). The present result shows that BPO supplementation to quails' diet increased the levels of immunoglobulins (IgG and IgM), and these findings agree with Kishawy et al. (2022), who reported that supplementation of 0.25 or 0.5 gBPOkg-1 in broiler diet improved immunity characteristics, as revealed by elevated lysozyme action, immune system antibody levels, and phagocytic metrics, in comparison with birds fed a diet without supplemental BPO. Moreover, Kishawy et al. (2022) stated that in broiler chickens, BP increases the IL-10 and IgA gene expression.

It can be proven that BPO supplementation in Japanese quails' diet can boost growth performance and antioxidant indices, enhance lipid profile and carcass traits, and reduce intestinal infections. Hence, BPO supplements may be used to improve the growth performance and health conditions of growing Japanese quails.