In parallel with the increased importance of ecological agriculture, today, the use of natural substances has gained prominence in the livestock sector. The plant essential oils and extracts prevent the invasion of the digestive tract by pathogenic microorganisms, increase the effects of digestive enzymes, strengthen immunity, and improve both the feed conversion rate and palatability of feed (Demir et al., 2005; Gumus et al., 2017a). In view of these effects, feed supplements of plant origin have gained popularity in livestock and poultry production for their growth promoting, production increasing, immunity boosting and health protective effects (Zhang et al., 2021). Two important plant supplements, which have found use in poultry production for their growth promoting and antioxidant effects, are thyme (Thymus vulgaris L.) and rosemary (Rosmarinus officinalis), both of which are environmentally friendly plants that belong to the family Lamiaceae (Soncu et al., 2020; Zhang et al., 2021).

The main pharmacological effects of thyme (Thymus vulgaris L.) are attributed to thymol and carvacrol, which are the most important bioactive compounds found in the structure of this plant (Grigore et al., 2010). The active substances found in thyme improve the digestive activity of several enzymes, such as protease, amylase and lipase, and thereby increase the feed conversion rate (Abdel-Wareth et al., 2012). Previous studies have pointed out to various biological and protective effects of thyme, including among others, antioxidant, antibacterial, anti-inflammatory, immunostimulatory and health protective activity (Gumus et al., 2017a; Zhang et al., 2021). While some reports have indicated dietary thyme supplementation to improve the performance parameters of poultry (El-Ghousein and Al-Beitawi, 2009; Toghyani et al., 2010), it has also been suggested that dietary thyme has no such effect (Saleh et al., 2014). It has also been reported that thyme essential oil improves meat quality (Bahrami et al., 2021).

Owing to its strong antioxidant activity, rosemary (Rosmarinus officinalis) is commonly used for both food aromatization and medicinal purposes (Demir et al., 2005; Soncu et al., 2020). Demir et al. (2005) reported that natural plant supplements, including thyme and rosemary, significantly increase the performance of broiler chickens and, thus, suggested that consumers could benefit from the antioxidant effect of these poultry products. It has also been indicated that the phenolic substances found in thyme and rosemary show a protective effect against oxidative stress by means of free radical scavenging activity (Gumus et al., 2017a; Zhang et al., 2021).

This study was aimed at determining the effects of different doses of thyme and rosemary essential oils incorporated into broiler rations on fattening performance parameters (body weight gain, feed intake and feed conversion rate) as well as on the quality criteria of breast and drumstick meat.

The search for natural supplements as an alternative to synthetic growth promoters has accelerated research on medicinal and aromatic plants and their extracts. Essential oils are listed among natural, safe and non-residual alternative dietary supplements of plant origin. Essential oils are reported to have antioxidant, anti-inflammatory and antimicrobial effects and to increase the activity of digestive enzymes and, thus, are suggested to be used as dietary supplements either alone or in combination, for the economical and safe production of animal products, the maintenance of animal health, and the increase of yields (Gumus et al., 2017a; Soltani et al., 2016).

The present study demonstrated that, overall, by the end of the study period, no statistically significant effect had occurred on the body weight, body weight gain, feed intake and feed conversion rate of broiler chickens that had received dietary TEO and REO supplementation. These results are in agreement with previous reports indicating no statistically significant difference to have occurred in the body weights and daily body weight gain of broiler chickens given 100 and 200 mg kg-1 of dietary TEO (Saleh et al., 2014) and 100 mg kg-1 of dietary REO (Abd El-Latif et al., 2013). Similar to the results of the present study, previous reports have also indicated no effect to have been achieved with dietary REO supplementation on body weight (Franciosini et al., 2016), daily body weight gain (Franciosini et al., 2016; Norouzi et al., 2015), feed intake (Franciosini et al., 2016; Norouzi et al., 2015; Rostami et al., 2015; Soltani et al., 2016) and feed conversion rate (Franciosini et al., 2016; Soltani et al., 2016) in broiler chickens. Likewise, the incorporation of thymol into broiler rations has also been reported not to have affected feed intake (Saadat Shad et al., 2016). On the other hand, there are also reports that differ from the present study and suggest that the supplementation of broiler rations with 250 mg kg-1 of thymol significantly increases body weight gain and the feed conversion rate (Saadat Shad et al., 2016); dietary supplementation with 1 % of REO significantly increases daily body weight gain and daily feed intake and improves the feed conversion rate (Al-Kassie, 2008). Furthermore, there are literature reports that suggest the supplementation of broiler rations with thyme to have significantly increased body weight and daily body weight gain (El-Ghousein and Al-Beitawi, 2009; Toghyani et al., 2010) and with 100 and 200 mg kg of REO to have significantly increased feed intake (Abd El-Latif et al., 2013). Moreover, apart from the above-mentioned literature reports, which suggest dietary thyme and rosemary supplements to either increase or not affect performance parameters, there are also other studies, which have reported statistically significant decreases resulting from the supplementation of broiler rations with 200 mg kg-1 of REO in body weight, daily body weight gain and the feed conversion rate (Abd El-Latif et al., 2013), with 0.5 % and 1 % of rosemary powder in body weight gain and the feed conversion rate (Rostami et al., 2015), with 3 g kg-1 of rosemary powder in body weight (Soltani et al., 2016), and with 1.5 % of rosemary powder in the feed conversion rate (Norouzi et al., 2015). Based on the results obtained, it is suggested that, despite the lack of any positive effect on performance parameters, dietary supplementation with TEO and REO could provide benefits as feed supplements through antimicrobial and digestive stimulant effects, particularly in the event of unfavourable environmental conditions and malnutrition. Furthermore, differences observed between the results of studies are attributed to differences in the composition, usage and doses of thyme and rosemary supplements administered, and the animal breeds used.

One of the most important factors known to affect meat quality is the microbial load of meat. Some microorganisms impair the quality and shorten the shelf life of meat and pose a risk to human health (Mastromatteo et al., 2010). It is well known that the microbial load of meat increases directly proportional to the length of the storage period of this product (Chouliara et al., 2007; Gumus et al., 2018). Thus, antimicrobial substances found in the structure of meat are highly important for the storage of it without spoilage. The antimicrobial and antioxidant activities of both TEO and REO have been known for a long time, and recently, their use for meat products has gained increased popularity (Amariei et al., 2016; Gumus et al., 2017a; Soncu et al., 2020). Indeed, previous research has demonstrated that essential oils extend the shelf life of meat through their bactericidal and bacteriostatic effects (Bahrami et al., 2021; Gumus et al., 2017b; Soncu et al., 2020). A previous study in quails demonstrated that dietary supplementation with 300 and 450 mg kg-1 of TEO decreased breast meat coliform, Lactobacillus spp., Pseudomonas spp. counts and TPAB levels (Gumus et al., 2017b). In a similar study conducted by Chouliara et al. (2007), it was determined that 0.1 % of oregano essential oil extended the shelf life of chicken meat stored at +4 ∘C from 5 to 8–9 d. Furthermore, Aksoy et al. (2011) reported that thyme extract showed an antibacterial effect against Pseudomonas spp. and TMAB in broiler meat. Another study demonstrated that dietary supplementation with 3.5 % and 7.5 % of thyme leaves produced an antibacterial effect against psychrotrophic and lactic acid bacteria in lamb meat (Nieto et al., 2010). In agreement with the literature reports referred to above, the present study demonstrated that dietary supplementation with TEO and REO tended to reduce the microbial load of breast meat and drumstick meat throughout the storage period.

Depending on the particular structure of a food product, its water content is involved in various biochemical and microbiological reactions. Water activity refers to the capability of muscle tissue to retain moisture and constitutes a major criterion influential on the palatability of meat (Zhou et al., 2010). It is indicated that while the maintenance of this criterion within the normal reference range depends on the integrity of the cell membrane of myocytes, the flow of intracellular fluid out of the cell impairs meat quality (Amariei et al., 2016). In the event of the disruption of the membrane integrity of myocytes, the hydroxyl group of the alcohols contained by phenolic compounds may constitute a strong barrier as they show less affinity to water (Amariei et al., 2016). Mehdipour et al. (2014) indicated that the supplementation of quail rations with 100 mg kg-1 of thyme extract significantly increased the water activity of drumstick meat. Amariei et al. (2016) also reported that thyme and oregano essential oils increased the water activity of minced beef and pork stored at +4 ∘C for 5 d. Different from these results, it has been reported that the supplementation of quail rations with 150, 300 and 450 mg kg-1 of thyme essential oil showed no effect on the water activity of breast meat (Gumus et al., 2017b). The present study demonstrated that dietary supplementation with TEO and REO produced a limited and variable effect on the water activity of drumstick and breast meat.

One of the critical traits used by the meat industry for the assessment of meat quality is the pH value of meat (Bianchi et al., 2005). In broiler chickens, breast meat is the first choice for the determination of meat pH level (Glamoclija et al., 2015). The pH level of meat is reported to range between 5.2–7.0 and to be affected by several factors, including among others, genetics, sex, pre-slaughter stress, slaughter technique, and storage (Ristc and Damme, 2010; Van Laack et al., 2000). The pH level of good-quality broiler meat has been reported to range from 5.9 to 6.2 (Ristc and Dame, 2010). According to some other literature reports, while the pH value of the highest quality of poultry breast meat falls within a range of 5.7–6.0, a pH value below 5.8 is associated with a paler meat colour and softer consistency (Glamoclija et al., 2015; Van Laack et al., 2000). In a previous study in quails, it was determined that dietary supplementation with 150, 300 and 450 mg kg-1 of thyme essential oil had no effect on the pH value of breast meat (Gumus et al., 2017b) In the present study carried out in broiler chickens, dietary supplementation with thyme and rosemary essential oils was ascertained to have maintained the pH level of both breast and drumstick meat within the favourable range on days 0, 2, 4, 6, 8 and 10 of storage.

Another criterion influential on the physical quality of meat is meat colour and is known to be affected, either positively or negatively, by the myoglobin content and pH value of muscle tissue (Mir et al., 2017). Depending on the post-mortem temperature and pH value, the extent of protein denaturation and physical appearance of meat affect the amount of light reflected from the inner and outer surface of meat (Lawrie and Ledward, 2014). While light scattering has a minimum effect on the a and b values of meat, its effect on the L* value is distinctive. A muscle tissue pH value of ≥6.0 is associated with minimal protein denaturation, low light scattering and a semi-translucent appearance, whilst a pH value of ≤6.0 is associated with a higher level of protein denaturation, increased light scattering and opacity in muscle tissue (Anadon, 2002). The normal reference range for the L* value is indicated as 50–56, such that a L* value below 50 is associated with darker meat colour and a L* value above 56 is associated with pale meat colour (Petracci et al., 2004). In the present study, dietary supplementation with TEO and REO was observed to have maintained the L* value of both breast meat and drumstick meat within the normal range. In a previous study in quails, the incorporation of TEO in the ration at doses of 150, 300 and 450 mg kg-1 was reported not to have affected the L*, a* and b* values of breast meat (Gumus et al., 2017b). Similarly, in a previous study carried out in broiler chickens, 300 mg kg-1 of dietary oregano essential oil (Kirkpinar et al., 2014) and 0.5 % of a dietary mixture of thyme and oregano essential oils were observed not to have affected the L*, a* and b* values of breast meat (Rimini et al., 2014). On the other hand, Al-Hijazeen et al. (2016) determined that the addition of 100, 300 and 400 mg kg-1 of oregano essential oil to raw broiler breast meat did not affect the a* value on days 0, 3 and 7 of storage but improved the L* value on only day 7. When added at doses of 300 and 400 mg kg-1, oregano essential oil was determined to have improved the b* value on days 0 and 7. In the present study, different doses of dietary TEO and REO were observed not to have shown any adverse effect on the colour parameters, namely the L*, a* and b* values of breast and drumstick meat throughout the storage period.

Free radicals generated as a result of metabolic processes cause oxidative reactions. The primary products of oxidative reactions are peroxides, whilst hydrocarbons, aldehydes, ketones, alcohols and organic acids are the secondary products of these reactions. These secondary products adversely affect the nutritional value, sensorial properties and shelf life of animal products (Cai et al., 2014). Antioxidants play an important role in the scavenging of free radicals. It is well known that antioxidants significantly reduce lipid peroxidation, which is considered the main indicator of free radical presence, in muscle tissue (Gumus et al., 2017a). The maximum tolerable level of thiobarbituric acid reactive species (TBARS), which has no adverse effect on chicken meat quality, has been reported as 4 mg malondialdehyde equivalents (MDA eq) per kg. In another study, in which chicken meat was applied TEO, TBARS levels were determined to range between 0.19–0.25 mg MDA kg-1 on day 1 of storage and to progressively increase until day 10 of storage but still remain below the maximum tolerable limit (Majdinasab et al., 2020). In another study, thyme and oregano essential oils were reported to reduce lipid oxidation in broiler breast meat (Rimini et al., 2014). Similarly, another study determined that the supplementation of broiler chicken rations with 120 ppm of rosemary essential oil and 500 ppm of thyme essential oil reduced lipid oxidation in meat (Abbasi et al., 2020). Likewise, the present study demonstrated that, excluding day 10 of storage, dietary supplementation with TEO and REO decreased TBARS levels in breast and drumstick meat throughout the storage period.

In conclusion, the incorporation of thyme and rosemary essential oils into broiler chicken rations was determined not to show any adverse effect on the performance parameters. Furthermore, dietary TEO and REO decreased TMAB, Enterobacteriaceae and Lactobacillus spp. counts and TBARS levels in breast and drumstick meat and partly affected the pH, water activity and colour parameters of meat. Based on the results obtained in the present study, it is suggested that the dietary supplementation of broiler chickens with thyme and rosemary essential oils could aid in protecting these animals from oxidative stress and maintaining their health.