**1. Introduction**

Feeding systems in animal production are using additives that improve growth. These additives modify the weight daily gain, composition and yield of carcass, nutritional value, stability, and shelf life of the meat. Nowadays, the consumers are demanding naturally raised, trusted, and organic meat and meat products. Hence, there is a need to look for alternatives to the use of synthetic growth promoters [1,2].

Monensin sodium (SM) is an antimicrobial ionophore produced by *Streptomyces cinamonensis* and is used in ruminants to improve feed efficiency [3]. Even though antimicrobial growth promoters in animals intended for human consumption are strongly limited in some countries [4,5], they are still used in the U.S.A. and Mexico.

Phytochemicals have properties that benefit not only the animals fed with them but indirectly might impact the consumers of the meat [6,7]. Oregano essential oil (OEO) has antioxidant and antimicrobial properties, due to its phenolic components, mainly thymol and carvacrol [8–10]. The active components (carvacrol and thymol) of OEO are potent antimicrobials affecting populations such as *E. coli*, *Staphylococcus aureus*, *Salmonella typhimurium*, protozoa, fungi, *Ruminococcus fibrisolvens,* and *Fibrobacter succinogenes*; this change in populations modifies ruminal fermentation, which is fundamental in the conversion of dietary nutrients to muscle tissue [11–13]. Specifically in sheep, there is evidence that carvacrol potentially decreases acetate concentrations, and it increases propionate and butyrate. Both are volatile fatty acids precursors of muscle and fat components in the animal [14].

Specific nutritional components in the lamb diet might directly a ffect the quality of the meat. Ruminant meat research has drawn considerable attention because ruminant meat contains some bioactive lipids, including n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA), and the fatty acid profiles of ruminant meat can be enhanced through dietary supplementation [15,16]. Polyunsaturated fatty acids (PUFAs) are not only essential nutrients for humans, but also significant in providing protection from cardiovascular diseases, inflammatory diseases, diabetes, some cancers and behavioral disorders [17–19]. Therefore, the consumption of adequate n-3 LC-PUFAs is crucial to maintaining a healthy body and for the prevention of chronic diseases [20]. Humans can obtain n-3 LC-PUFA or their C18 PUFA precursors from various sources including aquatic, farm livestock products, oilseeds, fruits, herbs, cyanobacteria, and microorganisms [21].

The formation of large amounts of saturated fatty acids (SFAs) in the rumen is a result of the biohydrogenation process when bacteria convert unsaturated fatty acids to SFA. So, the fatty acids (FAs) occurring in the rumen are highly saturated and take part in the absorption as well as deposition of the fat in muscles. The decrease in SFA and increase of health-beneficial fatty acids (PUFAs) content have been an important objective in ruminant meat studies. According to the Food and Agriculture Organization of the United Nations [22], the ratio of PUFA and SFA is a significant indicator for the nutritional evaluation of fat, with a recommendation of about 0.40. It is essential to increase the consumption of eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) in human diets, because the synthesis of these FAs from dietary α-linolenic acid (ALA, C18:3 n-3) is very restricted [23].

Dietary supplementation of OEO in pigs, chickens, and cattle rendered meat with lower microbial concentration and higher antioxidant capacity [24–30]. Essential oils have shown to decrease methane production [31] and biohydrogenation [32]. The FAs profile in meat may be a ffected by the form of the provided lipid. However, the exact influence of oregano essential oil on the FA profile of lamb meat is limited.

Limited information exists on the benefits of phytochemicals in the productive performance and carcass and meat quality of ovine and other small ruminants. Moreover, information is available in terms of OEO and oxidative stability, but little is known about the e ffects on the fatty acid profile of lamb meat. Here, the aim of this study was to investigate the e ffect of the dietary supplementation of three levels of oregano essential oil and sodium monensin on the oxidative stability, colour, texture, and the fatty acid profile of lamb meat (m. *Longissimus lumborum*).
