**1. Introduction**

Feed utilization is strongly affected by rumen fermentation, so nutritionists have focused on the physiology of the rumen and its microbiome. The entire rumen microbial community cooperates in the digestion and utilization of the feed, rather than the efforts of individual groups or strains to perform specific metabolic pathways such as fatty acids biohydrogenation. The shifts in the composition and abundance of ruminal microorganisms are affected not only by feed type [1] but also by the use of rumen modifiers such as plant secondary metabolites, including phenolic compounds and essential oils Essential oils (EOs) [2]. EOs are volatile, complex compounds extracted from different parts of the plants to protect plants against attacks by bacteria, fungi, or insects. Common EOs fall under two chemical groups: terpenoids and phenylpropanoids, which are synthesized

through secondary metabolism in plants [3]. Chemically, extracted EOs vary due to genetic determinants of the plant, stage of growth, and environmental factors [4]. Supplementing ruminants with EOs can improve their productive performance, rumen fermentation, health, and product quality (such as milk and meat) [3, 5–7].

As a result of the prohibition of antibiotics in the animal production sector, scientists have used plant EOs and phytochemicals as available alternatives to antibiotics [8]. The EOs generally have antibacterial activity against both gram-negative and gram-positive bacteria [9, 10]. The mode of action of EOs is based on their ability to disrupt cell walls and cytoplasmic membranes, leading to lysis and leakage of intracellular compounds [11].

Many gram-positive bacteria in the rumen are involved in biohydrogenation reactions of unsaturated fatty acids (FA) [12]. Manipulating the rumen fermentation process is an important function of EOs. [13], and impeding the biohydrogenation of n-6 and n-3 fatty acids in the rumen [14], which inhibits the biohydrogenation process in the rumen and increases the passage of polyunsaturated fatty acids (PUFA) into milk and meat, and this has positive effects on human health. Extensive literature supports the use of EOs as rumen modifiers; however, more studies are urgently needed to improve the quality of milk and meat by improving their PUFA content. In this chapter, we discussed the effect of EOs on the rumen microbiome and the characteristics of dairy and meat final products.
