The experiment aimed to evaluate the effects of varying levels of Laurus nobilis leaves [0% (control), 0.5%, 1%, 1.5%, and 2%] on the in vitro ruminal fermentation of a ruminant diet consisting of a 50% concentrate mixture, 40% berseem hay (Trifolium alexandrinum), and 10% rice straw (Oryza sativa). The in vitro incubation lasted 48 h, during which gas production (GP), methane (CH4), carbon dioxide (CO2), total and individual short-chain fatty acids (SCFA), and nutrient degradability were measured. The experiment utilized a randomized block design and consisted of two incubation runs. Gas chromatography analysis revealed that 1,8-cineole (81%) was the primary volatile compound in the L. nobilis leaves. The 0.5% inclusion level exhibited the highest (linear, p = 0.006) asymptotic GP and lowest lag of GP (linear, p = 0.002), while the 2% inclusion level had the highest lag of GP. The 2% inclusion level significantly lowered CH4 (linear, p = 0.003) compared to the control, and all levels of the leaves linearly decreased in the proportional CH4 production (p = 0.001), with the lowest value at the 0.5% inclusion level. The highest asymptotic CO2 production was observed with the 0.5% inclusion level (linear, p = 0.002), while the 0.5%, 1%, and 1.5% inclusion levels significantly increased (quadratic, p = 0.006) the proportion of CO2 compared to the control. The 0.5% inclusion level showed the highest (p < 0.001) degradable DM and fiber fractions compared to the control, whereas the 2% level decreased them. The 0.5% inclusion level resulted in the highest (p < 0.01) production of total SCFA, acetate, and propionate. Additionally, the 0.5% inclusion level demonstrated the highest (p < 0.05) metabolizable energy and microbial crude protein, while the 2% level reduced these measures compared to the control. It is concluded that L. nobilis leaves can be included at 0.5% of the ruminant diet (e.g., sheep) to improve ruminal fermentation and reduce CH4 production.
