¹Igdir University, Agricultural Faculty, Animal Science Department, TR-76100 Iğdır - TÜRKİYE DOI : 10.9775/kvfd.2026.36680 This study evaluated the chemical composition, in vitro fermentability, and anti-methanogenic potential of oak (Quercus spp.) (Quercus brantii, Quercus cerris, Quercus coccifera, Quercus ithaburensis subsp. macrolepis, Quercus infectoria, Quercus libani, and Quercus suber) leaves, focusing on condensed tannins and the effect of polyethylene glycol (PEG) treatment. Leaves from seven species were collected, dried, and analyzed for dry matter, crude protein, ether extract, crude ash, neutral detergent fiber, acid detergent fiber, and condensed tannins. Fermentation characteristics, methane production, metabolizable energy, and organic matter digestibility were assessed in vitro, with and without PEG. Chemical composition and tannin content varied significantly among species (P<0.001), with crude protein ranging from 10.25% to 17.41% and condensed tannins from 1.18% to 9.54%. PEG treatment significantly increased total gas production, methane production, metabolizable energy, and organic matter digestibility across all species (P<0.001), although methane percentage remained largely unaffected, suggesting tannins indirectly reduce methane rather than acting directly on methanogens. Among species, Q. cerris exhibited the highest gas production and digestibility after PEG, whereas Q. brantii showed the lowest fermentation efficiency. Species with higher tannin content displayed lower in vitro fermentability but greater methane suppression. Findings suggest oak leaves are a valuable feed resource that can enhance ruminant nutrition while helping control methane emissions. Utilizing tannin-rich leaves with PEG provides a dual benefit: improving feed efficiency and contributing to sustainable livestock management. Keywords : Quercus spp., Oak leaves, Condensed tannins, Polyethylene glycol (PEG), methane, In vitro gas production