This study was conducted to identify the nutritive and anti-nutritive composition, in situ rumen degradability, and the kinetics of in vitro gas production of Persian oak (Quercus persica) leaves harvested at three growth stages during the spring season. A tannin bioassay was also performed using polyethylene glycol (PEG 6000) as a tannin-complexing agent in a gas production test. Leaves were harvested in monthly intervals in spring, starting on April when leaves were at the early vegetative stage, and then May and June. As the leaf maturity progressed, crude protein decreased but total phenols, total tannins, and hydrolysable tannins increased. Condensed tannin concentration was not affected by maturity stage (Average=13.0 mg leucocyanidin equivalent/g dry matter (DM)). As leaf maturity increased, the rapidly degradable A fraction of DM increased. In vitro gas production, metabolizable energy, in vitro DM degradability, ruminal NH3-N, and short-chain fatty acid concentrations were greatest in leaves harvested at the early vegetative stage (April) compared with other months. Application of PEG increased in vitro gas production, metabolizable energy, in vitro DM degradability, and NH3-N and short-chain fatty acid production in the rumen fluid compared with no addition of PEG. Overall, oak leaves harvested at the early vegetative stage appeared to be a good source of forage for ruminants. However, as leaf maturity increased, ruminal fermentability decreased, which was improved with PEG addition. Manuscript profile