Experiments were conducted to study the chemical changes and microanatomical characteristics associated with maturity, rate and extent of DM digestion, behavior and extent of tissue digestion and mode of microbial attack during simulated ruminal digestion. Barley and millet plants were collected at four different stages of development: vegetative, boot, heading and mature. Berseem was harvested at three different stages of development: vegetative, bud and full bloom. For the first three stages of grasses and legumes, the plants were separated into three botanical fractions: leaf blades (leaflets), leaf sheaths (petioles) and stems. Grasses were harvested at mature stage and barley seeds were separated by thresher and millet seed by hand cutting the head. Straws were chopped in a hammer mill through a 2.5 cm screen and treated: 1) no added water (control), 40% added water, 2) alone or with 3) urea, 4) NH₄OH and 5) H₂O₂ at pH 11.5. Grasses showed higher (P<.05) cell wall constituents, xylose, p-coumaric and ferulic acid and lower (P<.05) IVDMD values for stems and leaf sheaths than leaf blades. Legume showed higher (P<.05) values for cell wall constituents and xylose for stems, compared to petioles and leaflets. At advanced maturity, scanning electron microscopy (SEM) showed increased lignification and decreased tissue digestion for leaf sheaths and stems, compared to leaf blades. Lignification was higher in millet leaf sheaths than barley leaf sheaths. Within berseem plant parts, petioles showed better digestion than leaflets and stems. The contents of ADF, cellulose and lignin were increased (P<.05) and hemicellulose was decreased (P<.05) in alkaline H₂O₂-treated straws, compared to untreated, NH₃- and urea-treated straw. Total phenolic acids were decreased (P<.05) in treated, compared to untreated straw. In vitro DM digestibility was increased (P<.05) for all chemical treatments, with larger (P<.05) increase for NH₃ treatment than alkaline H₂O₂ treatment. / Ph. D.