The objective of this experiment was to evaluate the effects of partial replacement of corn silage with long alfalfa hay and/or coarse chopped wheat straw on chewing activity and rumen fermentation in late lactating dairy cows fed diets with 50% forage on dry matter basis. Twelve late lactating Holstein primiparous cows including four cows equipped with a rumen cannula, averaging 210 +/- 20 d in milk and weighing 575 +/- 50 kg were randomly assigned in a 4x4 Latin square design. During each of four 21-d periods, cows were fed 4 total mixed diets that varied in the forage sources : 1) CS (50% corn silage), 2) CSW (35% corn silage + 15% wheat straw), 3) CSA (35% corn silage + 15% alfalfa hay), 4) CSWA (25% corn silage + 10% wheat straw + 15 % alfalfa hay). The physically effective neutral detergent fiber (peNDF) contents of the diets which were determined by using the Penn State Particle Separator were as 33.2, 36.3, 33.6, and 35.3% of DM for CS, CSW, CSA, and CSWA diets, respectively. The proportion of particles retained on the 19.0 mm screen of the CS was lower (P < 0.001) than in the others group (3, 14, 12, and 24 % of DM, for CS, CSW, CSA, and CSWA diets, respectively). Dry matter intake was not affected by the diets but peNDF intakes were lower (P < 0.001) with CS and CSA than other diets. Cows spent more time eating the CSW, CSA and CSWA diets compared with CS diet (P < 0.001). Cows fed with CSW spent more time ruminating than cows fed CS diet (P < 0.05). Total chewing time was higher (P < 0.01) in CSW (662 min/d) and CSWA (633 min/d) groups than CS (521 min/d). Mean rumen pH values, ammonia content (NH3-N), acetate/propionate ratio, and total concentration of volatile fatty acids in the rumen fluid were not affected by the treatments. However, the time (h/d) where the pH was under 5.8 value, was increased when cows fed the CS diet (P < 0.001). It is concluded that forage source may have an effect on the chewing activity and rumen pH related to the peNDF and fiber structure. The proportion of particles > 19.0 turn and peNDF may be used as predictor of chewing activity.