The effect of a surface open to air on the ensiling process was investigated in 15 × 60 cm PVC silos in four trials, three with alfalfa (30, 41, and 49% dry matter, DM) and one with whole-plant corn (35% DM). Silos were sampled 1, 2, 5, 7, 14, and 28 d post-filling at 5, 20, 35, and 50 cm from the open surface. At 5 cm, little fermentation occurred as oxygen contents were at 10% or higher, and ...