Round bale grass silage harvested early (D-value 757 g kg^-1 DM) or at a normal (D-value 696 g kg^-1 DM) time was used to study the effect of harvesting time, chop length and their interaction on chewing activity and digestibility by dairy cows. Six early lactating Norwegian Red cows were used in a 6 x 6 Latin square with 3-week periods. Chewing activity was measured using IGER Behaviour recorders, and digestibility was measured by total collection of faeces. The two silages were fed long (170 mm), coarsely chopped (55 mm), or finely chopped (24 mm median particle length). Cows were fed silage ad libitum and supplemented with 6 kg concentrate. Early harvested silage significantly decreased total ration eating (ET), rumination (RT) and chewing time (CT) per kg silage DM compared with normal harvested silage (CT = 38 vs. 46 min kg^-1 DM). Chopping of silage reduced CT significantly, mainly due to reduced ET, CT = 45, 41 and 39 min kg^-1 DM for rations with long, coarsely and finely chopped silage, respectively. Grass silage chop length did not influence diet digestibility, but there was a significant effect of harvesting time on digestibility. No interaction between harvesting time and chop length was found, neither for chewing activity nor digestibility.