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Physical structure of 14 maize silages was investigated; chewing activity was measured with eight cows over 4 d. Silages were harvested from the milk dough dent to the hard dough dent stages of maturity and chopped at lengths of 4 to 16 mm. Silages were fed for ad libitum intake and were supplemented with 2 kg of soybean meal. Eating, ruminating, and total chewing indexes varied from 16.5 to 25.3, 34.3 to 50.2, and 50.8 to 75.5 min/kg of silage DMI, respectively. Ruminating and total chewing indexes decreased as maturity stage increased. Eating index was not significantly affected by chopping to 4 to 8 mm lengths but was higher for the 16-mm maize. Ruminating index was higher with 8-mm than with 4-mm maize, but coarser chopping had no additional effect. Thus, total chewing index increased continuously as chopping length increased from 4 to 16 mm. Chemical (DM, crude fiber, NDF, ADF, lignin, and starch) and physical parameters (particle size, milling resistance, and density) and in vitro digestibility were used to derive equations to predict ruminating and total chewing indexes. In vitro digestibility and the fraction of vegetative plant particles retained on a 2.38-mm sieve were the main determinants of both chewing indexes.