The effect of the fiber content of roughage on energy cost of chewing was determined by indirect calorimetry. Four Holstein non lactating cows [779 +/- 71 kg body weight (BW)] were used in a cross over design with 14-day periods. Two cows were fed sugarcane silage as high fiber roughage (HF) and the other two cows were fed oaten hay as low fiber roughage (LF), along with soybean meal [0.5 g/kg BW on a dry matter (DM) basis]. The aNDFom and peNDF contents were different (718 vs. 542 g/kg DM and 554vs. 402 g/kg DM, respectively), whereas the physical effectiveness factors (pet) was similar between sugarcane silage and oaten hay (771 vs. 741 g/kg DM). The study was performed in open circuit respiration chambers over a 14-day period, consisting of a 9-day adaptation and a 5-day energy balance measurements. Energy cost of chewing per minute was determined using a multiple linear regression model, with heat production per 10 min as the dependent variable and duration of activities per 10 min as independent variables. DM intake (DMI) of roughage in HF was lower than that in LF (5.46 vs. 9.79 kg/day; P=0.006), whereas duration in total chewing (Eating + rumination) was higher for HF than for LF (120 vs. 77 min/kg DMI; P=0.006). Energy cost of rumination per unit DMI tended to be higher for HF than for LF (0.71 vs. 0.48 MJ/kg DMI, P=0.062), whereas energy cost of each eating and rumination per unit time was similar between treatments (17.7 vs. 18.4 J/min/kg BW for eating, P=0.272; 12.0 vs. 12.7 J/min/kg BW for rumination, P=0.285). Energy cost of total chewing per unit metabolizable energy (ME) was higher for HF than that for LF (143 vs. 9.0 MJ/100 MJ ME, P=0.009). These results indicate that fiber content in roughage possibly affects energy cost of chewing per DMI and consequently results in loss of ME available for production. (C) 2014 Elsevier B.V. All rights reserved.