To evaluate a newer indirect calorimetry system (ICS; GreenFeed, C-Lock, Inc., Rapid City, SD) able to measure carbon dioxide (CO₂), methane (CH₄), and oxygen (O₂) fluxes, 8 cross-bred beef steers (initial BW = 241 ± 4.10 kg) were used in a 77-d experiment to examine the energetic parameters calculated with the gas fluxes. Steers were individually fed in a pen equipped with Calan gates (hay) and they visited the head box of the ICS 4 times/d while consuming their supplement. After a 2 wk training period, steers were randomly assigned to 1 of 3 treatments in multiples of maintenance ME intake (MEx, 7.43 Mcal/d): 1.1 (n = 3), 1.4 (n = 2), and 1.8 (n = 3) (GE intake = 17.4, 23.1, and 31.4 Mcal/d, respectively). Diets were long-stemmed wheat hay (Triticum aestivum L.; 15% CP, 64% TDN) plus a daily supplement of 1.03 kg of alfalfa-based pellets (Medicago sativa L.; 14% CP, 69% TDN). On d 1 and 77, BW was recorded after a 17-h fast to access BW gain. Heat production (HP) was calculated from gas fluxes and predicted urinary N excretion [0.62 × N intake (g/d) - 3.72 × DMI (kg/d) - 3.93] using the Brouwer (1965) equation (HP (Mcal/d) = [3.866 × O₂ (L/d) + 1.200 × CO₂ (L/d) – 1.44 × N excretion (g/d) – 0.518 × CH₄ (L/d)]/1,000). Dependent variables were analyzed with the MIXED procedure (SAS Inst., Inc.; Cary, NC) and least squares means were separated with linear and quadratic contrasts. Both BW gain and HP linearly increased (P < 0.01) with MEx (9.2, 22.3, and 50.1 kg, and 11.4, 11.8, and 13.3 Mcal/d, respectively). Energy losses from CH₄ emissions (2.15, 2.24, and 2.24 Mcal/d) did not differ (P > 0.62) among MEx. The percentages of energy losses from CH₄ decreased linearly (P < 0.02) with increasing MEx (12.1, 11.1, and 8.9% of GE intake, respectively). Percentage of HP decreased linearly (P < 0.01; 66.1, 50.8, and 42.1% of GE intake, respectively) with increased MEx. The percentages of CH₄ and urinary energy loss, and HP relative to GE intake are similar to other reports for steer consuming forages (Kurihara et al., 1999; Br. J. Nutr. 81:227–234). These results indicate that measurements of gas fluxes with this indirect calorimetry system produce estimates that are similar to researchers using respiration chamber.