Cattle grazing wheat pasture have the potential to gain BW exceptionally well, but excessive nitrogen intake results in increased excretion and increased greenhouse gas (GHG) emissions. Supplemental concentrates with the addition of an ionophore given to ruminants grazing wheat is a potential practice for producers to increase nitrogen efficiency while decreasing GHG emissions. Therefore, the objective of this experiment was to quantify the effects of energy (2.95 kg/d) and lasalocid (200 mg/hd/d) supplementation on nutrient intake, energy metabolism, respiratory gas fluxes, and performance of grazing cattle. Methane emissions were not affected (P = 0.58) by treatment, hence methane intensity (P = 0.07) and yield (P <0.01) were reduced for supplemented cattle. Supplemented cattle had greater CO2 emissions (P = 0.04) and O2 consumption (P = 0.03). Average daily gain tended to be greater for supplemented cattle (P=0.09) compared to Control (1.22 and 1.00 kg, respectively); but no effect (P = 0.88) was observed with the lasalocid. Fecal output was greater for supplemented cattle (P < 0.01), but forage intake was lower (P < 0.01) and nutrient intake was higher (P < 0.01) for supplemented cattle compared to Control. Supplemented cattle had lower forage intake with greater CO2 emissions and O2 consumption, but lasalocid did not affect any parameter measured. For Year 1 performance each kilogram of supplement increased (P = 0.04) ADG by 73 g/d; however, lasalocid did not increase (P = 0.73) ADG (avg ADG = 1.7 kg). Total BW gain by each steer (kg) was increased (P = 0.04) 4.7 kg for each kilogram of supplement fed daily and again, lasalocid did not increase (P = 0.73) performance (avg total BW gain = 109 kg). For Year 2 performance each kilogram of supplement increased (P = 0.001) ADG by 58 g/d; however, lasalocid did not increase (P = 0.17) ADG (avg ADG = 1.4 kg). Total BW gain by each steer (kg) was increased (P = 0.001) 3.7 kg for each kilogram of supplement fed daily and again, lasalocid did not increase (P = 0.17) performance (avg total BW gain = 88 kg). Advisors/Committee Members: Elizabeth Kegley, Stacey Gunter, Paul Beck.