Beef cattle feedlots emit large amounts of the greenhouse gases (GHG) methane (CH4) and nitrous oxide (N2O), as well as ammonia (NH3), which contributes to N2O emission when NH3 is deposited to land. However, there is a lack of simultaneous, in situ, and nondisturbed measurements of the major GHG gas components from beef cattle feedlots, or measurements from different feedlot sources. A short-term campaign at a beef cattle feedlot in Victoria, Australia, quantified CH4, N2O, and NH3 emissions from the feedlot pens, manure stockpiles, and surface run-off pond. Open-path Fourier transform infrared (OP-FTIR) spectrometers and open-path lasers (OP-Laser) were used with an inverse-dispersion technique to estimate emissions. Daily average emissions of CH4, N2O, and NH3 were 132 (± 2.3 SE), 0, and 117 (± 4.5 SE) g animal−1 d−1 from the pens and 22 (± 0.7 SE), 2 (± 0.2 SE), and 9 (± 0.6 SE) g animal−1 d−1 from the manure stockpiles. Emissions of CH4 and NH3 from the run-off pond were less than 0.5 g animal−1 d−1. Extrapolating these results to the feedlot population of cattle across Australia would mean that feedlots contribute approximately 2% of the agricultural GHG emissions and 2.7% of livestock sector emissions, lower than a previous estimate of 3.5%.