Grasslands such as the South American Pampas play an important role in the global greenhouse gas (GHG) balance. Depending on edaphoclimatic conditions and management, these ecosystems can be sources or sinks of GHG.  Ecosystem processes in the Pampas need to be further elucidated while major land use changes are taking place in the region. The well-known Century model has been shown to simulate many key soil processes adequately, whereas its newer version DailyDayCent can simulate GHG fluxes in natural and agroecosystems. The objective of this study was to test DailyDayCent in a long-term grazing experiment in  the Northeastern Pampas. The experiment was established in 1986 at the Federal University of Rio Grande do Sul (UFRGS) Experimental Station in Eldorado do Sul (Southern Brazil). Field measurements of CO₂, N₂O and CH₄ fluxes have been conducted for over a year (June 2012-July 2013)  The experimental area (31 ha) is divided in paddocks of aproximately 1.5 ha which are grazed with steers at four forage allowance levels (4, 8, 12, and 16 kg dry matter per 100 kg live weight per day). Air samples were collected bimonthly using static chambers followed by gas chromatography analyses. Initial simulations with default parameters showed poor agreement with measurements. Parameter changes minimized differences between simulated and field data, especially soil C, biomass production and the general trend of grassland GHG dynamics. However, some transient N2O influxes and CH4 effluxes were not captured by the model. This initial simulation exercise indicates that the DailyDayCent model could be used to simulate GHG dynamics under alternative grazing regimes and climate change scenarios in Southern Brazil.