In order to intensify cattle utilization, embankments were constructed to avoid tidal ingressions in Samborombon Bay, Argentina, The objective of this study was to evaluate the effect of tidal suppression and cutting frequency of a salt marsh dominated by Spartina densiflora Brongn. Two paddocks of a commercial cow-calf operation farm, one prevented from tidal flooding and another exposed to overflow from natural tidal pattern (control), were the main plots of the nested design. The experiments were carried out during a dry (2008–2009) and a wet growing season (2012–2013). Two defoliation frequencies, simulating light and moderate grazing pressure, were performed in the subplots nested within each main plot. Soil organic matter and N content were lower and soil structural instability index was much higher in the embankment than in the control treatment. Soil salinity during the dry growing season was higher in the embankment than in the control treatment. Bare soil was higher under embankment treatment and high defoliation frequency exacerbated this response. Relative contribution of Spartina densiflora was lower under embankment than control treatment and the changes of floristic composition depended on the growing season. Aboveground net primary production (ANPP) in the wet growing season was almost 70% higher than in the dry growing season. Embankment reduced ANPP and high defoliation increased ANPP with respect to low defoliation frequency in the control paddock, to a much higher extent in the wet season. Dry matter digestibility of S. densiflora was not affected by treatments. Crude protein was higher in control paddocks under high frequency. Our results showed that tidal suppression by embankment was not effective to increase productivity and forage value of S. densiflora saltmarsh but caused soil and structural changes that may negatively alter ecosystem processes of this vulnerable grassland of high importance for biodiversity conservation.