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The effects of grazing on relative (g g-1 d-1) and absolute (g m-2 d-1) aboveground net primary productivity and senescence of coirón blanco (Festuca pallescens (St. Yves) Parodi) were investigated in northwestern Patagonia. Aboveground net primary productivity under ungrazed and grazed conditions was estimated by means of a simulation model fitted to biomass data. Relative and absolute aboveground net primary productivity was greater for grazed than ungrazed plants during the early growing season while the inverse occurred during the late growing season (reproductive period). Grazing reduced the relative and absolute aboveground senescence during the late growing season. Relative and absolute effective primary productivity, expressed as the difference between the corresponding primary productivity and senescence, were greater for grazed than ungrazed plants. This increase is in accordance with the grazing optimization hypothesis. However, the increase was not of sufficient magnitude to compensate for losses of green biomass caused by grazing. Results imply that current grazing intensities in this region reduce aboveground productivity of coirón blanco. It follows that a reduction in stocking rates would be necessary to increase aboveground primary productivity. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020