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Shrub encroachment in southwestern grasslands has negatively impacted ranching, soil conservation, and grassland dependent species. Past research suggested that a complex interaction between grazing, altered fire regimes, and changing climates maintained shrublands once they established. Understanding the ecological interactions that maintain shrub dominance is critical for grassland restoration efforts to be successful. To assess the impact of native herbivores on grass reestablishment in an arid shrubland, we established an herbivore exclosure experiment on a 10.92 ha parcel on the USDA-ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona. Cattle were removed from the site over 50 years ago, yet the intershrub areas remain devoid of an herbaceous layer. We hypothesized that at this site, herbivory pressure of native mammals is significant enough to suppress the reestablishment of an herbaceous strata, which provides a positive feedback for the continued displacement of grasslands by native shrubs. We constructed herbivore exclosures with 5 treatment levels which corresponded to herbivory pressure among size classes of mammalian herbivores. The levels were: small (e.g. kangaroo rats [Dipodomys merriami]), medium (e.g. desert cottontails [Sylvilagus audubonii]), and large (e.g. mule deer [Odocoileus hemionus]). Two control levels (total access and total exclosure) were included as well. We used a two-way repeated measures ANOVA with trial date and exclosure type as our factors, and found a significant difference in grass utilization among exclosure types (F(4,45)=14.38, p<0.001). A Tukey�s post-hoc analysis indicated that utilization was significantly higher in exclosures that allowed access to small and medium sized mammals, relative to our control (p<0.001). This pattern disappeared following the monsoon rains (p=0.96), suggesting that forage demand shifts significantly depending on season. Our results suggest that herbivory pressure from small and medium sized herbivores may impede the reestablishment of grasslands in an arid shrubland.