A dense mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb] Beetle) canopy suppresses understory vegetation. Rambouillet ewes with high and low dietary preferences for mountain big sagebrush were tested for their ability to reduce the cover of dense stands of sagebrush. Eighty ewes with high and low preferences for mountain big sagebrush were grazed in October on 8 pastures with a 33% shrub cover for 3 years. Even though near infrared reflectance spectroscopy measurements of feces indicated that high-preference ewes consumed up to twice as much mountain big sagebrush than did low-preference ewes (P < 0.005), there was no difference in the reduction of sagebrush canopy between the high- and low-preference ewes (P = 0.46). Indeed, grazing did not reduce mountain big sagebrush more than in the ungrazed pastures. However, ewes with a high preference for mountain big sagebrush consumed more antelope bitterbrush (Purshia tridentata [Pursh] DC.) (P < 0.05) than did low-preference ewes (length reductions of 30 cm and 10 ± 3.7 cm [mean ± SE], respectively). In this study, the selection of ewes with a dietary preference for mountain big sagebrush had the unintended consequence of selecting ewes with a dietary preference for antelope bitterbrush. Antelope bitterbrush is a desirable shrub in sagebrush steppe ecosystems, and reductions in antelope bitterbrush as a result of altered livestock preferences will reduce rangeland health. Animals selected with a dietary preference for one plant species must be screened to determine what other plants they will preferentially select to limit potential negative consequences for plant communities and ecosystems.