- This study evaluated the influence of cattle stocking density on botanical composition of diet, diet preference, and cattle performance on the Zumwalt Prairie in Northeastern Oregon. In each of two years, 192 cow-calf pairs (549.27 kg, BCS = 4.89) and 48 yearling heifers (383.34 kg, BCS = 5.02) were stratified by age and body condition, and randomly allotted to a randomized block design (four blocks) with the following treatments: 1) Control, no livestock grazing; 2) low stocking, 0.36 animal units (AU)/ha; 3) moderate stocking, 0.72 AU/ha; and 4) high stocking, 1.08 AU/ha for a 42 d grazing period spanning from late May to early July. Using ruminally-cannulated cows, diet composition and masticate samples were taken, in May and July, following 20 min grazing bouts and were analyzed for forage fiber and crude protein. Treatments had no influence on cattle weight change and body condition (P > 0.10). In regard to foraging efficiency, grams per minute and bites per minute were lower in the early collection than the late collection (21.51 g/min vs. 31.21 g/min; 16.71 bite/min vs. 26.71 bite/min) and higher in the control pasture than the grazed pastures (31.21 g/min vs. 12.44 g/min; 26.71 bite/min vs. 12.70 bite/min; P < 0.05). In addition, grams per bite decreased linearly with increased stocking density (P < 0.05). Forage fiber (ADF and NDF) was lower in May compared to July whereas CP was higher in May compared to July (P < 0.01). Stocking density did not influence diet quality (P > 0.20). Botanical compositions of diets were 91.98% grass and 7.95% forb species, with beef cattle showing strong relative preference for grass (RPI = 1.69) and weak relative preference for forbs (RPI = 0.18). Shifts in diet composition and preference were noted in regard to stocking density with Idaho Fescue, Prairie Junegrass, Hawkweed species and Hoary Balsamroot (P < 0.05). In summary, cattle stocking density had minimal effect on diet quality and cattle performance, but did influence foraging efficiency, botanical composition of the diet, and relative preferences of beef cattle grazing on a bunchgrass prairie.