Technical Abstract: Semi-arid rangelands are expected to be highly sensitive to global change, but few studies have explicitly investigated interactions between increased atmospheric CO2 and plant defoliation (such as occurs with animal grazing). This experiment subjected intact plant-soil cylinders from the Wyoming prairie to a 3-way factorial of CO2 (370 vs. 720 µl l-1), defoliation (non-clipped vs. clipped) and soil nitrogen (control vs. 10g m-2 added N) under nearly natural climate conditions. The functional groups C3-grasses, C4-grasses and forbs were investigated. Two- and three-way interactions on productivity and nitrogen resources were rare. Only the C3-grass had a positive (+50%) growth response to elevated CO2. Above-ground productivity was equal in defoliated and non-defoliated plants. Elevated CO2 consistently reduced “quality” of C3 and C4-grasses through reduced (-16%) tissue N concentration. Nitrogen-Use-Efficiency (NUE) was increased (+25%) under elevated CO2, but plant uptake of 15N from the soil was unaffected. In contrast, defoliation improved (+29%) tissue quality (%N) in all functional groups; however, NUE was reduced (-23%) by defoliation. Without additional N availability, forage quality with defoliation may not be maintained under increasing atmospheric CO2. Increased dominance of C3-grasses plus reduced forage quality will necessitate changes in grazing management practices in the northern mixed-grass prairie.