Field experiments were conducted in central Alberta from 1999 to 2003 to examine the affects of defoliation intensity and frequency, and associated grazing systems, on Canada thistle (CT) and forage abundance. Regardless of fertilization practices, deferred clipping of non-CT herbage maximized forage yield and minimized the density and biomass of adjacent CT stems. Similar responses were observed under a high intensity--low frequency (HILF) clipping regime implemented throughout the summer. Conversely, continuous clipping reduced forage and increased CT abundance. Frequent but light defoliation (SD) generally led to moderate levels of CT and non-CT herbage suggesting forage production and associated CT suppression was linked to the frequency rather than intensity of defoliation. In field scale grazing trials over 3 years, the HILF system nearly eliminated CT stems, prevented flowering, maintained greater forage quality of CT, and led to greater biomass of adjacent forage, compared to continuous and SD grazing systems.