The Multi-species RIM (Resistance and Integrated Management) model is used in this analysis to investigate the value of Roundup-Ready® canola in the simultaneous management of annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum). It is likely that the transgenic canola variety resistant to the non-selective herbicides glyphosate will soon be introduced in Australian agriculture. The perceived advantage of growing these crops is the potential to control post-emergent weeds with excellent broad-spectrum herbicides, and without the yield penalty evident in triazine-resistant canola (grown widely in WA). This may also help prolong the life of selective herbicides, to which ryegrass and radish can be highly resistant. Therefore, the introduction of genetically modified glyphosate-resistant canola will, other factors being equal, not only increase the options for weed control, but increase the yield of the vast canola crops grown in WA. Conversely, increased usage of the herbicide to which the new crop is resistant can result in the evolution of resistance to that herbicide in weeds. These trade-offs are discussed here.