Multi-species RIM, a bio-economic model that simulates the population dynamics of two annual weed species over a 20-year period, was used to investigate the value of transgenic glyphosate-resistant canola in the management of herbicide resistant annual ryegrass and wild radish in a Western Australian dryland farming system. The perceived advantage of growing this crop is the potential to control post-emergent weeds with the broad-spectrum herbicide glyphosate, and without the yield penalty evident in triazine-resistant canola. We found that the economic value of glyphosate-resistant canola is consistently higher than that of the commonly grown triazine-resistant canola. However, the benefits of glyphosate-resistant canola would need to be weighed up against potential risks to marketability (due to consumer resistance) and risks of increased weed resistance to glyphosate (due to increased selection pressure). (c) 2004 Elsevier Ltd. All rights reserved.