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Herbicides (chemicals that remove unwanted plant species) are widely used in world agriculture. Since 1945 a steady stream of new herbicides has been commercialised and readily adopted by Australia farmers such that almost all cropping systems rely on herbicides as an effective, cheap and flexible method of weed control. Herbicides have largely replaced the plough for weed control (minimum tillage) and have greatly contributed to soil conservation by limiting loss of topsoil after cultivation. Hence, there are many advocates for use of herbicides. However, there has been a dark side to excessive reliance on herbicides for weed control, namely herbicide-resistant weeds. Although herbicide-resistant weeds occur worldwide, their abundance in Australia is startling. What started in the early 1980s as a biological curiosity has now become a serious practical problem over more than two million hectares of the Australian cereal region (Figure 20.1). In this section we draw on the Australian experience with herbicide resistance to describe how plant species are able to profit from catastrophic changes in their environment. The grass weed Lolium rigidum will be focused on particularly as this species alone contributes more than 80% of all documented herbicide resistance in Australia.