20.1.1  Lolium rigidum in Australia: a very resistance prone weed!

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Figure 20.1 The area affected by herbicide-resistant weeds in Australia, showing a dramatic rise during the 1990s

L. rigidum in Australia provides a spectacular demonstration of herbicide resistance, as thousands of populations of L. rigidum over vast areas have evolved resistance to many herbicides. Why is this species so resistance prone? Key biological features are that L. rigidum is a highly plastic, fecund and genetically diverse cross-pollinated annual diploid grass. Furthermore, L. rigidum has been a valuable and widely established pasture grass across southern Australia since the nineteenth century. Since a shift towards arable crops began in the 1960s, this previously well-regarded pasture plant became Australia’s most important weed of crops. Because L. rigidum was once so favoured, it is still found across some 40 million hectares of southern Australia at densities of up to 4000 seedlings m–2. These huge, genetically diverse populations have now been exposed to strong selection pressure by sudden introduction of potent new herbicides, causing high mortality. Persistent application of herbicides might be expected to drive L. rigidum to extinction. Instead, it selects for resistant genotypes by the same mechanism whereby selection with antibiotics gives rise to resistant microbes (Figure 20.1).