20.1.2  Rapid development of resistance

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Figure 20.2 Acetyl-coenzyme A carboxylase (ACCase)- and acetolactate sythase inhibiting herbicides generate herbicide resistance in populations of Lolium rigidum growing in the cropping belt of Western Australia after only three to four applications. (Based on Gill 1995)

Many factors influence the development of resistance, including abundance, genetic diversity and biological characteristics of the weed species, characteristics and application of the herbicide, and genetic basis and expression of resistance genes. Crucial factors are initial frequency of resistance genes in field populations prior to herbicide use and intensity of the selection pressure. Clearly, with a higher initial frequency of resistant individuals there will be more rapid development of resistance and, conversely, if resistance genes are extremely rare then resistance takes many years to develop, or may never develop at all. Australian populations of L. rigidum can become resistant to a new herbicide applied once yearly after only three years! Thus, within three generations a population can change from being highly susceptible to being highly resistant (Figure 20.2). Large, genetically diverse populations of L. rigidum harbour high numbers of resistant individuals at frequencies reaching 0.01%. Selection with highly effective herbicides causes resistance levels to rise alarmingly.