20.2.4  Multiple resistance

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Multiple resistance occurs when several resistance mechanisms accumulate to confer herbicide resistance. This is a practical problem in a number of populations of L. rigidum because of simultaneous resistance to a wide range of herbicides. Commonly, after resistance to one herbicide chemistry has developed, the population is exposed to and develops resistance to a different herbicide. In extreme cases a number of mechanisms endowing both target site and non-target site cross-resistance are accumulated within the same individual. L. rigidum populations are superb accumulators of resistance mechanisms because resistance genes are quickly spread by out-crossing, producing a diverse progeny. Different resistance mechanisms can be inherited from each parent. In addition, genetic diversity in L. rigidum populations allows them to respond successfully to rapid changes in selection pressure.

A dramatic example of multiple herbicide resistance can be seen in a population of L. rigidum selected over 21 years with herbicides from five different chemistries. This population expresses resistance to nine different herbicide chemistries with five different target sites. The selected population contains two resistant target sites, ACCase and ALS, as well as enhanced metabolism of at least five different herbicide chemistries. Studies using inhibitors of cytochrome P450-dependent microsomal oxidases have established that at least four different herbicide-degrading enzymes contribute to resistance in this population. Therefore, this population contains at least six distinct resistance mechanisms conferring resistance to a wide range of herbicide chemistries. This biotype is the most dramatic example of multiple herbicide resistance worldwide!