Many of the guiding principles for the evolution and management of herbicide resistance were founded on an assumption of single gene traits, inherited in Mendelian fashion and exemplified by target site resistance. For many weed species and for many resistance traits, it is increasingly apparent that non-target site resistance (NTSR), and, in particular, herbicide metabolism, is the dominant mechanism of herbicide resistance. Whilst in most cases, the precise genetic basis for NTSR remains unresolved, it is clear that NTSR traits are polygenic, being inherited in a quantitative fashion, and leading to complex patterns of cross-resistance. NTSR traits do not evolve via the selection of rare mutations in herbicide target genes, but rather through the selection and recombination of standing genetic variation in defence- and metabolism-related pathways in weed populations. The underlying genetic basis and evolutionary dynamics of NTSR traits call into question a number of the resistance management guidelines that were established for single-gene resistance traits. NTSR traits, selected from standing genetic variation, may be preferentially selected by low herbicide doses. Where these traits lead to broad cross-resistance patterns, underpinned by a suite of plant defence genes, such as cytochrome P450s, it may even be the case that herbicide rotations and herbicide mixtures are not able to slow selection for resistance. Here, I will present results from a range of experimental, modelling and epidemiological studies which demonstrate that the increasing importance of NTSR mechanisms should cause us to re-evaluate the influence of herbicide dose, herbicide mixtures and rotations, stacked herbicide resistance traits and fitness costs on the evolution and management of herbicide resistance. In the face of NTSR mechanisms that confer broad patterns of herbicide resistance, it is ever more important to recognise the need for weed management strategies that integrate chemical, cultural, physical, biological and ecological approaches.

The evolution and management of non-target site resistance

256th National Meeting (2018)

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