Hubert Menne

1. HRAC Classification of Herbicides and Resistance Development

Theuse of herbicides has caused significant changes in the production systems of all major crops globally. The highly effective chemical control of weeds has replaced manual, animal, and mechanical weed control, has increased the productivity, and has also enabled the development of larger farm sizes and an improved subsistence for farmers. However, herbicides have not resulted in the extinction of weeds; rather, they cause – together with other influencing factors – a continuous selection of plants to occur which are able both to survive and to reproduce. As a consequence, these plants with such survival properties are able to become dominant and to become distributed over increasingly large areas. The first cases of herbicide resistance were reported around 1970, since then the resistance of both monoand dicotyledonous weeds to herbicides has become an increasing problem worldwide. At the end of 2010, the International Survey of Herbicide-Resistant Weeds recorded 348 herbicide-resistant biotypes with 194 weed species – 114 dicotyledonous and 80 monocotyledonous (I. Heap, 2010, personal communication, http://www.weedscience.com). The relatively steady increase in the number of new cases of resistance since 1980 accounts for the increasing importance of herbicide resistance in weeds in the major agricultural regions (Figure 1.1). During the period between 1970 and 1990, most documented cases of resistance concerned the triazines. The introduction of new herbicides with different sites of action (SoA) resulted in a shift, so that more recently both acetolactate synthase (ALS) and acetyl-CoA carboxylase (ACCase)-resistant weeds have been reported (Figure 1.2). Additionally, the rapid adoption of glyphosate-resistant crops in North and South America, and the use of glyphosate as a pre-sowing treatment in different cropping systems, have resulted in increasing cases of glyphosate resistance (I. Heap, 2010, personal communication, http://www.weedscience.com). The probability of resistance developing towards glyphosate had been expressed as being likely but underestimated, though less frequently in comparison with most other SoA classes [1].