HRAC Classification of Herbicides and Resistance Development

The first cases of herbicide resistance were reported around 1970. Since then resistance of monoand dicotyledonous weeds to herbicides has become an increasing problem world-wide. In March 2006 the International Survey of Herbicide-Resistant Weeds recorded 305 herbicide-resistant biotypes with 182 weed species – 109 dicotyledonous and 73 monocotyledonous weeds [1]. 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 (Fig. 1.1). In the period 1970–1990 most documented cases were concerning triazine resistance. The introduction of new herbicides with different modes of action (MoA) resulted in a shift, so that more recently ALSand ACCase resistant weeds have been reported (Fig. 1.2). 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 has resulted in increasing cases of glyphosate resistance [1]. The probability of resistance development to glyphosate had been expressed as being likely, though less frequently in comparison with most mode of action classes [2].