ABSTRACT
Chemical weed control began with the use of 2,4-D in the mid-1940s. Since then, a wide array of herbicides has been commercialized and that has greatly contributed to increased crop yields. Herbicide use in 21 major crops in the United States increased over 13-fold from 16 million kg in 1960 to 217 million kg in 1981. By 1980, over 90% of the corn, cotton and soybean areas were treated with herbicides compared to less than 10% of these crops planted in 1952 (Fernandez-Cornejo et al. 2014). With the introduction of several new, more specific and more effective herbicides, the cost of weed control with herbicides decreased relative to other control practices (labour, fuel and machinery). These benefits of lower production costs, higher crop yields and quality, and increased profit margins for farmers resulted in over-dependence on herbicides for weed management. Use of the same herbicide year after year has led to evolution of herbicide-resistant (HR) weeds. Development of herbicide resistance in weeds is widely recognized as a result of adaptive evolution of weed populations to repetitive use of same herbicide or class of herbicides (Jasieniuk et al. 1996). In response to selection pressure exerted by herbicides, weed populations change in genetic composition by selection of genes already present or arisen newly through mutation resulting in evolution of resistance (Délye et al. 2013; Jasieniuk et al. 1996). The first case of resistance to triazines was reported in 1968 (Ryan 1970). Since then, there has been an alarming increase in evolution and spread of HR weeds. As of 2017, globally, 252 weed species (147 dicots and 105 monocots) have evolved resistance
to 161 different herbicides representing 23 of the 26 known herbicide sites of action (SOA) in 91 crops in 68 countries (Heap 2017).
