ABSTRACT
In the mid-1970s, a linear increase of evolved herbicide resistance began that has continued unabated until the present time, now approaching almost 500 unique cases of evolved weed resistance. Different herbicide classes have been unequally used, so that selection pressure from the different classes has been very uneven. This makes it difficult to judge the risk of herbicide evolution for the little-used herbicide classes for which there has been little or no reported resistance. After the glyphosate patent expired in 2000, glyphosate became a less expensive, generic herbicide, making use of GR crops even more economically attractive. The phenomenal success of this crop/herbicide combination reduced the use and the value of the global market for other herbicides in maize, soybean, cotton, canola, sugarbeets, and alfalfa. Weeds have been much more resilient to the selection pressure imposed by herbicides than the pioneers of herbicide-based weed management would have envisioned.
