Evolution of Herbicide Resistance in Weeds.

Evolution of resistance to herbicides is a significant problem facing world agriculture.  Because herbicides are so efficient and effective for weed control, many growers have over time essentially replaced all other weed control tactics with herbicides alone.  This extensive reliance on a single weed control tactic has resulted in intense selection pressure for any traits enabling survival.  Herbicide resistance has evolved in many types of annual crops, including both selective herbicides and transgenic crops with resistance to otherwise non-selective herbicides. Herbicide resistance has also evolved in weeds found in perennial crops such as vineyards and orchards, fence lines, and no-tillage fallow, highlighting that herbicide resistance can evolve in any situation where a single herbicide mode of action is used as the sole weed control tactic.  Of particular recent importance, glyphosate resistance has now has been reported in 29 species globally.  Glyphosate resistance mechanisms reported to date include target site mutations in the EPSPS gene, amplification of the EPSPS gene, and reduced glyphosate translocation.  Glyphosate metabolism has recently been reported in Digitaria from Brazil.  EPSPS gene amplification has now been reported in six species.  Reduced translocation has been shown in several species to be due to sequestration of glyphosate in the vacuole.  The resistance mechanism in Ambrosia trifida is currently unknown and involves a rapid necrosis response in resistant individuals.  As for most herbicides, multiple mechanisms confer resistance to glyphosate, and these mechanisms can combine via cross-pollination so that a single individual carries multiple mechanisms.  High selection pressure occurs with intensive use of a single herbicide mode of action, meaning that any individuals carrying traits that confer greater survival and reproduction will produce more offspring and result in the continued evolution of herbicide resistance.  This process occurs through both the combination of known mechanisms and selection for any additional unknown mechanisms that are capable of conferring higher survival.  Non-target-site resistance mechanisms including enhanced herbicide metabolism are especially of concern, because these mechanisms can confer cross-resistance across completely different modes of action.  Greater diversity in weed control tactics is necessary, and practices that permit reliance on a single weed control tactic must be avoided, in order to disrupt the evolutionary processes that result in evolution of herbicide resistant weeds.  Future weed management programs must include diverse tactics such as pre-plant burndown treatments (knockdowns) with a different mode of action, pre-emergent herbicides, mixtures of different modes of action within a crop, removal and/or destruction of seeds produced on surviving plants, and rotation to different herbicide modes of action and crops from year to year.  Integrated tactics are necessary to reduce the weed seed bank density.  New innovations with novel modes of action are also urgently needed.  These actions will improve the sustainability of future crop harvests and help address the current problem of herbicide resistance.