Herbicide Resistance
This is a general review of the global adoption of genetically modified herbicide-resistant crops and the implications that these transgenic crops have on weed populations. Topics discussed include: aspects of the evolution of herbicide-resistant weed bio
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Herbicide Resistance Micheal D. K. Owen
9.1
Introduction
Herbicide resistance is not a topic or concern specifically focused upon the relatively recent introduction and adoption of genetically modified crops. In fact, the first case of herbicide resistance in weeds was reported in the scientific literature in 1970, but the occurrence of evolved resistance to herbicides in plants was suggested in 1956 (Harper 1956; Ryan 1970). Since the original report, over 300 herbicideresistant weed biotypes in more than 180 different plant species have been reported (Heap 2009). Generally, the recurrent use of an herbicide or herbicide mechanism of action imparts significant selection pressure on the weed population and provides an ecological advantage to those rare individuals within the population that have the heritable mutation conferring herbicide resistance (Owen and Zelaya 2005). The relatively recent introduction of crop cultivars with genetic modifications for herbicide resistance served to narrow the spectrum of herbicides used for weed control thus focusing on single herbicides (i.e. glyphosate) and increasing the potential for evolved herbicide resistance in weeds (Young 2006). It should be noted, however, that the genetically modified trait(s) typically does not impart selection pressure on the weed population, but rather the grower decision to utilize the herbicide causes the selection for resistance (Owen 2008a, b). However, despite claims that (due to a number of physicochemical characteristics related to glyphosate) glyphosate-resistant weeds would never evolve, the broad-scale adoption of genetically modified glyphosate-resistant crops globally has resulted in the evolution of glyphosate resistance in 16 weeds species, to date, and the rate of resistance evolution appears to be increasing at an increasing rate (Bradshaw et al. 1997; Heap 2009). This chapter provides a perspective of genetically modified herbicide-
M.D.K. Owen Agronomy Department, 3218 Agronomy Hall, Iowa State University, Ames, IA 50011 USA e-mail: [email protected]
F. Kempken and C. Jung (eds.), Genetic Modification of Plants, Biotechnology in Agriculture and Forestry 64, DOI 10.1007/978-3-642-02391-0_9, # Springer-Verlag Berlin Heidelberg 2010
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resistant crops, the implications of grower adoption of the genetically modified herbicide-resistant crops and the impact of this adoption on weed communities.
9.1.1
Overview of Adoption
Since the commercial introduction of genetically modified crops in 1996 (for methods, see Chaps. 1, 2), the area planted to these cultivars has increased globally at an increasing rate (Anonymous 2006). In 2006, approximately 100 million hectares of genetically modified herbicide-resistant crops were planted worldwide and an estimated 80% had the genetically modified trait conferring glyphosate resistance (Service 2007). The use of these cultivars increased an estimated 12% in 2007 and represented 114.3 million hectares and included 23 countries (Anonymous 2006). The primary countries t
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