Is There a Need for Stewardship or is Killing Weeds Good Enough?
Since the introduction of glyphosate resistant (GR) soybean cultivars in 1996, GR crop have become the most rapidly and globally accepted agronomic practice in the history of agriculture (Anonymous 2007; Service 2007). In the United States (US), more than one billion cumulative acres of genetically engineered (GE) crops have been planted with most of these represented by GR crops (Gianessi 2005; Anonymous 2007; Service 2007; Gianessi 2008). Monsanto reported that their 2006 market share of glyphosate resistant crops (GRC) included 71.6 million acres of the soybean area, 34 million acres of corn, 11 .3 million acres of cotton, and 5.7 million acres of canola in the US and Canada (Anonymous 2006b). They anticipate that their GRC market share in US corn will approach 60 million acres by 2010 (Anonymous 2006a). In Iowa, GR soybeans are planted on an estimated 97% of the acres and GR corn is approaching 75% of the acres. All of the GR crops likely receive at least one and more often, two or more applications of glyphosate for weed control. Growers like the perception of simplicity and convenience of the GR-based crop systems, the consistent control of weeds and the lack of crop injury from glyphosate. However, despite the appearance of a successful weed control program, the soybean producer often fails to recognize the cost of simplicity and convenience: given the widespread adoption of GR crop systems, weed populations are changing and there is a critical need to include glyphosate stewardship in GR crop systems. The concurrent use of glyphosate in GRCs has resulted in the evolution of weeds that are resistant to glyphosate (Figure 1). There are now 14 different weed species that have glyphosate resistant biotypes (Heap 2004). Nine glyphosate resistant weed species have been identified in the US and six were confirmed glyphosate resistant since 2004. Importantly, it is apparent that weed populations are evolving resistance to glyphosate at an increasing rate. While alternative herbicides may still be effective and provide control of the glyphosate resistant weeds, resistance to many of the alternative herbicides also exists. Currently 183 herbicide resistant weed species are identified and many of these weeds are found in Iowa (Heap 2004). Thus it is critical to understand the implications of current weed control practices on future weed problems and assess the need to implement stewardship proactively to protect the sustainability of GRCs.