The Current State of Herbicidal Weed Control

The recent adoption of GM crops resulted in a dramatic impact on the use of herbicides for weed control in Iowa. Particularly, glyphosate-resistant soybeans dominate the varieties that are planted and anticipated increases in grower adoption of glyphosate-resistant corn have resulted in the unprecedented use of one herbicide being applied to most of the row crop acres in Iowa. There are a number of important benefits for the use of glyphosate-resistant crops and the concomitant applications of glyphosate. The benefits that can be objectively established include broad-spectrum weed control, the consistency of weed control, minimal concerns for crop injury, and the low cost of glyphosate. Another benefit that is difficult to substantiate is the favorable environmental profile of glyphosate-based cropping systems. However, growers also tout as important benefits for the glyphosate-based systems the simplicity of weed control, the flexibility of application timing, and the ability to control large weeds. Significant risks are also a consideration of the glyphosate-based systems, and in fact several of the aforementioned perceived benefits could be better described as risks. The risks include the alleged simplicity of the glyphosate-based system, the selection pressure imposed on weed communities, pollen introgression of the glyphosate resistant trait into non-glyphosate resistant corn, and issues relating to application timing. One other consideration of the adoption of glyphosate-based technology has been the slowed development of new herbicides which may be needed to overcome the risks associated with the glyphosate-based systems. These benefits and risks, as well as the development of existing and new herbicides will be discussed in this paper. The corn and soybean herbicide effectiveness charts have been included as a reference.