Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.
The purpose of this paper is to introduce the basic tenets of weed management, to define herbicide resistance and tolerance and how they affect crop production and are affected by management practices, and to present the environmental impacts of herbicide-resistant crops. This paper will summarize aspects of herbicide resistance in five different sections: (1) a description of basic weed science management practices and concepts, (2) definitions of resistance and tolerance in weed science, (3) environmental impacts of herbicide-resistant crops, (4) strategies for management of weed species shifts and herbicide-resistant weeds and adoption by the agricultural community, and (5) gene-flow potential from herbicide-resistant crops.