Growers often do not detect resistance until the herbicide resistant biotype comprises 30 to 40% of the weed population.Weed scientists have established the maximum number of various weed species that can be tolerated before significantly decreasing yields – the economic threshold. Although growers are encouraged to wait until the weed reaches the economic threshold before implementing control measures, the wisest strategy for managing weed resistance is to act proactively and not wait until the problem is recognized (Yancy, 2005). Such a strategy centers on reducing the selective pressure imposed by the herbicide. “Selection pressure is the key,” says Dr. Alan York of North Carolina State University. “You have no way of knowing whether that ‘one in a 100 million’ plant is present or not. You must assume it might be present and your resistance management strategy becomes one of reducing selection pressure on that plant.”

Managing agroecosystems to minimize the development of resistant weeds relies upon changing weed control measures regularly. As a result, new control methods continually need to be evaluated and implemented, so that farmers can stay one step ahead of weed resistance problems.

Herbicide resistance or weed spectrum shifts become a problem most often when a single herbicide or herbicides with the same mode of action are used repeatedly on a weed population over several years. Therefore, weed resistance is often associated with monoculture cropping systems. In addition, relying only on herbicides to control weeds and neglecting other methods, such as crop cultivation techniques, accelerates the development of herbicide resistant weeds. Therefore, the risk of development of resistant weeds is highest in corn and soybean agroecosystems, if herbicides, especially those with the same mode of action, are used as the sole tactic for weed management (Owen, 2001; Heap, 2005). 

Herbicides are very important tools for farmers, and they should be used properly to preserve their effectiveness. Any weed management option that reduces herbicide-imposed selection pressure will reduce the rate of resistance development to the herbicide. By adopting options, such as mixtures of herbicides with different mode of action and crop rotation, selection pressure for resistant weeds can be reduced and the usefulness of herbicides preserved. Generally, the best approach to resistance management is to use Integrated Weed Management (IWM) practices. IWM utilizes a range of weed control methods and includes the following tactics:

• Avoid using the same herbicide or herbicides with the same mode of action multiple times per year or year after year.
• Use tank-mixtures consisting of different herbicide types with overlapping weed spectra.
• Use crop rotations because different crops allow different cultural and tillage options that compete much differently with weeds. 
• When using herbicides, use full label rates and tank mix partners.
• Use clean seeds and clean equipment to minimize spread of weed seed.
• Monitor fields after herbicide applications for appearance of resistant weeds.
• Control weeds before they form seed.
• Where practical, use cover crops and other methods to reduce weed seeds in soil.