At MSU, Bill Dyer, a weed physiologist, Meghan Trainor, an Extension weed associate, and I work on this issue. Weed resistance is not simply lack of weed control in a single year. Lack of weed control may be due to unfavorable climatic conditions, weed emergence after herbicide application or error in application. Weed resistance to a herbicide occurs when herbicide use selects for individual plants that have increased tolerance to the herbicide.
True resistance is passed on from parent plant to the seed.
Smart use of herbicides to prevent additional resistance is important, because there are only four different ways herbicides act to selectively control grass or broadleaf weeds in wheat. When mutation at any of these four sites of chemical action occurs, so can resistance to a herbicide. The only way to confirm herbicide resistance is to test weed response to a number of herbicide doses, usually in the greenhouse. Figure 1 shows where those resistances were confirmed. It is logistically impossible to test all weed escapes for herbicide resistance, so careful inspections in the field and thorough documentation of application conditions should be done when resistance is suspected.