A number of weed species that once were susceptible to and easily managed by certain herbicides have developed resistance. These weeds no longer are controlled by applications of previously effective herbicides. To date, at least 53 species of weeds are resistant to at least five different herbicide families. Herbicide resistance probably developed through the selection of naturally occurring biotypes of weeds exposed to herbicides over a period of years. A biotype is a population of plants within the same species that has specific traits in common.
Resistant plants survive, go to seed, and create new generations of herbicide-resistant weeds.
Mechanisms for resistance vary depending on herbicide family. Resistant biotypes may have slight structural differences from their susceptible counterparts that eliminate sensitivity to certain herbicides. Sulfonylurea-resistant plants have a modified ALS enzyme that prevents herbicide binding. Regardless of the mechanism for resistance, becoming familiar with the herbicide mode of action can help design programs that prevent the introduction and spread of herbicide-resistant weeds. Management programs for herbicide resistance should emphasize an integrated approach that stresses prevention.