Wild oats have developed resistance to several wild oat herbicides used in Minnesota. Herbicide resistance develops through the selections of naturally occurring weed biotypes that have an inherent ability to tolerate the herbicide. The term “biotype” refers to plants within a species that have a slightly different genetic makeup from the general population.

Selection for change in weed populations begins when a small number of plants (a biotype) within a weed species have a genetic makeup that enables them to survive a particular herbicide application. Where this difference in genetic makeup originated is not clear. However, herbicides are not known to directly cause the genetic change (i.e. mutation) that allows resistance. The resistant biotype, therefore, is present in low numbers in natural populations and when a herbicide is applied, most of the susceptible weeds die but the few resistant weeds survive, mature, and produce seed. If the same herbicide continues to be applied and, the resistant weeds reproduce, the percentage of the weed population that is resistant will increase.

It is difficult to predict exactly which weed species will have biotypes resistant to a given herbicide. However, we have learned from previous pesticide resistance problems that the occurrence of herbicide resistant weeds is linked directly to the herbicide program used, the weed species present, and the crop management practices employed.

Regardless of how weed resistance develops, it is important to know the herbicide mode of action to plan weed control programs that prevent the development and spread of resistant weeds.

Bev Durgan Extension Weed Specialist