How resistant weed populations increase
The weeds in your field may look the same on the outside, but they may not have the same processes happening on the inside. These weeds are called “biotypes” and some of these biotypes may be resistant to certain herbicide Groups. If the same herbicide Groups are used repeatedly on the same field every year, then the biotypes which are resistant will remain in the field.
Squeezing crop rotations can favour herbicide resistance
Growing the same crop too often could mean repeating the same herbicide program too often. That means using the same herbicide, from the same Group, repeatedly, rather than rotating to herbicides with different modes of action. Since the same herbicide Groups can be used on commonly-grown crops in your area, the same population of weeds will be repeatedly exposed to the same herbicide Group, presenting greater opportunity for resistance to develop.
Overuse of modes of action
Herbicides impart a very high level of selection pressure on a resistant weed population – for example, an effective wild oat herbicide will control every wild oat in a field except the few plants that are resistant. Initially, it appears that a herbicide has worked so well that it's tempting to continue to use it. Over several years, though, repeated use of the same herbicide or group gives resistant weeds the opportunity to set seed, multiply and establish themselves in a field. Once established, moving across a field with a combine, a cultivator or even a planter/drill can spread those resistant seeds.
Don't let your
Weed seed deposition
Controlling weeds before they produce seed will help reduce the spread of resistant plants. Simply put, don’t let your weeds go to seed.