How resistant weeds spread
In any given field on your farm, weeds number in the thousands, maybe tens or hundreds of thousands. Long before resistance became an issue, chances are that only a few possessed a genetic mutation that would enable them to survive herbicide treatment.
Over the past 50 or so years, agriculture has become increasingly reliant on herbicides for weed control. It 's a given that herbicides are the most effective means to control weeds, and the growth of conservation tillage practices has increased that dependence on herbicides even more.
Overuse of modes of action
Highly effective herbicides (modes of action) impart a very high level of selection pressure on a resistant weed population - in other words, an effective wild oat herbicide will control every wild oat in a field except the few plants that are resistant. Initially, it appears this herbicide has worked so well that it's tempting to continue to use it. Over several years, repeated use of the same herbicide or mode of action gives resistant weeds the opportunity to set seed, multiple and spread throughout a field.
The warning signs
It can take many years to recognize that you might have a resistance problem. An early warning sign is a small number or a patch of healthy weeds that apparently escaped control. Sprayer malfunction might have been the cause; however, if you have no reason to suspect your equipment and especially if the herbicide met your expectations elsewhere in the field, those healthy weeds could be the beginning of a resistant population.
The importance of field records
You can back up field observations by examining your field records. Have you used this herbicide before? Was it effective in the past but appears to be losing its punch? How many years have you applied it to the field with the suspicious patch? If your records reveal that you have used a particular herbicide or mode of action repeatedly in a field, then the surviving weeds may well be resistant.
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