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Weed Resistance

Many Agronomic Factors Can Cause The Problem

By Amanda Huber
Southeast Editor

Once upon a time, it was thought that weeds would not become resistant to herbicides. Both scientists and producers speculated on what could happen if weeds did become resistant, but that unpleasant thought was mostly brushed aside, and everyone hoped and prayed it wouldn’t happen. Now, we know differently, and today’s reality is that herbicide resistance is a major concern in the cotton industry.

Did the phenomenon of herbicide resistance just happen recently? Not at all. In fact, the first case of herbicide resistance was reported in 1968, says Ken Smith, University of Arkansas Extension weed specialist, who spoke at the recent Beltwide Cotton Conferences, in Nashville, Tenn.

“Since that time, more than 180 species have developed resistance to one or more of 19 herbicide families,” Smith says.

Resistance apparently is caused by selection pressure and mutations. Selection pressure is ‘survival of the fittest.’ As for mutation, Smith says, there probably was, at some point, a mutation. But, he notes, there is no evidence that herbicides cause weeds to change, mutate or become resistant.

Examine Weed Management Program

Although there is no evidence that herbicides cause weeds to become resistant, resistance is more likely to develop in areas of widespread use of the same herbicide family.

When trying to determine whether there is a risk of herbicide resistance, producers should examine the diversity of their weed control program, says Jason Ferrell, University of Florida Extension weed specialist.

“The risk of developing resistance is much greater if the same herbicides are being used each season for weed control,” says Ferrell.

“For example, consecutive cotton crops that rely on glyphosate for 100 percent of the weed control are much more at risk than areas under a diverse crop rotation scheme or cotton crops that use many different herbicides for weed control.”

Not getting proper coverage on each weed is one reason for herbicide failure, which farmers may confuse with resistance. Ferrell says herbicide failure can occur for many other reasons: drought or adverse growing conditions; improper mixing; improper calibration; incompatibility between different chemicals in the spray tank; or failure to use a proper surfactant system.

“In all of these cases,” he says, “you should see a similar failure across the entire field and on all weed species.”

Year Two Is Critical

Smith says “year two” is the critical year to watch.

“In year two, you will see spots of weeds that are smaller, not widespread, more contained to one area,” he says. “They will look as though they likely came from one or two plants the year before. These spots must be dealt with. Because, in year three and beyond, there is no way to control it.”

Essentially, resistance in the field cannot be definitively determined.

“Suspected resistant plants will need to be removed to a greenhouse and tested against a known susceptible population,” says Ferrell. “This will often require the assistance of scientists within the university Extension service to make a call on the presence and level of resistance.

“If you suspect resistance, it’s time to examine your weed management program to see if proactive changes can protect your operation.”

Contact Amanda Huber at ahuber@onegrower.com or (352) 486-7006.

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