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Weed resistance marches on

Herbicide-resistant weeds continue to gain ground in Nebraska in 2011, prompting University of Nebraska specialists to call for more integrated weed management programs that include rotating products with different modes of action.

“The sky isn’t falling on commonly used and effective weed control products, but we need to get this in front of the radar screen for farmers,” says Lowell Sandell, UNL Extension weed science educator.

At a glance

Last year saw an expansion of weeds resistant to herbicides.

Repeated use of the same products and modes of action is the main cause.

Integrated weed approach can aid in sustaining popular products.

First appearance

The resistance issue first cropped up in Nebraska in 2006 when UNL confirmed glyphosate-resistant marestail in at least two eastern Nebraska soybean fields.

That was followed in 2011 with confirmations of giant ragweed resistant to glyphosate in Butler and Washington counties; 2,4-D-resistant waterhemp found in southeast Nebraska; waterhemp resistant to HPPD-inhibiting products such as Callisto, Laudis and Impact found in a seed cornfield; and glyphosate-resistant kochia in Keith County.

Palmer amaranth has shown resistance to glyphosate in Southern states, and that resistance could evolve in local populations of Palmer amaranth. The weed occurs in the southern tier of Nebraska counties now.

“We’ve always talked of resistance management in an abstract way, thinking the resistance issue was something occurring in the South or the eastern Corn Belt,” Sandell says. “Nebraska is relatively new to this phenomenon, but it’s coming home to roost here.”

The weed-resistance problem stems from repeated use of the same products and the same modes of action. Continuing to do so will increase the likelihood of putting the effectiveness of products like glyphosate and other herbicides at risk, according to Sandell.

“Glyphosate is part of an incredibly effective and convenient weed control system, but if we allow weeds to develop resistance by using only glyphosate, it puts that effectiveness in jeopardy,” he adds.

In cases where effectiveness of herbicides has lagged, farmers often have pushed up the rates to maintain weed control, but Sandell says that won’t bring adequate control, and it will only increase the resistance problem.

New chemistries lacking

Advances in herbicide technology through the years have allowed farmers to “move on” by taking advantage of new chemistries that developed. “Any time we had a problem with a certain chemistry, something better always came along over the past 30 or more years,” Sandell says. “But that’s not the case now. New technologies such as dicamba- or 2,4-D-resistant crops will come on line and be very helpful, but neither of those two products are new chemistries. They are expanded uses of existing chemistries. New active ingredients will come on to the market, but in existing modes of action, not new modes of action.”

Strategy

Alternative chemical control products can be equally effective, but timing is critical. “For instance, if the label states target weeds at 2 inches or less, it means just that,” Sandell says.

“University scientists have promoted integrated weed control systems from the start,” he adds. “These systems may not be the most economical, but we are seeing the resistance problems emerge that university people predicted.”

Sandell and his UNL colleagues offer recommendations to avoid additional resistance development:

First, know the weeds present in your fields and then use weed management programs that are both economically effective and will reduce the potential development of glyphosate resistance.

Rely more on preemergence products with glyphosate if control of certain weeds is a problem.

Rely on tankmixes of glyphosate and postemergence herbicides.

Work with your chemical dealer, consultant or agronomist to better understand weed resistance to herbicides and the best weed management programs to avert additional problems.

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GROWING PROBLEM: Lowell Sandell, UNL Extension weed science educator, says Nebraska may be relatively new to the problem of herbicide-resistant weeds, vs. other states, but it’s now becoming more of a state concern.

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OBVIOUS RESISTANCE: This photo shows 2,4-D-resistant waterhemp on the left and dead 2,4-D susceptible waterhemp on the right. Photo in UNL greenhouse was taken six weeks after application. University of Nebraska-Lincoln photo

This article published in the February, 2012 edition of NEBRASKA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.