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The Drawback to Simple and Convenient Systems

Mike Owen has seen some good
things happen in crop production during his career as an Extension weeds
specialist at Iowa State University (ISU). Bad things often come to his mind
first, though. He pointed out several examples to those attending this month’s
ISU Integrated Crop Management Conference.


“When I got here, woolly
cupgrass was not on anyone’s radar,” he says. “It became a huge issue in the
Midwest within 10 years.”

the mid-1990s debut of glyphosate-tolerant systems curbed many woolly cupgrass

This hasn’t been the case with
all weeds, though. “In the 1980s, no one knew what common waterhemp was,” says
Owen. Now, this weed has biotypes that resist multiple herbicide modes of


The corn-soybean rotation generally
held corn rootworm in check in the 1980s. No more. Extended diapause in
northern corn rootworm and variant populations of the western corn rootworm have
emerged in more areas to nix crop rotation as a control measure.


Little was known about maladies
like soybean cyst nematode, white mold and Sudden Death Syndrome in soybeans in
the 1980s. Now, they consistently threaten Midwestern soybean production.

“All of these have
happened in a short period of biological time,” Owen says.

So what gives?

These maladies and others are rooted
in the success of the current corn and soybean management system. Practices like
repeated reliance on a single herbicide mode may be simple and convenient.
Repeated use, though, places selection pressure on that rare weed within a
population that resists a herbicide. Each subsequent use of the herbicide
enables the surviving weed to multiply until the herbicide no longer controls
the weed.

“We are essentially creating the
problem due to lack of diversity,” he says.

This applies to all herbicides
and herbicide-tolerant systems. For example, glufosinate-tolerant (LibertyLink)
systems are an alternative to glyphosate-tolerant systems. Yet, weed resistance
to glufosinate can develop if it is repeatedly used. In 2009, researchers in
Malaysia confirmed glufosinate-resistant goosegrass that infests oil palm and
rubber plantations.

Management Steps  

One way to manage herbicide weed
resistance is to rotate herbicide modes of action. By itself, though, it’s not
a cure-all. Owen notes a farmer friend of his used herbicides other than
glyphosate in his corn. He countered this the following year in
glyphosate-tolerant soybeans with two glyphosate applications.

“His corn was clean, but his
soybeans were a mess,” says Owen. Ultimately, tall waterhemp surfaced so
resistant to glyphosate that even a 96-ounce per acre application—several times
the recommended level—did not kill it.

Owen adds rotation of modes of
action is a good component of a resistant management strategy. Still, it’s only
one part of a successful program.

“When you rotate year-in and
year-out, what you gain is one year. So, if it takes five years for glyphosate
resistance to develop in a particular field, rotating with something else other
than glyphosate every other year will result in 10 years before you see a major
resistance problem. The whole idea of rotating modes of action is good as far
as it goes, but it only delays the inevitable evolution of resistance to

why an integrated strategy involving other tactics like variable herbicide
application timing, herbicide tank mixes, and
primary tillage needs to be considered.


Fix it now

There’s an old saying that goes
“if it isn’t broke, don’t fix it.”

Well, when it comes to weed
control and herbicide resistance, the opposite is true.

“If it’s not broken, that’s exactly
the time you should fix it,” says Owen. If you keep doing the same thing, you
end up with problems.” 

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