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4 ways to squelch herbicide-tolerant weeds

By John Pocock

Corn and soybean growers who
don’t currently use an integrated weed-management program are most likely in a
pickle when it comes to potential herbicide-resistant weed problems – and not
just with glyphosate, the active ingredient in Roundup, says Mike Owen, Iowa
State University Extension weed specialist.

“Glyphosate resistance is a
potential issue for most corn and soybean acres in the Midwest,” says Owen.
“There are also about 19 other herbicide mechanisms of action, however, for
which weeds have evolved resistance. In the Midwest, these mechanisms of action
include triazines, ALS inhibitors, and PPO inhibitors. In parts of the U.S.,
we’ve had herbicide-resistant weeds for over 30 years.”

But when it comes to
herbicide resistance in Iowa, the four weeds that researchers and farmers are
typically most concerned about are common waterhemp, common lambsquarters,
giant ragweed, and marestail/horseweed, says Owen. “Those are the ones that I’d
be looking at the closest,” he says. “In particular, some common waterhemp
populations are resistant to multiple herbicides: glyphosate, triazine
products, ALS inhibitors, and PPO inhibitors.”


Farmers who want to be
proactive in their weed-control strategies need to be scouting, emphasizes
Owen. “Herbicide resistance in weeds is simply Darwinian evolution in
fast-forward,” he says. “So, if you don’t know what’s out in your fields, you
can’t make a good judgment on what to use to control them.”

In addition to regularly
scouting fields, farmers can employ four other strategies to help minimize the
threat from potential herbicide-resistant weeds in corn and soybeans, says


1. Start out with a
soil-applied, early preplant product.

Use one that gives residual
control for the key weeds competing against crop yields that are also present
in your fields. “You might want to take a look at some triazine herbicides for
use in corn (atrazine, for example) or soybeans (metribuzin, for instance),”
says Owen. “It just depends on the specific weeds that you’re trying to

2. Tankmix glyphosate with
another postemergence product.

You want a product that’s
effective on weeds that might be evolving resistance to glyphosate. “Some
possible herbicides to rotate with glyphosate in corn would be 4-HPPD
inhibitors like Callisto, Impact, Balance Pro, or Laudis,” says Owen. “Those
are just some suggestions for use, not endorsements,” he says.

3. Rotate your herbicides
and your crops.


“Rotating LibertyLink with
Roundup Ready traits might be a good management strategy, as long as the weeds
that you’re concerned about can be controlled by Ignite herbicide,” says Owen.
“Again, what you use is all predicated on the weed that you’re trying to

Any product that is
advertised to be a simple answer to weed resistance is inaccurate, says Owen.
“Managing weeds – whether herbicide-resistant or herbicide-sensitive – requires
thought, planning, and flexibility,” he points out. “The greater the diversity
of the weed-management programs, the more successful they will be economically,
ecologically, and environmentally.”

4. Consider some secondary
tillage. Rotary hoes are one option.

“The adoption of
conservation tillage programs as a result of glyphosate-resistant crops is a
benefit for growers at many levels,” says Owen.

“However, the best option for managing glyphosate-resistant weeds that have
adapted to conservation tillage – like marestail, for example – is to add some
tillage into the crop-production system on specific fields that have that weed
problem. Tillage is not the best option for the environment or for managing the
farmer’s time or cost of production. But it is the most effective solution to
herbicide-resistant weeds,” he says.

Only those fields with
herbicide-resistant weed problems will need some tillage, and other tactics
should also be used on those fields besides tillage, adds Owen. “Fields can be
quickly returned to conservation tillage,” he says. “The key ecological
principle is to break up the system, thus, placing the adapted weed at a

Weeds will adapt not only to
herbicides but also to other production practices, such as tillage, notes Owen.
“If it’s simple, it’s wrong,” he says. “Mother Nature can figure out how to
overcome simple weed-management programs pretty quickly. So, basically farmers
need to focus on diversifying their overall weed-management program in order to
minimize the threat from herbicide-resistant weeds.”

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