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Herbicide rotation sparks performance

By John Pocock

Josh Smith, Auburn,
Illinois, planted some LibertyLink soybeans for the second consecutive year in
2010. It was also the first year he planted all his soybean acres to

“LibertyLink soybeans yield
well, and the Ignite herbicide does a good job on the broadleaves that have
been hard for us to control the last couple years,” says Smith, who raises corn
and soybeans on about 1,000 acres. “We have some waterhemp that’s been pretty
tough to kill with Roundup. We also have some buttonweeds, or velvetleaf, that
seem like they might be developing some herbicide resistance too.”

Herbicide resistance is a
real worry now, especially in soybeans, emphasizes Smith. “In the past, we
would normally apply two passes of Roundup,” he says. “Now, we’ve rotated to
LibertyLink soybeans and Ignite herbicide, which uses a different mode of

By rotating
glyphosate-tolerant crops with LibertyLink crops that are tolerant to Ignite (a
glufosinate-containing herbicide), farmers can help to prevent
herbicide-resistant weed development, says Andy Hurst, Bayer CropScience,
Ignite product manager. “We can’t continue to spray 97% of the soybean acreage
in the U.S. with glyphosate without also seeing a rapid expansion of weed
resistance,” he says. “Ignite herbicide and LibertyLink traits currently
provide the only non-selective herbicide alternative to glyphosate and
glyphosate-tolerant technology. So now, farmers truly have a means to rotate
their herbicides and hopefully preserve the utility of glyphosate as a
weed-control product.”

Ignite controls many
troublesome weeds, including weeds that are now showing resistance to Roundup,
confirms Jeff Gunsolus, University of Minnesota Extension weed scientist.
“Ignite offers fairly broad-spectrum weed control, and it also does well on
glyphosate-resistant broadleaves like common and giant ragweed and common
waterhemp,” he says. “So, much of the appeal with LibertyLink soybeans is that
it will control the more difficult broadleaf weeds with a different chemistry
than glyphosate.”

While still popular among
farmers, glyphosate is steadily losing effectiveness due to overuse, says

“Herbicide diversification
is the key to preventing resistance,” he says. “Don’t just go to one system and
keep using it – that’s what we’ve been doing with the glyphosate system, and
now we’re seeing resistant weeds that are causing us headaches.”

Thus, farmers who rotate glufosinate-tolerant
and glyphosate-tolerant systems should also seek ways to ensure that weeds
won’t adapt to this strategy either, warns Gunsolus. “If you decide to use this
combination of herbicide products, a chemical rotation with other products is
still important,” he says. “That’s where preemergence and postemergence
tank-mix partners come into play.”

Timing is also key, says
Gunsolus. “With Ignite herbicide and LibertyLink soybeans, you really need to
target the smaller weeds (2 to 4 inches tall) to be successful,” he says. “This
is especially true for difficult-to-control weeds such as common lambsquarters
and glyphosate-resistant broadleaves that emerge at different times during the
growing season. So the preemergence herbicides, such as Authority First, Sonic,
Valor, Gangster, Enlite, Prefix, and OpTill, will help take out the early weed
flushes and allow you to get more value out of Ignite as a post-emergence

Simple Systems Fade

Simple weed-control systems
are ideal but will eventually break down, says Bayer’s Hurst.

“Roundup Ready tremendously
simplified weed management for growers,” he says. “However, weed resistance has
now become an issue with glyphosate in many states, particularly in the South,
with the explosion of glyphosate-resistant palmer amaranth. In the South, weed
resistance is now on the verge of being disastrous for row-crop production.”

As a result, Bayer is
working to facilitate a new, industry-wide initiative to promote good,
integrated weed-management programs for growers, says Hurst.

“This new initiative is all
about supporting the rotation of crops and the rotation of herbicide modes of
action,” he says. “We emphasize the need to use some preemergence soil residual
herbicides with a postemergence program, and we support using a herbicide with
the same mode of action only twice on the same field in a two-year time frame.

“The point that we’re really
looking to stress is that for the first time, growers have the opportunity to
alternate herbicide-tolerant traits or stacked herbicide-tolerant traits
together to enable an effective rotation of glyphosate and Ignite herbicides on
multiple crops,” he adds. “With LibertyLink and Ignite, we provide growers with
excellent-performing soybean, cotton, and corn varieties and efficient weed

Profit Breakers

Glyphosate-resistant weeds
have now become profit-breaking problems for southern farmers, says David
Nichols, who raises corn and soybeans on about 1,700 acres near Ridgely,

“We’ve learned from
experience that if we let resistant weeds like palmer amaranth get away from
us, then it’s game over,” says Nichols, who is past president of the Tennessee
Soybean Growers Association. “So our main effort is to make sure we start clean
and then stop that second flush of weeds right after planting with a
preemergence herbicide.”

Marestail resistance has
been a problem for Nichols for about the past four to five years, and palmer
amaranth resistance has been a problem for about the past two years, he says.

“Since weed resistance is
building in our area, I treat every field as if it is resistant,” says Nichols.
“For us, it’s too late if our first postemergence product doesn’t work well.
After that, the weeds will just become too large to kill with any herbicide
that’s currently on the market.”

In addition to rotating
LibertyLink with Roundup Ready technology on about one quarter of his soybean
acres, Nichols says he’s also been using Ignite along with his preemergence
corn herbicides on some acres.

“We’re trying to change up
our herbicide chemistry and switch away from atrazine and glyphosate sometimes
so that we can keep those products effective when we use them,” says Nichols.
“Lately, we have been using more Ignite in place of glyphosate as a burndown
ahead of soybeans to try to control resistant palmer amaranth and marestail.
We’ve also been using Callisto, Laudis, and Halex as our postemergence products
to rotate away from using glyphosate in corn.”

After corn harvest in late
winter or early spring, Nichols applies a burndown like dicamba or Sharpen to
control resistant marestail ahead of soybeans. At planting or right behind the
planter, he applies a preemergence soybean herbicide in a tankmix with Ignite
or Gramoxone. He follows up with Roundup as a post-emergence soybean
application on his Roundup Ready soybean acres.

“If we see any weed escapes
of palmer amaranth or pigweed, we’ll use Flexstar or Ultra Blazer as early as
we can,” says Nichols. “So to stay on top of weed-resistance management, we
have to stay real close on our scouting.”

Just 10 or 15 years ago,
Nichols sprayed only one or two applications of Roundup postemergence with no
preemergence product. “Now we may apply as many as two burndown applications
and one preemergence application before we ever get to our postemergence
program,” he says. “Resistant weeds are really causing us a lot of extra costs
in manpower and herbicides, but we’re hoping to stay on top of it now.
Resistant palmer pigweed is a real game changer.”

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