TOWANDA, Ill. — Football coaches enter each season with a
playbook outlining strategies to defeat their opponents, and farmers are no
different as they try to notch wins over weeds.
Like opposing teams, weeds have evolved over time,
necessitating changes in management strategies.
Jeff Bunting, Growmark Crop Protection Division manager,
reviewed the evolution of weed resistance and major changes in herbicide usage
during Soy Capital Ag Services’ plot day.
Prior to glyphosate’s introduction, the herbicide Pursuit
with its active ingredient imazethapyr was applied to a high percentage of
acres. It also was during that period that resistant waterhemp was found in
Illinois and Iowa.
“Waterhemp was one of those challenging weeds that caused
the evolution in the use of glyphosate,” Bunting said.
“Resistance is here to stay. It all started back in 2000
with marestail. Horseweed being the major issue, and now we have giant ragweed,
waterhemp, Palmer amaranth. All of these weeds are making us look at our system
to see how we can manage that going forward,” he said.
Bunting noted the next era of post-emergence herbicides
awaits regulatory approval.
“Enlist Duo and the Roundup Ready 2 Xtend system are
game-changers. These will allow us to look at our processes and our programs in
a whole different light if and when they do become deregulated for us to use in
Illinois fields,” he said.
“Our crop protection products have changed in the last 15
years. The things we did in the past, we don’t do today, which is probably a
good thing, which goes back to the evolution of the resistance and the programs
we need to control those weeds.”
There also have been changes in application rates.
“In the past, we’ve come out with maybe the 22-ounce rate or
the 28 ounces of glyphosate usually in that first shot. Over the last two,
three, four, maybe even five years, we’re seeing slow gradual increases in
glyphosate rates,” Bunting said.
“The 22-ounce rate is now the half rate that we’re using.
We’re going out there with the 40 ounces of glyphosate per acre in that first
shot, and it’s really challenged the overall system in regard to how to approach
The expansion of glyphosate-resistant weeds also has
necessitated the use of other modes of action, a management tool that has been
utilized for the past few decades.
Atrazine was the No. 1 herbicide used on corn acres in 1996,
and Dual, dicamba, harness, Beacon, Bladex and others also were in the mix that
year, Bunting said.
“In 2005, the No. 1 herbicide was still atrazine. In 2010,
the No. 1 herbicide was Roundup and No. 2 was atrazine,” he said.
“When you think about the evolution of products, we started
bringing in different chemistries back in 1996 to 2010 and slowly bringing
glyphosate into the overall weed management picture.”
On the soybean side, Pursuit was the most-used herbicide in
1996, and glyphosate took over by 2006.
“Glyphosate was on 92 percent of the soybean acres in 2006.
Second at 7 percent was 2,4-D,” Bunting said.
“So I’m not surprised today to realize from a U.S.
perspective we have 12 species of weeds resistant to glyphosate going back to
the overall use of glyphosate the last couple of years. Is that sustainable?
“The No. 1 herbicide used in 2012 was also glyphosate, but
the good news is you have a lot of different chemistries that are now in that
mix, including Valor, Authority, 2,4-D, dicamba, and all those products now are
used on a much higher scale than what they were in 2006. That is taking the
pressure off the glyphosate to get a better foothold on those activities.”
In an effort to control tough weeds such as marestail,
waterhemp, giant ragweed, lambsquarter and velvet weed, growers are leaning more
toward a fall herbicide application.
“They’re getting a head start on maybe the marestail
population with some 2,4-D or dicamba. Our sales of 2,4-D in 2012 were about 50
percent higher than the year before that,” Bunting said.
However, farmers switched strategies this year due to the
“Growers didn’t want to wait seven days to plant according
to what the label said, so they took 2,4-D and dicamba out of their weed
management plans,” Bunting said.
“That put a lot of pressure on the post side to get things
cleaned up. So they used a higher rate of glyphosate and maybe some different
combinations of other active ingredients because the 2,4-D wasn’t in that
He said one of the brightest spots in weed management
changes has been the higher use of residual herbicides on soybean acres.
“A couple of years ago, they talked about the Roundup Ready
rate and the half rate of the product. Today, we’re seeing higher rates on those
soybean acres,” he said.
“We’re seeing the full rates based on the population, soil
type and those situations we have because we need those higher rates to control
those weeds that are out there.
“The last couple years on the post applications, we’ve also
seen some sort of residual thrown in that tank with the glyphosate.”
Bunting stressed the importance of monitoring fields for
weeds such as waterhemp topping the crop canopy.
“Keep an eye out for those resistant populations, especially
now as we’re seeing some of the Palmer amaranth coming in from the south. That
is one you definitely want to keep an eye on,” he said. “Be thinking about
managing those populations, especially on the residual side of the
He said the new weed control products in the research and
development pipeline awaiting regulatory approval “are the next game-changer —
it will be similar to the impact we saw with Roundup Ready in the late 1990s.”
Bunting said there are other herbicide products in the
pipeline that won’t be available in the marketplace until around 2016-2018
“It’s going to get pretty complicated out there. There will
be a lot of things out there that will make us look at our programs to make sure
that we do things right,” he said.
“Utilize the technologies that we have today. Utilize those
residuals. Utilize those fall applications, the soybean residuals.
“Look at your use rates. Look at those rates in the
soil-applied to make sure we take the pressure off that glyphosate or that 2,4-D
or dicamba when it does get deregulated.
“If you happen to miss a waterhemp, your only option right
now is to use a PPO type chemistry on that or get your garden hoe and cut them
out. There are some areas where that is being done, especially in the southern
and western parts of Illinois where they have a lot of Palmer amaranth that they
can’t control with chemistry.”
The other challenge is: If a field has waterhemp or
marestail, growers should not assume the weed is resistant to one product.
“There is a lot of cross-resistance out there where you have
a marestail plant that you can’t use an ALS chemistry on, you can’t use a
glyphosate, you might not be able to use an atrazine, so a lot that is out
there, too, with regard to the complexities,” Bunting said.
He recommended keeping records of what herbicide chemistry
was applied and, if there are signs of resistance, another product should be