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Glyphosate-resistant waterhemp moves into Corn Belt (Friday, Jan. 10, 2003 -- CropChoice news) -- Plant Health Progress, 12/12/02:
First it was horseweed (marestail) in Delaware, confirmed resistant two
years ago to glyphosate-based herbicides such as Touchdown® and Roundup®.
Then the same weed proved resistant in Tennessee, spreading to about 500,000
acres this year, and Ohio researchers say as many acres are likely infested
between southwest Ohio and south central Indiana.
Now, at least one weed scientist is ready to add waterhemp to the list of
glyphosate-resistant weeds.Iowa State University researchers have tested
waterhemp plants from a number of locations around the state. Individual
selected plants survived an average of 2.6 times the labeled rate of
glyphosate, with some plants able to shrug off even higher rates.
Mike Owen, extension weed management specialist with Iowa State University,
says there¹s enough evidence for him to label these individual weeds
Œresistant.¹
³Our work demonstrates that the glyphosate resistance in these waterhemp
plants is heritable,² he says. ³It¹s passed on from one weed generation to
the next, although the exact mechanism and frequency is still unknown. We do
not feel that there is a major concern for glyphosate resistance in
waterhemp at this time.²
In similar waterhemp studies conducted by Reid Smeda, weed scientist with
University of Missouri, waterhemp plants from Missouri and Illinois survived
up to eight times the labeled rate of glyphosate. For now, Smeda is calling
these weeds Œinsensitive¹ to glyphosate. ³It¹s more than a natural
tolerance, but we don¹t have enough evidence yet to call it Œresistance,¹²
he says. ³Even if we¹re not saying resistance, we¹re not controlling
waterhemp as well as we did six years ago,² adds Bryan Young, assistant
professor of weed science at Southern Illinois University. ³We¹re using
higher rates of glyphosate and more applications.²
Regardless of semantics, says Owen, the problem in a grower¹s field is the
same: ³You can¹t kill the weed with the labeled amount of glyphosate.²
Roundup Ready System at Risk
For most growers in the 2003 season, the waterhemp and marestail problems
may not amount to anything more than a chink in the armor of glyphosate, the
largest selling herbicide in the U.S. and the world.But weed scientists in
the Corn Belt say glyphosate resistance will spread, and growers should take
steps now to manage it. Resistance could spoil the effectiveness of Roundup
ReadyÒ (RRÔ) technology, which is now used on about 75% of the soybeans in
the U.S.
It was only a few years ago that waterhemp developed resistance to the most
popular soybean herbicides at that time, a family of chemistry known as
ALS-inhibitors. The timing coincided with the introduction of RR soybeans,
which quickly eclipsed ALS-inhibitors and solved the problem - at least for
a while.
This time around, however, there¹s no major new technology waiting if a
problem develops.
³There¹s no new silver bullet around the corner,² says Owen. If glyphosate
fails, the only alternatives will be older herbicides already on the market,
different management tactics, and mechanical control strategies.³Without
glyphosate in the arsenal or waterhemp, we would go back to narrower windows
of application, intensive scouting, and tank-mixing,² says Tom Hoverstad,
weed scientist at the University of Minnesota. ³We would be evaluating crop
injury versus control, and if our conventional herbicide didn¹t work, we
wouldn¹t always be able to apply it again.² One piece of good news: weed
experts say that because of the way the weed reproduces, resistant waterhemp
isn¹t likely to spread as rapidly as horseweed has. Also, most growers in
the Midwest grow RR soybeans but not RR corn, so they¹re rotating herbicide
modes of action annually. That¹s one real key to preventing or slowing the
spread of resistance. Horseweed (marestail) spread from a few fields in
Tennessee in 2000 to an estimated 500,000 acres in Tennessee and Kentucky in
2002. On the East Coast, glyphosate-resistant horseweed spread from six
Delaware fields to over 50,000 acres in Delaware, Maryland, Virginia, and
New Jersey - all in the same three-season span.
In Ohio, extension weed specialist Jeff Stachler is now reporting
glyphosate-resistant horseweed. ³We have nine populations from four counties
in southwest Ohio surviving four times the labeled glyphosate rate,² says
Stachler. The glyphosate-resistant horseweed is also in Indiana, halfway
between Indianapolis and the Ohio River. He¹s calling for strong resistance
management tactics.
Resistance vs. Weed Shifts
Likewise, Hoverstad says he¹s seeing more waterhemp than ever before in
Minnesota. ³Velvetleaf also seems to have an edge over glyphosate,² he
notes. ³But this is probably the result of weed shifts and not true
resistance.² Like resistance, weed shifts are caused by repeated glyphosate
use. But there¹s no genetic change in the weed population, just a build-up
of weeds that are naturally more tolerant to glyphosate chemistry.
³Glyphosate has never been as consistent on weeds like black nightshade,
yellow nutsedge, and velvetleaf,² says Bob Hartzler, weed management
specialist with Iowa State University. ³In a RR system where you¹re relying
heavily on glyphosate and little else, these weeds can survive, produce
seed, and increase in density.²
In Ohio, dandelion is a good example. Stachler reports that it¹s gone from a
minor weed to a major problem on just about every acre of no-till soybeans.
³It¹s even hard to control it with two tillage passes,² he says. He¹s also
seeing a shift to common lambsquarters in RR soybeans, where glyphosate no
longer gives consistent control of the weed. Dallas Peterson, weed
specialist with Kansas State University, says he is seeing more morningglory
where farmers grow continuous RR soybeans. Like his counterparts in Ohio and
Minnesota, he says this is more of a weed shift than resistance. ³But the
end result is the same,² he notes. ³We¹re left with a weed that our current
program no longer controls.² What Causes Resistance?
Researchers say the culprit in glyphosate resistance and weed shifts is
continuous use of glyphosate in Roundup Ready cropping systems. In
Tennessee, it¹s continuous RR cotton, or RR cotton rotated with RR soybeans.
In Delaware, it¹s continuous RR soybeans. These rotations allow for
glyphosate use every year, with few if any other herbicides in the rotation.
³In the Corn Belt there hasn¹t been as much continuous glyphosate because
many growers have been rotating RR soybeans with conventional corn,² says
Hartzler. But an increase in RR corn threatens to upset that balance.
Introduced in 1999, RR corn took off slowly, but Hartzler says it¹s gaining
popularity in pockets of the Corn Belt, especially where corn is grown for
domestic use instead of export.
³If farmers keep planting more RR corn, we¹ll be in a continuous glyphosate
situation too,² Hartzler says. ³Granted, many farmers would probably use a
pre-emergence treatment on RR corn and that would offer us some protection
against resistance, but probably not enough. There would still be a lot of
selection pressure.²
Rotate to Fend Off Resistance
Despite the potential consequences of resistance, researchers say resistance
management is a tough sell with some growers. They like the simplicity of
Roundup Ready technology and want to use it every year. Many figure they¹ll
make a change after resistance becomes a problem. But once that happens, the
damage is already done.
³Once you apply glyphosate or any herbicide, the clock starts ticking,² says
Smeda. ³If you want to protect the chemistry, you must be proactive.² The
only way to preserve glyphosate is to avoid overusing it, says Hartzler.
He advises growers who plant RR soybeans to avoid RR corn, and vice versa.
³It¹s just too easy to fall into a trap of overusing glyphosate when you¹re
growing one RR crop after another,² says Bryan Young, assistant professor of
weed science with Southern Illinois University.
KSU¹s Peterson agrees. ³If you¹re relying heavily on glyphosate in RR
soybeans, then we encourage you to use conventional weed control systems in
corn,² he says.
Rotation figures heavily in resistance management guidelines issued by
Syngenta for growers using glyphosate-based herbicides - including
Syngenta¹s Touchdown, which is commonly used in RR crops.
³We¹re advising growers to use no more than two applications of Touchdown
or any other glyphosate-based herbicide over a two-year period,² explains
Chuck Foresman, technical brand manager at Syngenta. ³If they¹re using two
sequential sprays on RR soybeans or corn, then we recommend using
conventional seed and herbicides the following year.²
Corn vs. Soybeans
Which crop should be Roundup Ready? Hartzler leans toward soybeans. ³First,
growers are already using RR technology in soybeans but most haven¹t started
yet in corn,² he says. ³Second, we have good alternatives for waterhemp
control in corn, but there is no existing chemistry or new chemistry in
development that would give us the same flexibility for controlling
waterhemp in soybeans.²
Corn is also more susceptible to early-season weed competition than
soybeans, notes Hoverstad. That makes post-emergence glyphosate weed control
a better fit for soybeans.
For now, weed specialists say the biggest challenge will be convincing
growers to use less RR technology instead of more. ³We can¹t keep using
glyphosate until it doesn¹t work anymore, because there¹s nothing else out
there,² concludes Stachler. ³It¹s essential to start addressing resistance
problems now -- before we wind up with super- weeds.² | |
