9-Misc: Are Roundup Ready Weeds in Your Future II
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TITLE: Are Roundup Ready Weeds in Your Future II
SOURCE: Iowa State University, USA, by Bob Hartzler
DATE: Jan 29, 2003
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Are Roundup Ready Weeds in Your Future II
January 29, 2003 - Author note: I first used this title for a paper
presented at the 1998 Crop and Pest Management Short Course, St. Paul,
MN. The following paper was presented at a recent meeting in St. Joeseph, MO.
Weed management generally doesn't generate much interest among the
general public, but the topic of glyphosate resistant weeds was recently
featured on the front page of the Des Moines Register and New York Times.
By now most Midwest farmers have experience with at least one herbicide
resistant weed species, so resistance is not a new problem.
The question is whether glyphosate resistance should be treated different
than resistance to other herbicides. This talk will review where we stand
in terms of glyphosate resistance and potential impacts of this problem.
The first documented case of glyphosate resistance was reported in 1996
involving rigid ryegrasss in Australia.
The resistant biotype was obtained from an orchard near Orange, New South
Wales, Australia. The site had intensive selection pressure, with two or
three applications per year of glyphosate for 15 years. Roundup was used
to control weeds within rows of trees. Greenhouse research confirmed that
this population was 7 to 11 times more resistant to Roundup than
susceptible populations. Since the original report several additional
glyphosate resistant weed populations have been identified: rigid
ryegrass in a wheat production system in Australia and in California,
Italian ryegrass in Chile, goosegrass in Malaysia and horseweed
(marestail) in the east, midwest and southeast U.S. Resistance in
goosegrass is due to an altered target site, whereas the mechanism of
resistance in the other weeds is currently under investigation.
The identification of glyphosate resistant horseweed is the first case of
a weed developing resistance in Roundup Ready soybeans. The resistant
biotype first appeared in Delaware in 2000 and since has spread as far
west as Indiana and has been identified in the Southeastern U.S. where
Roundup Ready cotton is grown. The first population originated in a field
in which glyphosate was the only herbicide used in Roundup Ready soybeans
in 1999 and 2000. Prior to 1999 glyphosate had been used infrequently as
a preplant burndown herbicide in no-tillage systems. The horseweed
biotype has exhibited 8 to 13 fold resistance to glyphosate.
There has been considerable discussion whether waterhemp should be
included on the list of glyphosate resistant weeds. Waterhemp populations
with individuals capable of surviving 'normal' use rates were identified
in Iowa and Missouri the first year that Roundup Ready soybeans were
marketed. Research in the greenhouse and laboratory has shown that the
enhanced tolerance/resistance exhibited by these plants is controlled
genetically, rather than environment, coverage, and other management
A student working for Mike Owen at Iowa State University has been able to
increase the tolerance in one of these problem populations by 3.5 fold
after two generations with recurrent selection (see article).
In academic terms, the research at both Missouri and Iowa could be
interpreted to support that glyphosate resistant waterhemp is indeed
present in the Midwest. Both groups have identified waterhemp populations
that survive higher levels of glyphosate than most populations, and they
have documented that the trait can be passed on to new generations.
However, I believe there are a few important pieces missing, and in my
mind prevent the problem waterhemp populations from being classified as
First, the populations were identified the first time glyphosate was used
as a postemergence herbicide in the field (i.e. the first year RR
soybeans were marketed). Resistance is defined as the ability of a plant
to survive a dose of herbicide that was toxic to the original population.
This implies that the resistance is identified after selection pressure
has weeded out the susceptible individuals. Since the problem was
identified the first time glyphosate was used for general weed control, I
feel these populations fail to meet the criteria of being selected from
the original population. Second, I am not aware of these problem
populations increasing following continued use of glyphosate.
I am not aware of any farmers or dealers in Iowa who have given up on
controlling waterhemp with glyphosate. Higher rates of glyphosate are
currently being used than when RR soybeans were first introduced, and the
percentage of RR soybean fields treated with a preemergence herbicide has
increased dramatically. Waterhemp may be largely responsible for both of
these occurrences in many fields. However, growers are still relying on
glyphosate to control waterhemp postemergence, and I'm not aware of
anyone who adds a diphenylether to glyphosate to control waterhemp in RR
soybeans. For these reasons, I do not think waterhemp should be included
in the list of weeds resistant to glyphosate. However, the research has
documented that within the waterhemp gene pool there is the potential for
resistance to develop, and it is something that should be watched closely.
Impact of glyphosate resistant weeds
There has been a lot of talk lately about the potential impacts of
glyphosate resistant weeds. Some persons have described them as super
weeds, and there have even been inferences that the presence of
glyphosate resistant weeds could reduce the value of farmland. There is
no question that the development of glyphosate resistant weeds will
increase the cost of weed management for farmers, but the question is by
how much. I think there are several possible scenarios: some situations
would have relatively little impact whereas others would pose a major
problem for farmers. Which scenario develops depends upon the
characteristics of the resistant weed, primarily the effectiveness of
alternative tactics on this species and how quickly the weed spreads.
For most weed species we have alternatives to glyphosate that are highly
effective and provide good flexibility in application timing. For these
weeds a farmer could simply add another herbicide to glyphosate to
control the resistant species. In this situation, the primary impact of
the glyphosate resistance is the added cost of the additional herbicide,
otherwise the farmer could use the identical weed management program used
prior to the development of the resistant velvetleaf population. Farmers
who rotate Roundup Ready corn and Roundup Ready soybeans already do this
by using Select or a similar herbicide to control RR volunteer corn in
The more costly scenario would involve a weed for which the alternative
herbicides have limited flexibility in application timing. A weed species
that requires postemergence applications to be made before weeds reach a
4-inch height would have a major impact on weed management systems. In
this situation, the loss of application flexibility would present a
greater cost to many growers than the additional herbicide expense.
The continued growth in farm size increases the importance of the
application flexibility provided by glyphosate.
Since the first report of glyphosate resistant rigid ryegrass in 1996,
four additional resistant species with this trait have been identified.
While not quite one new species per year, this rate of development
suggests that we will continue to see new resistant biotypes. Eventually
one of these weeds will appear in Iowa and surrounding states, and my
guess is it will happen sooner than later. However, the ability to
survive glyphosate does not create a 'super weed', and there is no reason
to use scare tactics to try and change farmers' perceptions and practices.
I believe it is makes good sense for farmers to implement a long-term
plan to reduce the selection pressure placed on weeds by glyphosate. The
simplest way to do this is to avoid planting continuous Roundup Ready
crops. Using additional modes of actions with glyphosate provides
alternative selection pressures on certain weeds, and in some situations
this will reduce the likelihood of resistance. However, since we do not
know which weed is likely to develop resistance, it is impossible to know
whether the alternative mode of action is reducing glyphosate selection
pressure on the appropriate speces.
Thus, we believe an annual rotation of herbicides should be the
foundation of resistance management.
With the manner that glyphosate is being used in the Midwest, resistance
is inevitable. When resistance develops, we will need to control these
biotypes with existing herbicides - no new modes of action are coming
down the pipeline in the foreseeable future. The large number of
alternative products for use in corn and soybean will reduce the impact
of glyphosate resistance, but there can be significant costs associated
with the problem. The need for application flexibility in today's
agriculture increases the cost of glyphosate resistance compared to
previous cases of resistance experienced by Iowa farmers. Because of
this, evaluating weed management programs in terms of selection pressure
placed on weeds should be an important component of crop management planning.
Prepared by Bob Hartzler, extension weed management specialist,
Department of Agronomy, Iowa State University
For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
Common chemical and trade names are used in this publication. The use of
trade names is for clarity by the reader. Inclusion of a trade name does
not imply endorsement of that particular brand of herbicide and exclusion
does not imply nonapproval.