GENET archive


9-Misc: Morning glories and Ragweeds start to develop glyphosateresistance

                                  PART I
-------------------------------- GENET-news -------------------------------

TITLE:  Morning glories creeping their way around popular herbicide, new
        UGA research reports
SOURCE: University of Georgia, USA, by Phil Williams
DATE:   23 Aug 2004

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Morning glories creeping their way around popular herbicide, new UGA
research reports

Athens, Ga. - Morning glories are beloved mailbox flowers all over rural
America, but to farmers, they are something else: a noxious weed that can
lower yields and choke harvesting combines. For some 30 years, however,
the herbicide glyphosate has kept morning glories quite effectively out
of farm fields.

Researchers at the University of Georgia have identified morning glory
families that are tolerant to glyphosate - noxious vines that could cause
problems for the country's farmers.

"Our study suggests that serious and immediate consideration should be
given to developing regional strategies for managing the evolution of
tolerance in morning glories," said Regina Baucom, a doctoral student at
UGA who directed the research.

Baucom and UGA assistant professor of genetics Rodney Mauricio co-
authored the study, which is being published this week in the Proceedings
of the National Academy of Sciences. The research was funded by the
National Science Foundation and a research grant from Sigma Xi.

The tolerance of some morning glories to glyphosate is a naturally
occurring trait, not something caused by the application of RoundUp®, and
other herbicides that contain the chemical, which is used on millions of
home lawns and gardens as well as farm crops. The problem is that the
chemical does kill most morning glories quite effectively so that the
tolerant ones could be the "last weed standing" and leave farmers without
an effective means of control.

The current study does not address the practical concerns of agriculture
however. Rather, it examines genetically how morning glories - both those
that are not killed by glyphosate and those that are - lose or maintain
the ability to produce offspring for future generations.

The issues are complex. The use of herbicides and pesticides has allowed
dramatic increases in food production in the past century, but, as the
paper in PNAS points out, the repeated use of herbicides exerting strong
selection pressure on crop weeds has led to more than 250 documented
cases of herbicide resistance, and "this process is likely to accelerate
with increased reliance on herbicides."

Glyphosate has been available since 1974, but to date only six cases of
glyphosate resistance in plants have been reported out of the 250
documented cases of herbicide resistance. The makers of the best-known
glyphosate herbicide developed RoundUp-Ready® canola, corn, cotton,
soybeans and sugar beets - crop varieties that aren't harmed by
glyphosate, which means it can be used to kill weeds and increase yields.

"Our interviews with farmers in the Southeast suggest that morning
glories can tolerate applications of glyphosate," said Baucom, "and, in
some cases, increasing concentrations of the herbicide have been required
to control it."

Such an increase in tolerance to the chemical gives researchers a unique
opportunity to study the evolutionary genetics of a novel trait and may
help them and others slow the rate of evolution of tolerance in morning

What Baucom and Mauricio found was that, in at least one natural
population of morning glories they studied, there is a substantial
genetic variation for tolerance, meaning that the "evolutionary door" is
wide open. For evolution by natural selection to succeed, there must be
genetic variation with a population and a significant selective force.
This study is a case-in-point of evolution by selection - human-mediated
evolution, similar to the evolution of bacteria resistant to antibiotics.

"Given the continued presence of glyphosate, the number of tolerant
individuals should increase within the population over time," the
scientists reported, "as might the overall level of tolerance of the
population." The fact that glyphosate is a relatively recent tool in the
fight against weeds led the scientists to conclude that the tolerance
trait in this wild population was naturally occurring - not caused by use
of the herbicide.

The presence of genetic variation, however, does not in itself guarantee
that tolerance to glyphosate will evolve. The requirement also exists of
"net selection" for tolerance, and it is acted upon by two components:
fitness costs and benefits. The "benefit" of being tolerant must outweigh
any sort of "cost" of being tolerant, much akin to the theory of economic
cost/benefit models.

In the ecological realm, however, the production of offspring can be
compared to making money. For example, in the face of glyphosate
application, if the benefits of being able to tolerate the chemical
outweigh the costs, then the tolerant individuals will produce offspring
for future generations and the susceptible individuals will not. Costs
are thought to be caused by diverting important nutrients and resources
away from reproduction into the trait(s) conferring the ability to be
tolerant. Costs are evident only in an environment in which the benefit
of tolerance is not needed, that is, in an environment without
glyphosate. Thus, if the benefits of tolerance outweigh the costs, then
glyphosate-tolerant plants can increase in the population by the action
of selection.

In fact, this research has shown that there is positive directional
selection for tolerance to glyphosate, meaning that by applying
glyphosate, those that are tolerant to the herbicide produce more seeds
than those that are susceptible (given that susceptible individuals
either die or produce almost no seed). Perhaps more key for the farmer,
however, is the finding that in an environment devoid of glyphosate,
tolerant families produce many fewer seeds or offspring than susceptible
families. This is evidence of a fitness cost of tolerance, and this
information can be used in managing or controlling the further evolution
of tolerance in morning glories by arguing for not spraying RoundUp® in
certain years. Since the issues are so complex, new strategies will have
to be considered to control increasing numbers of glyphosate-tolerant

"Hers [Baucom's] is the first demonstration of a fitness cost of
tolerance to glyphosate," said Mauricio. "This finding, along with an
analysis suggesting a critical evolutionary threshold has been crossed,
will be of broad interest to scientists and policymakers."

Morning glories are not at the level of such nuisance weeds as musk
thistles in crops, but they are still a widespread problem for farmers.
The new evidence for genetic variation of tolerance in morning glories,
however, points toward a potential problem with no easy solutions.

"For glyphosate, such strategies could involve something as simple as
periodically spraying with alternate herbicides, as long as there is
little cross-tolerance with glyphosate," said the authors. "If, however,
there is cross-tolerance with other causes of plant damage, such as hail,
herbivores or pathogens, alternative spraying regimes may not be a viable
mechanism for controlling the evolution of glyphosate tolerance."

Writer: Phil Williams, 706/542-8501,
Contact: Regina Baucom, 706/542-1417,

                                  PART II
-------------------------------- GENET-news -------------------------------

TITLE:  'Highly suspicious' plants found in north-central Arkansas
SOURCE: Delta Farm Press, USA, by David Bennett
DATE:   30 Aug 2004

------------------- archive: -------------------

'Highly suspicious' plants found in north-central Arkansas
Is ragweed the next glyphosate-resistant weed?

Be it fate or selection - natural or otherwise - it appears common
ragweed has become the latest glyphosate-resistant weed. Found in a 22-
acre patch of north-central Arkansas dryland, no-till soybeans, the hardy
ragweed has survived heavy, and repeated, shots of Roundup. While lab
work has yet to be completed, Extension researchers agree that all
indications point to resistance.

"We're not confirming anything, but we couldn't be more suspicious," says
Bob Scott, Arkansas Extension weed specialist. "To be honest, we hadn't
expected to announce this before absolute verification. But we've seen so
much evidence that, even if these plants aren't resistant, and we're able
to kill them if we spray earlier, this is a problem weed because of its
tolerance. With no-till crops everywhere, that's of major concern regardless."

On June 17, Randy Chlapecka took a call from a worried farmer just up the
road from Newport. The producer reported some ragweed appeared unaffected
after Roundup applications. Chlapecka, Jackson County Extension agent,
then called Scott.

"This producer is not new to the farming game - he knows what he's doing
and he does it well," says Scott. "He pretty much sprays his crops
automatically. So many days after planting, he puts the first Roundup
shot on and so many days after that, the second shot goes on."

The field has been in a grain sorghum/soybean rotation for around a
decade. Roundup has been used for that entire period.

As the field is no-till, there was a pre-plant application. It's
uncertain if the ragweed had emerged prior to that spraying. However,
Scott says the 1.5-pint June application got good coverage.

"So we walked this field. What we found were scattered ragweed in various
stages of control. Amid all the dead weeds, we found very healthy, lush

Scott says he's called out on similar problems regularly. In most fields
it's usually misapplications, a spray boom set too high, the application
rates too low - all kinds of things allowing escapes. But this field
didn't appear to have any of those problems: the situation immediately
set off alarm bells.

Except for a small section cordoned off for tests, another 2 quarts of
Roundup was applied to the 22 acres. Two weeks later, Scott and Chlapecka
checked the progress. Even when pulling into the field from the road,
says Scott, "It was obvious the 2-quart application hadn't done what it

Leaf burn

Surviving 1.5 pints of Roundup causes "a lot" of necrotic and burnt
leaves. The common belief is that makes subsequent Roundup applications
less likely to work. The plant has been toughened up and doesn't take the
herbicide up as it should. That could be a factor here, says Scott, "but
with the plants looking as good as they do after sprayings, it's very odd.

"Right now, this ragweed actually looks like it's being segregated right
in front of you. There are plants of all different sizes and health. The
plants are stair-stepped and you'll see them dead as a doornail right
beside ones that are bright green and happy."

After the 2-quart application didn't provide expected control, Scott and
Chlapecka set up several tests. "On the (cordoned off) section, we went
with 2, 4 and 8 quarts of Roundup Original MAX. In a section where the
farmer had applied the 2 quarts, we came back and put out an additional
2, 4 and 8 quarts. So, in this field today, we have ragweed with
applications of 1.5 pints followed by 2 quarts followed by another 2
quarts and they're still putting on seed. The 4- and 8-quart rates
injured the soybeans but failed to completely control the ragweed."

Researchers will evaluate the ragweed next year and spray earlier. "It's
worth mentioning that for a 12-inch ragweed, the proper rate of Roundup
is 22 ounces," says Scott. "If the ragweed is 18 inches, the rate is 32
ounces. So if you spray a 12-inch ragweed using only a pint, you probably
won't get a kill. A 1.5-pint rate may or may not kill a 12-inch ragweed."

For that reason, Scott admits he's "a bit concerned that we're jumping
the gun. But, by now, these weeds have had so much Roundup on them and so
many plants in the field that look unaffected that it's beyond the
theoretical. The plants might burn up a little and then will send out new
growth. The bad thing is they're still producing seed."

Monsanto is well aware of the problem ragweed, says Scott. The company
already has sample plants in St. Louis being evaluated.

Chlapecka has investigated several other ragweed calls. None have proven

"He thinks the ragweed in those instances was already very large when it
was first sprayed," says Scott. "Those fields are probably just
experiencing partial control. The only thing that makes us believe that
isn't what happened here is the various level of control. There are dead
plants literally right beside plants that are green and full of seed."


Ragweed is a very similar plant to marestail, a confirmed glyphosate
resistant weed. "They're not genetically similar," says Scott, "but they
are in growth habits and physiology. Like marestail, ragweed gets a woody
stem and grows tougher with age. It also likes to grow in the same type
of areas that marestail likes."

If proven to exist, resistant ragweed isn't going to change agriculture
profoundly. Ragweed isn't a huge problem in any major crop and there are
herbicides besides Roundup to control the weed.

The larger issue, says Scott, is the path agriculture is traveling. "Two
years ago, on the front page of Delta Farm Press was a headline saying
'Horseweed is resistant.' Now, two years later, all indications are we've
got resistant ragweed. What's going to be next? Will it be pigweed?"

If, indeed, it is pigweed - as many experts believe - Scott says trouble
looms. "I'll go on the record: it's a matter of when, not if, pigweed
becomes resistant. Tall waterhemp or lambsquarter would also be bad news.
In some areas - like Jackson County - where there's already Scepter/DNA-
resistant pigweed, producers would be in a pickle because there's not
another good option. If that scenario plays out, we'd be knocked back at
least 20 years as far as herbicides."

In the coming weeks, Scott and colleagues will be on the 22 acres
harvesting ragweed seed and taking soil samples "to grow plants from the
seed bank. We'll then grow the plants in the greenhouse and compare those
to a population of ragweed that's definitely Roundup susceptible. We'll
then run all kinds of tests in cooperation with Dick Oliver (a weed
biology specialist at the University of Arkansas) and Ron Talbert (a
resistance management specialist also with the university). By Christmas,
we should have a definite answer on this. I really hope this ragweed is
just tolerant but I wouldn't put any money on it."


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