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Glyphosate resistance continues to build

More weeds species a concern for researchers.


November 13, 2007
By Ralph Pearce


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8aAt the 2005 Southwest Agricultural Conference at Ridgetown College, Dr. Bill
Johnson, a professor of weed science from Purdue University in West Lafayette,
Indiana, made a presentation on the progress of glyphosate-resistant Canada
fleabane. Among his observations he noted weed escapes migrating northward,
near Findlay, Ohio and Fort Wayne, Indiana.

But according to a growing number of researchers and weed specialists, glyphosate-resistant
Canada fleabane is just one species, and the concern with resistance must be
broadened to include others. Ignoring the problem or dismissing it as a greater
concern for growers in the US than those in Canada is overlooking the obvious:
glyphosate resistance is inevitable.

More and more evidence
Already across the US midwest and through the east, the number of resistant
species is increasing, including Palmer amaranth, a pigweed species, in Georgia
and North Carolina; common waterhemp and common ragweed in Missouri; and glyphosate-resistant
Canada fleabane across much of the eastern US. "It's not a matter of 'if'
it's going to happen here in Ontario, but 'when', and it's going to be extremely
important that all farmers adopt responsible glyphosate use to minimize and
delay the selection for these glyphosate-resistant biotypes," says Dr.
Peter Sikkema, an assistant professor in field crop weed management at Ridgetown
College.

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For several years, Sikkema has warned of glyphosate resistance developing in
various weed species, in spite of the current nature of farming. That reality
demands growers use what works in the 'here and now', intent more on paying
the mortgage today instead of looking ahead to 10 or 20 years from now. There
is also the continuing tendency to trust that the industry will come up with
something new in the nick of time. "That's just not the case," declares
Sikkema, adding that there are other herbicides that are being developed. "But
glyphosate is such a unique molecule there isn't anything as good as it."

History may look back critically
In fact, according to Sikkema, there is no molecule that approaches glyphosate's
flexibility, efficacy and crop safety, and he cites it as a one in 100 or 200
year discovery. "And because it's such an amazing molecule, it's incumbent
upon every weed scientist to protect that technology for future generations,"
he says. Taking it one step further, Sikkema believes history will look somewhat
critically at what he sees as a lack of effort on the part of weed scientists
and warnings of weed resistance. "It's my guess that in the future, weed
scientists at the turn of the century, including myself, will be judged harshly
for not being more outspoken about the potential for the development of glyphosate-resistant
weeds and encouraging growers to use prudent management practices. We should
be advising growers to not purchase Roundup Ready hybrids or varieties for more
than 50 percent of their acreage in any given year."

Happening first in the US
Dr. Bill Johnson agrees with Sikkema's assessment of glyphosate overuse. Based
on his research at Purdue University, he notes the northward migration of a
weed like Canada fleabane but says its resistance to glyphosate is only part
of the picture. "As we move further north, lamb's quarters and giant ragweed
control issues become larger, and I'm sure Ontario growers are going to have
lamb's quarters control issues in the very near future, because they're pretty
prevalent in northern Indiana, northern Ohio and Michigan," says Johnson.
He notes the giant ragweed issue is equally critical, especially since finding
a number of escapes in fields that have been sprayed with glyphosate as many
as three times. "In Indiana and Ohio, that one may have a bigger impact
on soybean yields than soybean rust ever will, if glyphosate-resistant giant
ragweed becomes widespread. And we don't have anything for glyphosate-resistant
giant ragweed because so much of it is already ALS (Group II) resistant."

That comment does not win Johnson many friends among field pathologists, but
he says growers will have sprays to control Asian soybean rust when it arrives.
Another comment Johnson hears often is that he and other weed scientists are
anti-Monsanto or anti-Roundup Ready. "Nothing could be further from the
truth," he says, echoing many of Sikkema's observations about glyphosate's
attributes. "We're the biggest supporters of it, and for that reason, we
want to keep the technology available as long as possible."

What must change
Among the many changes Johnson believes must occur before the tide turns on
glyphosate resistance is the same sense of anticipation Sikkema and others cite
regarding new chemistries. "US growers have this perception that the spigot
will be turned on and another molecule will be spit out, or that there are new
herbicides out there, when in fact, we're just re-mixing old compounds,"
explains Johnson. "But the probability of having a new mode of action for
soybeans or corn in the next five years is non-existent."

Based on conversations Johnson has had with manufacturers, herbicide development
is no longer the first priority, but third behind developing input traits in
the plants themselves, and equipment enhancements. "The good news is that
glyphosate-resistant weeds will change that," says Johnson. But how long
will it take to see that change remains a question.

Legislation not an option… yet
The last resort to encourage a reduction or the delay of glyphosate-resistance
is through legislation, and neither Johnson nor Sikkema want to see that. Although
in Canada, the Canadian Food Inspection Agency has succeeded in imposing specific
guidelines for growing Bt corn, Sikkema points out that European corn borers
are mobile but weed seeds generally are not. And while the Bt corn guidelines
have been effective, growers have continued to put too much selection pressure
on glyphosate.

Johnson adds that in the US, the Environmental Protection Agency (EPA) is aware
of the situation with glyphosate resistance but it knows growers do not want
more rules and regulations forced on them.

Specific recommendations
In early 2005, the Ontario Weed Committee, a group of individuals from government,
academia, private sector and independent agri-business professionals, developed
a document entitled Responsible Glyphosate Use in Field Crops. It contains
seven recommendations for growers using glyphosate and glyphosate tolerant crops
(Roundup Ready), based on a similar document first developed by the University
of Wisconsin.

The recommendations are simple and straightforward, and will help with responsible
stewardship of glyphosate and its technological applications. Dr. Peter Sikkema
is a member of the Ontario Weed Committee, and a dvocates growers follow the
'seven steps of responsible glyphosate use':

  1. Use 'Roundup Ready' technology in fields where it will have the greatest
    economic benefit;
  2. Rotate 'Roundup Ready' crops with conventional or other herbicide tolerant
    crops as well as rotate herbicides with different modes of action;
  3. Always use the full labelled rate of glyphosate;
  4. Tank-mix glyphosate with residual herbicides where appropriate;
  5. Use cultivation where appropriate;
  6. Scout for and report any suspected glyphosate-resistant weeds to 1-877-424-1300,
    and
  7. Control and prevent the spread of weed escapes.

For more information on herbicide resistance, go to: