Seed & Chemical
An action plan.
November 12, 2007 By Top Crop Manager
Weeds resistant to glyphosate continue to crop up around the world. Researchers
have identified resistant biotypes in seven countries, with Spain the most recent
addition in 2004. In the US, glyphosate resistant biotypes are now present in
10 states, the most common one being Canada fleabane (known in the US as horseweed).
So far, none have been reported in Canada. And industry stakeholders are doing
all they can to keep it that way.
Over the past five years, Roundup Ready soybean varieties have become the norm,
claiming well over half of all soybean acres. Current estimates suggest Roundup
Ready corn acreage at about 15 percent of Ontario's corn crop in 2004.
The first case of a glyphosate-resistant broadleaf weed occurred in Delaware
in 2001 after three years of using only glyphosate for weed control in a continuous
no-till Roundup Ready soybean cropping system. That means somewhere between
six and 12 glyphosate applications over a three year period.
"There's a huge opportunity to make your weed management 100 percent glyphosate,"
says Dr. Al Hamill, weed scientist with Agriculture and Agri-Food Canada at
Harrow, Ontario. "Economics could certainly drive you in that direction.
And that would be a very costly mistake."
Hamill is concerned that growers are complacent about the potential loss of
effectiveness of glyphosate because in the past, when resistance developed,
new chemistry was available to take care of it.
"But that's not going to happen this time," says Hamill. "The
well is dry. Herbicide chemistries are not coming as quickly as they used to
and I doubt if there will ever be another herbicide with the environmental friendliness
of glyphosate that does the kind of job it does. Glyphosate was developed 30
years ago when regulatory requirements were much different and the cost of development
was a fraction of what it is today."
Alternatives to glyphosate
Glyphosate-resistant Canada fleabane is already prominent in
10 states in the US. And it is likely only a matter of time before it
appears in Ontario. Field trials were conducted in 2003 to evaluate herbicides
with different modes of action that effectively control Canada fleabane
Classic, First Rate, Amitrol 240 and Broadstrike Dual II Magnum all provided
excellent control as a tank-mix partner with glyphosate or as an alternative
"These are actually much better than the 2,4-D ester tank-mix they're
recommending in the states," says Mike Cowbrough, weed management
lead with the Ontario agriculture ministry. "The tank-mixes we're
suggesting are safer on the bean crops, more economical and provide better
Get with the program
The Ontario Weed Committee
recently struck a subcommittee to develop suggested use patterns to delay the
development of resistance here, especially given the increasing popularity of
Roundup Ready crops. The committee comprises 15 members including agronomists,
crop advisors, representatives of grower groups, farmers and manufacturers.
The committee has developed a common sense approach to weed management designed
to delay the development of glyphosate resistance. "In general, we're suggesting
a four step process," says Mike Cowbrough, weed management lead with the
Ontario agriculture ministry. "And the first one is to know what you have."
Scout fields to determine the weed spectrum present and take note of population
intensities and time of emergence from year-to-year. "Small changes in
weed population dynamics can be addressed before they become a huge problem,"
The second step is to develop a long-term approach to weed management. This
includes crop rotation, herbicide rotation and having a back-up plan. Rotation
between Roundup Ready, conventional crops or other herbicide tolerant crops
such as LibertyLink is a key component of sustainable glyphosate use. Grow Roundup
Ready crops where they will be of most value, economically and to manage weeds.
"The committee has suggested several crop rotation and herbicide application
options, including tank-mixes, that minimize risk of resistance development,"
says Cowbrough. "A number of effective and economical tank-mixes are currently
registered for use in Roundup Ready corn and soybeans, including atrazine +
Marksman for corn and Classic + Assure for soybeans."
Not all farmers will develop a long-term plan. And even the best made plans
can fall off the rails. In those cases, the committee suggests 50 percent of
cultivated acres should receive an 'in-crop' application of a non-glyphosate
herbicide with a different mode of action.
The committee is recommending risk assessment to evaluate how well a weed management
plan addresses herbicide resistance. The Herbicide Resistance Action Committee,
a global industry-based organization with representation from all the major
crop protection companies, has developed a risk assessment guide.
The fourth and final step is to be on the lookout for suspected weed resistance.
Growers should scout their fields for unusual weed escapes. Look for weed species
that should have been controlled but are healthy while other susceptible species
have been controlled. Growers who are suspicious can report them to 877-424-1300.
Stop press on ragweed
Late in December 2004, reports began surfacing that glyphosate-resistant
common ragweed had been discovered in central Missouri. Apparently some
plants in a 20 acre section of the field were able to survive applications
of up to 10 times that used effectively on the susceptible population.
The Bottom Line
The need to protect glyphosate is certainly a large issue with the increase
in herbicide tolerant crops. It's becoming to easy to spray all the crops with
one chemical. Our approach is to use glyphosate as a management tool. Field
histories are checked and fields with problem weeds are planted to a Roundup
Ready crop. We also tank-mix other chemistries to maintain zero tolerance to
resistant weeds. Grahame Hardy, Inkerman, Ontario.
The key to farming successfully is crop rotation and this means rotating chemicals
too. We have peaked at 75 percent of our soybean acres as Roundup Ready, 25
percent IP conventionals. With a corn-soybeans-wheat rotation, this means our
in-crop use of glyphosate is one year in eight.
Some are forced to shorten rotations in some years by conditions and that is
understandable and others are tempted to grow soybeans continuously, but just
imagine what it would be like if we had velvetleaf develop resistance to glyphosate
and conventional chemical groups!
It is also important that the proper rate of glyphosate is used: if you cut
rates you're only helping a weed develop resistance.
It is in the best interests of glyphosate companies to help fund research in
conventional chemicals. There are none of these in the pipeline now and we are
going to need them so that glyphosate can be around longer. Lennie
Aarts, Wainfleet, Ontario.
Glyphosates are not any different than any other herbicide, so the secret to
success is still rotation. Leo Guilbeault, Belle River,