Features Herbicides Seed & Chemical
Weed resistance to herbicides a disturbing trend

Agricultural producers are facing the serious issue of escalating weed
resistance to herbicides. The numbers of resistant weed species,
locations affected, and herbicide modes of action involved are all on
the rise. Compounding the problem, some weeds have developed resistance
to multiple modes of action, making their control that much more
difficult, if not impossible.

Dr. Mike Owen, University of Iowa, works in the US Midwest corn/soybean belt, where the majority of acres are planted to glyphosate-tolerant crops and there is high pressure for the selection of glyphosate-resistant weeds.
Conference participants learn about the development of herbicide-resistant weeds throughout the Americas and discuss how to maximize the long-term sustainability of herbicides.

There is an urgent need to develop and use tools and methods that will
make the herbicides that are currently available more sustainable over
the long term. For this reason, close to 300 participants from 18
countries met for a three-day conference in Miami in late-January 2010
to discuss weed resistance issues. The meeting was hosted by Bayer
CropScience and attended by representatives from industry,
universities, governments, and the media.

Presentations of weed resistance cases from Canada, the US, Mexico,
Brazil, Argentina and other areas drove home the fact that weed
resistance is an increasing problem. Resistant weeds are present in all
major crops, e.g., corn, soybeans, rice, cotton, cereals, orchards and
vines, and for virtually all classes of herbicide. Despite this, “the
growers’ perception is that the resistance threat is low,” said Dr.
Mike Owen, agronomist and weed extension specialist at Iowa State
University. “It is not the herbicides, but the way they are used that
cause problems,” stated Owen, a statement that was echoed by other
industry experts during the conference.

As an extension specialist, he is trying to get growers to recognize
that proactive management would effectively eliminate the resistance
problem. He thinks that growers have been misled by the convenience and
simplicity of some herbicide systems and have disregarded the risks of
neglecting herbicide and crop diversity. “Growers want to take steps,
but many are unwilling to initiate programs because they don’t want to
spend time planning and spend money for different, more costly
herbicides,” he said.

“Growers need to start using mixtures and diversity measures before
their current technology fails on a large scale, not after,” stated
another presenter.

The longer growers wait to address the weed resistance issue, the worse
the problem becomes. The chemical industry will not be able to develop
new herbicides and modes of action to keep up with resistant weeds. “We
need to recognize herbicides as precious chemical resources,” said Dr.
Stephen Powles, plant and crop scientist at the University of Western
Australia and director of the Western Australia Herbicide Resistance
Initiative. “Glyphosate was a one in 100 year discovery,” he said,
likening it to the discovery of penicillin. “We should do everything in
our power to keep it working.”

Maximizing herbicide sustainability
“Weed resistance is a global problem, but it is controlled locally on
each farmer’s field,” emphasized Dr. Harry Strek, head of integrated
weed management and resistance biology for Bayer CropScience. He noted
that herbicide companies could help growers by providing better
herbicide information and tailored management recommendations.

The consensus is that main way to lessen and manage weed resistance
development is to use diversity in both herbicide application and crop
rotation. This was stressed by various presenters; it will take more
planning and more work by growers, and will likely have higher costs.
It will also require the registration of additional useful tank mixes
by herbicide companies. 

One way to facilitate herbicide rotation might be through mandatory
labelling of herbicides with a clear indication of the herbicide group
or mode of action, said Dr. Hugh Beckie, Agriculture and Agri-Food
Canada weed scientist and adjunct professor at the University of
Saskatoon, Saskatchewan. He noted that such voluntary labelling is
being tried in Canada. He also suggested that close management of
resistant patches within a field to prevent new seeds from entering the
seed bank could help in containing resistant populations.

Speakers also stressed the importance of using the correct strength of
herbicide products. For example, participants learned that much of the
early glyphosate resistance that developed in Brazil arose before
glyphosate-tolerant crop technology was made legal. During that time,
growers were using the technology without supervision or company
technology support, said Dr. Pedro Cristoffoleti, weed scientist at the
Luiz de Queiroz College of Agriculture, University of São Paulo,
Brazil. He suggested that application rates that were too low likely
caused the rapid development of glyphosate resistance. Fewer
glyphosate-resistant weeds have been found since the technology was
legalized, he added. Insufficient application rates are problematic
because they leave weeds behind and facilitate the selection of
resistant individuals. 

Herbicide must also be applied at the right time in the weed life
cycle. If applied too late, weeds are too big to be killed completely
and can grow back, facilitating the selection of resistant individuals.
For some weeds, the correct timing of application can be a matter of a
few days’ window, so growers must keep on top of scouting weed
development. “The era of total post weed control is over, and I don’t
think it’s coming back,” said Dr. Larry Steckel, University of
Tennessee extension weed specialist for row crops. He noted that
glufosinate, a broad-spectrum, non-selective herbicide, is working with
glufosinate-tolerant crops (e.g., LibertyLink) in some areas of
Tennessee where glyphosate resistance is a serious problem. However, he
cautioned that growers should make sure not to make the same mistakes
they made with glyphosate by neglecting herbicide and crop diversity or
else they will end up with no way of controlling some of the major,
problematic weeds.