|
Minnesota Crop News > 2001-2008 Archives
December
20, 2004
Selection
of Glyphosate-Resistant Weeds
Jeff Gunsolus,
University of Minnesota, Bev Durgan, University of Minnesota, Chris Boerboom, University
of Wisconsin
Kevin Bradley, University of Missouri, Bob Hartzler, Iowa State
University, Mike Owen, Iowa State University
Bill Johnson, Purdue University , Jim Kells, Michigan State University, Mark Loux, Ohio State University
Dawn Nordby, University of Illinois, Christy Sprague, Michigan State University, Bryan Young, Southern Illinois University
It is well known that glyphosate-resistant horseweed
(also known as marestail) populations have been selected
in Roundup Ready soybean and cotton cropping systems. Resistance
was first reported in Delaware in 2000, a mere 5 years
after the introduction of Roundup Ready soybean. Since
that initial report, glyphosate-resistant horseweed is
now reported in 12 states and is estimated to affect 1.5
million acres in Tennessee alone.
A person could ask if this is any indication of what
might lie ahead. On one hand, it has been proposed in a
popular advertisement that glyphosate-resistant weeds are
unlikely to occur if glyphosate is frequently used, as
long as glyphosate is applied at full rates. The comments
in this advertisement, in part, are based on several long-term
university studies of Roundup Ready cropping systems. However,
the question that a person should ask about these studies
is whether or not they can prove that resistance will or
won’t happen. It is our belief that these studies
are not large enough to test if resistance will develop.
For example, the University of Wisconsin has a 7 year Roundup
Ready cropping system trial. This trial has horseweed in
the no-till plots. Despite burndown and in-crop treatments
with glyphosate, glyphosate-resistant horseweed has not
developed in these plots. Since this trial did not find
glyphosate-resistant horseweed, does this mean that glyphosate-resistant
horseweed cannot develop? Does it mean that the resistant
horseweed in Ohio or Tennessee or Delaware is not truly
resistant? Obviously not. Small scale trials cannot prove
that some event will not occur when a larger scale is considered.
In total, these Roundup Ready cropping system trials may
only be testing continuous glyphosate use on perhaps 50
acres, which is an extremely small fraction relative to
total glyphosate. The true real test to determine if a
rare event like glyphosate-resistant weeds will develop
is actually being tested on the tens of millions of acres
of Roundup Ready corn, soybean, and cotton that are sprayed
each year.
Many weed scientists across the Midwest have warned of
the potential for additional glyphosate-resistant weeds
if a “high selection pressure” is maintained.
In this case, “high selection pressure” refers
to the repeated use of glyphosate without interruption
by herbicides with other modes of action or other weed
management practices. This potential was confirmed this
week at the North Central Weed Science Society Meeting
where glyphosate-resistant common ragweed weed was reported.
This is the first report of glyphosate-resistance for common
ragweed. It was identified in a Missouri no-till soybean
field that has been in continuous soybean production (with
some double crop wheat) for many years and in Roundup Ready
soybean since 1996. The common ragweed in this field had
a high selection pressure for glyphosate with one or more
glyphosate applications per year. This is the second example
of a glyphosate-resistant weed that has developed in a
Roundup Ready cropping system with high selection pressure.
We do not know which glyphosate-resistant weed will be
the next to develop or when it will occur, but high selection
pressure will likely result in additional cases of resistance.
Midwest weed scientists believe in the value that glyphosate
and Roundup Ready crops offer to growers. We hope that
growers and crop advisors will evaluate how they use glyphosate
and the Roundup Ready technologies to gain the value of
these technologies without increasing the risk of resistance.
Ideally, we recommend:
- Alternating glyphosate use with other herbicide modes
of action between years.
- Incorporate appropriate integrated weed management
practices such as soil-applied herbicides and cultivation
with the use of glyphosate.
- In no-till systems, tank mix glyphosate with another
mode of action like 2,4-D in burndown treatments.
|
