Bismarck Farm and Ranch Guide, ND
04-13-07
Adding diversity is key to effective weed management strategy
By ANDREA JOHNSON, Assistant Editor
A plant can have great genetic diversity that allows it to adapt readily to its
environments.
This is certainly true for some weed species found in corn and soybean fields.
For some weeds, even pulling them out of the soil may not be enough to stop
them. Roots can work their way back into the soil if the soil is wet. Roots left in
the ground can start to grow again. The wind can blow seeds from uprooted
plants to produce a whole new crop of weeds.
If pulling out a weed is not enough to stop it, is it any wonder that weeds can
adapt to herbicides?
Syngenta Crop Protection recently hosted a web seminar entitled “Glyphosate
Resistance in the Midwest - What Lies Ahead?”
Glyphosate is sometimes called the world's greatest herbicide - a one-in-100year-discovery. This herbicide has so many attributes - it can control larger
weeds better than many other herbicides. It is inexpensive, safe to use and
provides broad-spectrum weed control.
First introduced in the mid-1970s, glyphosate was used primarily for burndown
and perennial weed control in corn and soybeans. Glyphosate-resistant weeds
were not an issue.
Then in 1996, Roundup Ready crops were introduced. Glyphosate has been
widely used for both burndown and in-crop weed control ever since. This has
significantly increased the number of acres where glyphosate is used and
increased the potential for selecting glyphosate-resistant weeds in the Midwest.
“I no longer call it the Corn Belt. I call it the glyphosate belt,” said Stephen
Powles, University of Western Australia professor and director of the Western
Australia Herbicide Resistance Initiative. “It is clear that weeds are developing
resistance to glyphosate. We need to use diversity in the system.”
Powles said that weed resistance is a real challenge in Western Australia
because of monoculture farming practices. He doesn't want to see the same
thing happen in the United States.
To fight weed resistance, he recommends herbicide diversity, tillage, and other
non-herbicide tools that make economic sense.
Currently there are several reports of weeds showing resistance to glyphosate in
the United States. Rigid ryegrass was reported in California in 1998. Glyphosate
resistant horseweed has been found in 16 states since 2000. Italian ryegrass and
common ragweed were first reported as glyphosate resistant in 2004 in several
states.
Palmer pigweed in Georgia and waterhemp in Missouri were reported in 2005.
“Glyphosate resistant weeds can occur even though the gene for resistance is
rare,” said Chris Boerboom, University of Wisconsin Extension weed specialist.
“But the number and geographic distribution of most glyphosate-resistant weeds
is limited. This means there is still time to adopt good management practices.”
Adding diversity is the key to weed management programs, he said.
Boerboom listed the following weed management practices that avoid continuous
and extensive use of glyphosate and lessen the potential for developing
glyphosate-resistant weeds:
- Rotate between Roundup Ready and conventional crops or crops with other
types of herbicide resistance. Use Roundup Ready crops and glyphosate when it
offers the greatest economic and management value.
- Rotate glyphosate with herbicides that have different modes of action. Apply at
full rates.
- Apply a residual herbicide before glyphosate or tank mix another herbicide with
glyphosate.
- Avoid making more than two glyphosate applications to a field over a two-year
period.
- If glyphosate is used as a burndown treatment and in-crop in the same year,
tank mix the glyphosate applied in the burndown treatment with an herbicide that
has a different mode of action. The in-crop glyphosate application should still be
rotated with other herbicides in other years.
- Use cultivation and other mechanical weed management practices.
- Apply glyphosate at labeled rates and at the correct stage of weed and crop
growth to reduce the risk of poor control.
- Scout fields regularly, identify weeds present, and mark their locations on a
map to watch for weed population shifts and the development of resistant weeds.
The information for this article came from the Syngenta web seminar and a
bulletin entitled, “Facts about Glyphosate-Resistant Weeds” by Chris Boerboom,
University of Wisconsin, and Michael Owen, Iowa State University. For more
information from Syngenta, visit http://www.resistancefighter.com.