West Lafayette, Indiana
July 13, 2009
Using a diverse herbicide
application strategy may increase production costs, but a
five-year Purdue University
study shows the practice will drastically reduce weeds and seeds
that are resistant to a popular herbicide.
Excess usage of glyphosate-resistant crops has led to weeds,
such as marestail, that also are resistant to glyphosate, the
herbicide used in Roundup. Bill Johnson, a Purdue associate
professor of weed science, said changing management practices
can almost eliminate resistant marestail and its viable seeds in
the soil.
"Another herbicide application is expensive, and it means more
trips across the field," Johnson said. "But we can reduce the
population and density of resistant weeds, which increases the
crop yield potential."
The results of Johnson's five-year study were published in the
journal Weed Science.
Marestail, also known as horseweed, was the first weed to
develop resistance to glyphosate. Other weeds also are adapting,
Johnson said, reducing the effectiveness of products such as
Roundup, the most widely used herbicide on the market.
It is Roundup's popularity that is contributing to its
diminished effect. Johnson said farmers have come to rely on
Roundup Ready crops that resist glyphosate as an easy way to
control weeds. But overuse of any herbicide allows weeds to
adapt and develop resistance.
Johnson's study found that farmers should diversify the
herbicides they use. Using a variety of herbicides in addition
to Roundup before planting and alternating between Roundup and
other herbicides in corn can significantly reduce marestail.
Fields that had three resistant weeds for every susceptible weed
while using only Roundup and Roundup Ready crops saw weed
populations drop to one resistant weed for every six susceptible
weeds while rotating herbicides as Johnson suggests. That
rotation also may lead to a 95 percent decrease in the number of
viable marestail seeds in the soil.
Continuing with only Roundup and Roundup Ready crops can
intensify the problem, Johnson said.
"Glyphosate-resistant marestail develops very quickly in a
field. Populations reach staggering levels of infestation in
about two years after it is first detected," Johnson said. "For
us, marestail being the first weed that developed resistance
showed that a weed-management system that is solely reliant on
glyphosate is starting to break down. However, a system that
incorporates other herbicides with glyphosate can be sustainable
for quite some time."
The Indiana Soybean Alliance, BASF, Dow AgroSystems, Monsanto
and Syngenta funded Johnson's study. His next step is looking at
management strategies that reduce the prevalence of other weeds
that have built up resistance to glyphosate.
ABSTRACT
Influence of Weed Management
Practices and Crop Rotation on Glyphosate-Resistant Horseweed
Population Dynamics and Crop Yield-Years III and IV
Vince M. Davis, Kevin D. Gibson, Thomas T. Bauman, Stephen C.
Weller, William G. Johnson
Horseweed, Conyza
canadensis, is an increasingly common and problematic weed
in no-till soybean production in the eastern cornbelt due to
the frequent occurrence of biotypes resistant to glyphosate.
The objective of this study was to determine the influence
of crop rotation, winter wheat cover crops (WWCC), residual
non-glyphosate herbicides, and preplant application timing
on the population dynamics of glyphosate-resistant (GR)
horseweed and crop yield. A field study was conducted from
2003 to 2007 in a no-till field located at a site that
contained a moderate infestation of GR horseweed
(approximately 1 plant m-2). The experiment was a split-plot
design with crop rotation (soybean-corn or soybean-soybean)
as main plots and management systems as subplots. Management
systems were evaluated by quantifying in-field horseweed
plant density, seedbank density, and crop yield. Horseweed
densities were collected at the time of postemergence
applications, one month after postemergence applications
(MAP), and at the time of crop harvest or 4 MAP. Viable
seedbank densities were also evaluated from soil samples
collected in the fall following seed rain. Soybean-corn crop
rotation reduced in-field and seedbank horseweed densities
versus continuous soybean in the third and fourth years of
this experiment. Preplant herbicides applied in the spring
were more effective at reducing horseweed plant densities
than when applied in the previous fall. Spring-applied,
residual herbicide systems were the most effective at
reducing season-long in-field horseweed densities and
protecting crop yields since the growth cycle of horseweed
in this region is primarily as a summer annual. Management
systems also influenced the glyphosate-resistant and
glyphosate–susceptible (GS) biotype population structure
after four years of management. The most dramatic shift was
from the initial GR:GS ratio of 3:1 to a ratio of 1:6 after
four years of residual preplant herbicide use followed by
non-glyphosate postemergence herbicides.
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