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Herbicide resistant wild oats needs control

Whether the wild oats are resistant to herbicides or not, a study by Hugh Beckie
with Agriculture and Agri-Food Canada at Saskatoon shows the importance of controlling
wild oats patches. Left unchecked, patches in Beckie's study expanded by an
average of 330 percent. However, when patches were prevented from setting seed,
the research showed that the growth of the patches was as little as 35 percent
over a six year period.

The research highlights the need to incorporate patch management into herbicide-resistance
management strategies. However, Beckie says that in a survey of 158 growers
in Alberta in 2001 and Manitoba in 2002, about 75 percent in both provinces
indicated that they do not control herbicide resistant weeds in suspected patches
before harvest or limit seed spread at harvest. He says that the present adoption
of this weed management practice is low despite a majority of growers who indicated
that they scout fields after herbicide application to check for uncontrolled
weed patches.

"There is a disparity between awareness and action," says Beckie.
"Maybe that's because there hasn't been much research into the effectiveness
of patch management on preventing the expansion of herbicide resistant patches."

Beckie's research changes that. It was conducted on a 160 acre field near Carrot
River, Saskatchewan. The field had been part of a 1995 weed survey and based
on the grower questionnaire, it was rated as high risk for Group 1 or Group
2 weed resistance. In 1996, the wheat field was sprayed for wild oats with a
Group 1 herbicide and later that summer, two patches of wild oats were identified
as having resistance to Group 1, Group 2, or Group 25 herbicides. In late 1997,
the field was re-surveyed for herbicide-resistant wild oats and four patches
were identified.

Subsequently, the field continued to be normally managed by the grower. Each
summer over the six year trial period, the perimeter of the four patches was
marked and mapped to show the extent of the growth of the patch once the wild
oats had headed out. Patches 1 and 2 were the check patches which did not receive
any treatment other than what the grower conducted on the entire field. The
percentage shedded seed in each untreated patch was determined immediately before
swathing to estimate the extent of seed shed at the time of harvest.

The experimental treatment in patches 3 and 4 was the prevention of seed shed
by clipping panicles from wild oats plants and bagging them. The two patches
were monitored periodically to ensure no seed was shed. Beckie says that clipping
seed mimics grower options for preventing seed shed by mowing, tillage, or non-selective
herbicides. Those management options are recommended by many extension specialists,
if a patch occupies a small area less than one acre.

Untreated patches spread rapidly
Beckie says that untreated patches 1 and 2 expanded at an average annual rate
of 31 and 38 percent, respectively. Over the six year period, patch 1 had expanded
by 270 percent and patch 2 by 390 percent. However, the rate of patch expansion
varied by year. For example, patch 1 expanded by nearly 50 percent from 1998
to 1999 and again from 1999 to 2000, but only by four percent between 2001 and
2002.

"2002 was a very dry year, so production of seed was relatively low and
the patch expansion was correspondingly low," explains Beckie. Relatively
high seed production and extensive seed shed before harvest in 1999 and 2000
would have resulted in significant natural seed dispersal. Good herbicide efficacy
and hot, dry conditions in the summer of 2002 resulted in low panicle density
and almost complete seed shed. These conditions may have restricted expansion
of patch 1, located on a broad upper slope, more than patch 2 located in a lower-slope
area with greater soil moisture and having a greater wild oats panicle density.

Relatively rapid expansion of patch 2 in 2001 may have been aided by harvesting
operations in canola as the majority of wild oats spikelets contained seeds.
There was a tendency of these patches to preferentially expand east to west
vs. north to south over the six year period, which is likely attributed to the
same seeding and combine harvester direction maintained each year by the grower.
"Wild oats can travel 150 metres or more with a combine, which can help
establish daughter patches of resistant wild oats," says Beckie.

Even though the unclipped patches 1 and 2 spread by up to 390 percent over
the six years, Beckie says that he was surprised the expansion was not greater.
He explains that the farmer's management helped to prevent the spread. During
the cereal harvests, almost all of the wild oats plants had shed their seed,
resulting in less movement during harvest. Canaryseed harvests were also later,
allowing high seed shedding as well. And Beckie says that there was very good
control of resistant wild oats during the HT canola years. "These all had
implications on the amount of seed shed and spread of wild oats during harvest,"
explains Beckie.

Beckie says that seed movement would be expected to be greater in fields with
more intensive soil disturbance by tillage or seeding equipment, or when the
crop is harvested before the shedding of wild oats seeds. These conditions would
favour faster expansion of wild oats patches or result in daughter patches.
Although rarely adopted, chaff collection would also effectively mitigate seed
movement by the combine harvester.

Preventing seed shed with clipping slows spread dramatically
Seed shed prevention through clipping of wild oats heads greatly reduced the
rate of area expansion of patches 3 and 4. Patch 3 expanded at an average annual
rate of four percent and patch 4 at a rate of eight percent. After six years,
the treated patches had expanded 35 percent in contrast to average expansion
of 330 percent for untreated patches.

The slow, limited expansion of clipped patches over the six year period likely
resulted from movement of seed in soil by equipment at the time of direct seeding.
Wild oats persistence in the seedbank combined with movement of crop residue
at or near the soil surface by the knife openers likely facilitated seed movement.

"We've seen that low disturbance direct seeding can result in greater
patch stability than high disturbance seeding," says Beckie.

In looking at overall management of wild oats, Beckie says that the grower
was able to effectively manage herbicide-resistant wild oats in this field because
of available alternative herbicides. However, the withdrawal from the market
of the Group 25 herbicide, Mataven, means that Group 1 or 2 herbicides are now
called upon more often for wild oats control. And while HT canola is an important
tool in managing herbicide-resistant wild oats, that can pressure growers to
tighten canola rotations which brings its own management issues.

For growers who have the option of mapping out wild oats patches with GPS,
Beckie's research also shows that the relatively stable patches observed in
this study suggests that a single weed map may be used for multiple years. The
resulting map can then be used for patch management of wild oats, whether that
is spraying with a non-selective herbicide, or harvesting for greenfeed or silage
prior to seed set.

"These tools can be important for farmers with multiple resistance who
rely on Group 1 and 2 herbicides," summarizes Beckie. "If they can
contain the patches in the field, then it will help sustain production on that
land." -30-