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Broadleaf weed impact on your crop

Severe infestations can hurt yield in wheat, so consider how to integrate broadleaf weed control into grassy weed herbicide selection.


November 27, 2007
By Top Crop Manager

Topics

For years, growers have focussed on green foxtail and wild oats in their search
for higher yields in wheat. And rightly so, considering the significant yield
impact those grassy weeds can have on cereals. While recent weed surveys conducted
across the prairies showed that green foxtail and wild oats remain key weeds,
the surveys also show that some broadleaf weeds are moving up on the list of
concerns.

 96a
Controlling broadleaf weeds before they become a problem can help
increase wheat yields.
Photo Courtesy Of Arysta LifeScience Corporation.


In the 2001 Alberta Weed Survey, four broadleaf species moved into the top
20, including cleavers, annual sowthistle, pineappleweed and narrow-leaved hawk's
beard. In Manitoba's 2002 Weed Survey, annual sowthistle, volunteer canola,
cleavers, dandelion, kochia and round-leaved mallow moved into the top 20. And
in Saskatchewan's 2003 Weed Survey, volunteer flax and narrow-leaved hawk's
beard are new to the top 20.


Jay Bruggencate, an Agri-Coach with Agri-Trend Agrology, says that more broadleaf
weeds are popping up on the radar screen. While they are not widespread concerns,
the broadleaves do warrant a second look in field scouting and herbicide selection.


"In my area through central Alberta, I'm seeing stork's bill and cleavers
more often. Hempnettle continues to increase, and the other background weeds
are always there," explains Bruggencate. He says that growers should be
cognizant of their grassy weeds, but also aware that some broadleaf weeds may
need special attention.


Weeds recently on the increase across western Canada are white cockle and nightflowering
catchfly and more surprisingly the annual form of biennial wormwood is becoming
a challenge to control in Manitoba.

Broadleaf weeds can hurt yield

While yield loss information has not been widely published, some research was
conducted in the 1980s by John O'Donovan, who was then at the Alberta Environmental
Centre at Vegreville. For instance, it shows that 10 wild buckwheat plants per
square metre can reduce yield in wheat by only three percent. While this is
not hugely significant, the presence of this weed at harvest is. Similar data
is also posted on the University of Manitoba's Plant Science Department's web
site.


Looking at the Alberta Weed Survey results from 2001 for example, over half
the fields surveyed had wild buckwheat, and those infested fields had a density
of 6.1 plants per square metre in wheat fields. And remember, these fields were
surveyed in the summer, after spraying, so infestation levels could have been
even higher.


Denise Maurice, technical development manager of crop protection with Agricore
United, says she is seeing some shifts in weeds as well. Specifically, in Alberta,
of particular concern is the increase in cleavers and dandelion, while kochia
is spreading northwards. "Generally, farmers have focussed on green foxtail
and wild oats because yield losses due to broadleaf weeds aren't as great as
grassy weeds," explains Maurice.


Using Alberta's hempnettle and lamb's quarters data as an example, Maurice
says that 50 plants per square metre would cut wheat yields by 21 percent. But
the Alberta Weed Survey showed that hempnettle was present in almost 19 percent
of the fields at a density of only 6.5 plants per square metre. According to
the economic threshold equations developed by O'Donovan, the 6.5 plants would
result in less than five percent yield loss in wheat if the crop and the weeds
emerged at the same time.


"But a factor to remember is that broadleaf weeds exist as a mixed community
so the yield losses from broadleaf weeds are a result of many different species
impacting yield," explains Maurice.

Wheat Weed density (plants/square metre)
10 20 30 40 50 60 100 160 240
Grasses Percent yield loss
Wild oats 7 12 17 22 26 29 41 53 62
Green foxtail <1 <1 <1 2 2 2 5 7 11
Broadleaf weeds Percent yield loss
Hempnettle 5 10 14 18 21 23 31 39 45
Lamb's quarters 5 10 14 18 21 23 31 39 45
Wild buckwheat 3 6 8 10 12 14 21 29 36
Based on recommended seeding rates and the crop and weed
species emerging at the same time. If the weed species emerge prior to crop
emergence, yield losses will be greater than indicated in the chart.
AgroManager on Weeds 2, Agricore United; John O'Donovan,
Research Scientist, Alberta Research Council, Vegreville, Alberta; Guide
to Crop Protection 2005; Weeds, Plants, Diseases, Insects – Saskatchewan
Agriculture, Food and Rural Revitalization.

Targetting broadleaves

Bruggencate says that another reason that farmers spent a lot of time on wild
oats control was that the herbicides available 15 years ago were not as effective
as today's new products. Today, with many more products, and many more tank-mix
options, growers are able to also focus on broadleaf weeds more effectively.


In the past, growers focussed on grassy weeds and stuck to their tried and
true products, such as Refine Extra or Buctril M, because those products worked
and continue to work on weeds like wild buckwheat.


However, some growers are now paying more attention to difficult to control
broadleaf weeds, where the older herbicides may not be as effective. "In
many cases, especially with difficult broadleaf weeds, farmers are now choosing
the broadleaf weed product and then a wild oats tank-mix," Bruggencate
says.


Another trend that Bruggencate has noticed is that farmers are sometimes choosing
a split application, going with a grassy weed herbicide first and then following
up with a broadleaf herbicide. He says weed control may be improved because
grassy weeds generally emerge earlier than broadleaf weeds, so a split application
can target each weed population at the best time.


Perennial weeds also factor into the selection process and control of Canada
thistle, dandelion and quackgrass may also influence the decision to tank-mix
or apply a split application in wheat.


"There are now in-crop solutions for some of these harder to control weed
species, even Canada thistle and dandelion can be targetted in-crop or pre-seed.
The broadleaf weed spectrum is becoming an important factor in herbicide selection
and tank-mix decisions," says Maurice. "Application timing is also
a consideration and the reason some of the hard to control species have taken
a foothold is that they may have fallen outside the typical application window
of many products."


A new tool for the control of certain broadleaf weeds introduced in Canada
for 2006 is Everest herbicide. Everest is known for its control of green foxtail
and wild oats, and has recently been labelled in Canada for control of redroot
pigweed, wild mustard, stinkweed, shepherd's purse, volunteer canola (non-Clearfield
varieties) and green smartweed.


Patrick Haikal, a researcher with Arysta LifeScience, says that his company's
research on Everest indicates a good fit on certain broadleaves. "We know
that Everest has activity on many broadleaf weeds and is registered for control
on numerous broadleaf weeds in the US and Canada. We are working with the PMRA
to seek registration on additional key broadleaf weeds in Canada," says
Haikal.


Haikal also adds that products like Everest can be sprayed early for grassy
weed control in wheat and will also control later flushes with the soil activity.
Then, the farmer can come back and spray his broadleaf weed herbicide to target
specific problems. Like many grassy herbicides, Everest can also be tank-mixed
with numerous broadleaf herbicides if a single pass application makes more sense.


Maurice says that in the end, focussing on the worst weeds, broadleaf, perennial
or grassy weeds, comes down to assessing what is in the field. She says that
looking at weed species, weed infestation levels, time of emergence and yield
loss potential should help growers determine their main driver weeds.


Haikal concludes by explaining that a herbicide like Everest, that does not
show antagonism but rather synergy with its tank-mix partners, will provide
even greater flexibility for growers targetting broadleaf weeds and help sharpen
the control of their current broadleaf tank-mix partners. -30-

Top 10 weeds by province
Alberta weed survey results, 2001
Weed species Rank Frequency (percentage) Density (plants/sq.m)
Wild buckwheat 1 54.3 7
Wild oats 2 45.6 10
Chickweed 3 21.2 31
Canada thistle 4 40.7 3
Stinkweed 5 30.6 6
Cleavers 6 18.4 8
Lamb's quarters 7 23.7 5
Hempnettle 8 18.8 7
Green foxtail 9 11.4 18
Dandelion 10 22.7 2
Source: Alberta Weed Survey; Julia Leeson,
Gordon Thomas, Linda Hall.
Manitoba weed survey results, 2002
Weed species Rank Frequency (percentage) Density (plants/sq.m)
Green foxtail 1 65 23.8
Wild oats 2 56.6 11.7
Wild buckwheat 3 57.7 2.8
Barnyard grass 4 24.1 12.0
Canada thistle 5 41.4 1.8
Redroot pigweed 6 27.9 3.4
Lamb's quarters 7 24.6 2.9
Pale smartweed 8 27.1 1.9
Dandelion 9 20.6 1.5
Volunteer canola – Argentine type 10 16.6 2.7
Source: Manitoba Weed Survey; Julia Leeson,
Gordon Thomas, Todd Andrews, Kim Brown, Rene Van Acker.
Saskatchewan weed survey results, 2003
Weed species Rank Frequency (percentage) Density (plants/sq.m)
Green foxtail 1 43.5 24.4
Wild oats 2 50.9 7.4
Wild buckwheat 3 50.8 2.7
Canada thistle 4 34.8 2.1
Lamb's quarters 5 22.0 5.9
Redroot pigweed 6 17.3 6.6
Stinkweed 7 18.0 5.5
Kochia 8 16.9 4.2
Spring wheat/durum 9 10.7 7.7
Russian thistle 10 11.8 4.5
Source: Saskatchewan Weed Survey; Julia
Leeson, Gordon Thomas, Clark Brenzil.


 


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