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Weed control in winter wheat an emerging issue?

Current conditions may drive more to spray.


November 14, 2007
By Ralph Pearce


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24aThe issue of whether to invest in a herbicide program for winter wheat is difficult.
Although there is not always a yield advantage from applying a broadleaf herbicide,
there are other benefits including ease of harvest, a cleaner sample at harvest
and reduced weed pressure in subsequent crops in the rotation.

Yet 2007 could be the year when a herbicide application will reap the rewards
of higher yields, and for a number of reasons. First, there is the aforementioned
crop condition. Much of the crop went into ground that was wet and subsequently
compacted. As a result, that smaller crop – 450,000 to 500,000 acres compared
to 1.1 million acres planted in 2005 – will likely emerge in poorer condition
and in stiffer competition with broadleaf weeds than in years past. Economically,
that wheat has a higher price per bushel, boosting its value and the need to
protect those fewer acres as early as possible.

"As prices go up, the relative cost of your herbicide goes down,"
says Dr. Peter Sikkema, assistant professor of field crop weed management at
University of Guelph-Ridgetown College. "I would suggest that more farmers
should spray, simply because prices are higher and you can justify your application
far easier on an economic basis."

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Sikkema joined Mike Cowbrough, weed specialist for field crops with the Ontario
Ministry of Agriculture, Food and Rural Affairs, in a two year study of broadleaf
weed control in winter wheat. Their findings, under what might be considered
'good to excellent' conditions in 2004 and 2005, showed an average yield advantage
of only 3.5bu/ac across 10 sites. "We may have greater pressure from winter
annual and bi-annual weeds, simply because the winter of 2006/07 was so open,
to start," says Sikkema, adding that winter conditions could worsen and
reduce that potential. "The warm start to winter allowed those weeds to
get bigger and we could have less winter-kill and consequently, more weeds present
at the time of herbicide application in the spring."

No sense to data or practices
For Cowbrough, the issue of weed management in winter wheat can be frustrating
because of the lack of clear data showing the benefits of spraying. In terms
of thresholds, weed versus crop densities, planting dates and ultimate yield
advantages, reality and theory seldom seem to meet. In many situations where
spraying should have indicated an advantage, it did not, and vice versa.

"From a yield perspective, it's not all that clear what the variables
are that would allow you to make that decision on whether or not to spray,"
says Cowbrough, who agrees with Sikkema's recommendation to spray in 2007, if
only to protect a higher value wheat crop, and because the economics make it
easier to justify the cost. "In some instances, we had very high populations
of species and it had no impact on yield, and in other cases, we had lower populations
of perennials and winter annuals, and it had a significant impact on yield.
Looking at all the data, if you have fields with a high percentage of winter
annuals and perennials, there will be a greater benefit to weed management than
one that strictly has annuals. Density should matter, yet our data suggests
it doesn't matter a lot."

An example that Cowbrough cites in the study was a field under high pressure
from wild buckwheat. He anticipated a significant difference between the treated
and untreated, yet when the yield data returned, there was only a 0.2bu/ac difference.
Another field had relatively low populations of dandelion, field horsetail and
chickweed, yet showed a 10bu/ac advantage.

Cowbrough is not trying to say growers should ignore the potential benefits
of spraying, he is simply asking growers to justify their applications according
to the individual field conditions and weed species. He can offer counter-arguments
to most of the traditions and long-held ideals of spraying, all while conceding
to a few weedy challenges, including ragweed and perennial sowthistle. "Ragweed
sits up high, it sticks on the cutterbar, you can't go as fast and it also increases
moisture," explains Cowbrough. "Perennial sowthistle is another one
that impedes harvesting and is difficult to control with a herbicide application.
The timing doesn't work because perennial sowthistle doesn't come up until past
the appropriate window to apply the herbicide."

Fewer acres, more being sprayed
From a private sector perspective, Blair Bossuyt, Ontario sales manager for
Nufarm Agriculture in Aylmer, Ontario, agrees the spring of 2007 will see a
higher percentage of wheat being sprayed, despite the lower acreage across the
province. He agrees with Cowbrough and Sikkema on the rationale for not spraying
when conditions are good, namely competition with wheat's density. "But
we'll find that when planting gets later like it was last fall, the range of
growers spraying herbicides will increase," he predicts, noting price will
also have an impact on the decision making process.

Other issues that Bossuyt mentions as concerns for the coming growing season
include an increase in cleavers in Ontario, which Sikkema suggests is a result
of contamination of western feed grains sold in Ontario. Control of cleavers
could be helped once Trophy (fluroxypur + MCPA ester) is registered for eastern
Canada, hopefully by 2008. "Right now, we have it registered in western
Canada, we have a couple of opportunities to get it through a minor use or through
the regular registration process," says Bossuyt.

One agronomic trend he believes growers will consider is a return to underseeding
wheat fields with red clover, a practice that has been curtailed with the earlier
planting dates in the past four or five years. "Where you tend to get the
best catch of clover is in the more intermediate crops, so with the crop we're
into potentially, this year, because of late planting and probably reduced stands,
we'll see better stands of red clover," says Bossuyt. "The other thing
is that we're going to have more compaction issues, so red clover is going to
be looked at a little more than in the past, just to break that soil down."

For clarification purposes, Bossuyt notes the only herbicide that can be used
on winter wheat underseeded to red clover is bromoxynil + MCPA Ester (Mextrol
on the Nufarm label). He states this to avoid any confusion of using the wrong
herbicide, like 2,4-D and Estaprop Plus, in this situation. -30-