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Fall tank-mixes can impact crop

Tolerance of winter wheat to pre-plant and pre-emergence herbicides researched.


November 13, 2007
By Mike Cowbrough and Dr. Peter Sikkema

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30aRecent research with certain herbicides, most notably Classic, that are tank-mixed
with glyphosate and applied either before or shortly after winter wheat planting
can result in yield losses of up to 34 percent. That is according to field trials
conducted at the Huron Research Station and the Ridgetown campus of the University
of Guelph during the 2004/05 growing season. Reductions in grain yield could
be attributed to crop injury and reductions in plant stand.

Winter annual and perennial weeds that emerge with a winter wheat crop will
have the greatest impact on grain yield. Pre-plant or pre-emergence glyphosate
applications are effective at controlling a broad spectrum of weeds that are
present at application while having no adverse affects on the winter wheat crop.
However, several growers have been challenged with weeds that are not well controlled
by glyphosate, such as cinquefoil, red clover, alfalfa, wild carrot and field
violet. Therefore, producers are interested in glyphosate tank-mixes to improve
the control of challenging weeds and prevent emergence of late emerging winter
annual, biennial or perennial weeds (like dandelion).

At four different locations, a number of herbicides were tank-mixed with glyphosate
and applied either before or shortly after winter wheat planting (one to six
days after planting). Evaluations of visual crop injury were taken throughout
the growing season. Table 1 provides a summary of both visual crop injury and
grain yield for each herbicide treatment. The information in Table 1 illustrates
that glyphosate alone or glyphosate + Refine Extra did not result in any crop
injury in 2005 while all other treatments caused some degree of crop injury.

Table 1. Visual crop injury (percentage) and grain yield
(bu/ac) of winter wheat that was treated with various herbicide treatments
at the Huron Research Station near Exeter, Ontario, and the Ridgetown Campus
of the University of Guelph in Ridgetown, Ontario, in 2004/05.
Pre-plant treatment Percentage of visual crop injury* Grain yield
(bu/ac)
Glyphosate alone 0 74
Glyphosate + Refine Extra 0 78
Glyphosate + Banvel II 2 76
Glyphosate + 2,4-D ester 3 72
Glyphosate + 2,4-D amine 4 71
Glyphosate + Distinct 8 62
Glyphosate + Amitrol 240 16 64
Glyphosate + Classic 33 49
*Percentage of visual crop injury ratings shown were taken
between 200 and 219 days after herbicide application (between May 31 to
June 1, 2005).

The level of crop injury caused by certain herbicides, namely Classic, Amitrol
and Distinct resulted in significant reductions in yield. The most injurious
treatment was the tank-mix of glyphosate + Classic (Guardian – see photo).
Growers have often inquired about the use of this tank-mix prior to planting
wheat to provide residual control of seedling dandelion. Unfortunately, the
residual component of Classic will significantly reduce the height and plant
stand of winter wheat (see photo).

It should be noted that Amitrol 240 is a product that is registered for the
control of dandelion and annual weeds when applied on the day, or one day prior
to planting winter wheat. Amitrol 240 caused significant reductions in grain
yield when compared to the control treatment (glyphosate) at one of the four
trial locations. The reasons for this are unclear, since Amitrol was applied
one day prior to planting winter wheat at the location where the greatest level
of crop injury and yield reduction was observed. More trial data is needed to
help fully understand what factors may increase the risk of Amitrol injury.

Lastly, tank-mixing glyphosate with 2,4-D ester or Banvel II did not result
in any appreciable crop injury or significant yield losses although the addition
of 2,4-D ester did significantly reduce crop height at one of the four trial
locations. The tank-mix of glyphosate + 2,4-D amine did not result in any appreciable
crop injury but a significant reduction in yield was observed at one of the
four trial locations. Currently it is recommended that the phenoxy herbicides,
such as Banvel, 2,4-D, MCPA, are not to be applied prior to planting winter
cereals and this preliminary research would support that recommendation based
on the appearance of crop injury and in one case, significant reductions in
grain yield.

In summary, the majority of weeds present prior to planting winter wheat are
adequately controlled by glyphosate alone and based on this preliminary research
there is more risk of increased crop injury and decreased grain yield associated
with tank-mixing a residual herbicide with glyphosate than any possible improvements
in weed control. Classic should never be applied prior to planting winter wheat.

*Mike Cowbrough, OMAFRA in Guelph and Dr. Peter Sikkema, University of
Guelph in Ridgetown, Ontario.