Control of Difficult
to Manage Weeds in Ontario Corn
the past three years, OCPA has provided funding ($15,000 per year) to
Peter Sikkema, Ridgetown Campus of the University of Guelph, to evaluate
various herbicides for control of 'difficult-to-manage' weeds. Generally,
the species studied are perennial weeds, and are most prevalent in reduced-till
cropping systems, although they may occur in conventional tillage programs
as well. Although many of the species studied are relatively isolated
in their occurrence (i.e., they do not affect a large proportion of
Ontario's corn acreage), nevertheless, where they do occur they often
cause serious yield loss and make crop management much more difficult.
The development of effective weed management strategies in a corn/soybean/wheat
rotation may mean the difference between profit and loss for growers
who have these weeds on their farms.
summary of the results follows. Where a particular weed species was
tested in both corn and soybeans in 2000, information on control in
soybeans has been included as well. (Some weeds, such as three-seeded
mercury, biennial wormwood and dandelion, were tested in soybeans but
not in corn, so results are not presented here).
Glyphosate applied alone preplant in no-till corn provided poor full
season control of spreading atriplex, while all the glyphosate tankmixes
evaluated provided good to excellent full season control (glyphosate
+ Converge, glyphosate + Fieldstar, glyphosate + Banvel II and glyphosate
Glyphosate applied alone preplant in no-till soybeans provided poor
full season control of spreading atriplex, while excellent control of
spreading atriplex was obtained with preplant tankmixes of glyphosate
+ Broadstrike Dual or glyphosate + Sencor/Lexone. Postemergence, glyphosate
in Roundup Ready soybeans provided excellent control of spreading atriplex
while in conventional soybeans, none of the herbicides available provided
consistent control, although Pinnacle has shown more activity than the
other herbicides evaluated.
In trials conducted over the past 10 years at Ridgetown College, none
of the postemergence herbicides available have provided consistent control
of swamp smartweed. Herbicide programs including dicamba (Banvel II,
Distinct and Marksman) have provided the best control.
Excellent full season control of volunteer hemp in corn was obtained
with atrazine, Distinct, Marksman, Pardner + atrazine and PeakPlus.
Glyphosate and Liberty provided excellent control of volunteer hemp
in Roundup Ready and Liberty Link corn respectively.
Bentazon provided excellent control in conventional soybeans, while
good control was obtained with Blazer and Pinnacle.
Glyphosate applied alone preplant, plus all the glyphosate tankmixes
evaluated (Converge, Fieldstar, Banvel II and Marksman), provided excellent
control of Canada fleabane in corn. The best postemergence control of
Canada fleabane in corn was provided by the dicamba-based herbicides
(Banvel II, Distinct and Marksman). Glyphosate provided excellent control
in Roundup Ready corn, and Liberty provided excellent control in Liberty
FirstRate and Classic provided the best postemergence control of Canada
fleabane in conventional soybeans while glyphosate provided excellent
control in Roundup Ready soybeans.
All the preemergence (Dual II Magnum, Primextra II Magnum, Frontier,
Prowl and Converge) and post-emergence (Elim, Accent and Ultim) treatments
that were tested provided good to excellent control of crab grass in
All of the postemergence treatments (Venture II, Assure II, Excel Super,
Select, Poast Ultra and Roundup Transorb) evaluated for crab grass control
in soybeans provided good to excellent control with the exception of
the late application of Poast Ultra.
The preemergence application of either Primextra II Magnum or Converge
provided good control of sandbur in corn at only one of two locations.
Other preemergence treatments (Axiom, Dual II Magnum, Frontier and Prowl)
provided poor control. The postemergence applications of Elim, Ultim
and Accent provided good control at both locations.
Management of bur cucumber in corn should begin with a soil-applied
application of atrazine, but this will not provide full season control.
This can be followed with a postemergence application of Pardner + atrazine,
which will provide almost full season control of this weed. Data from
Pennsylvania State University indicates that PeakPlus and Summit also
have activity on this weed.
Bur cucumber can be controlled in Roundup Ready soybeans with glyphosate.
The best control in conventional soybeans has been with Classic, Pinnacle
The tankmix of Ultim + Striker (Ultimax) has provided fairly consistent
control of field horsetail in corn. There has been some variability
in the results which is thought to be due to different biotypes across
All of the preplant herbicide treatments evaluated (glyphosate or Amitrol
240 tankmixed with Broadstrike Dual) provided good to excellent full
season control of field horsetail in no-till soybeans.
All of the herbicide treatments evaluated (Ultim + Banvel II, Ultim
+ Distinct, Ultim + Marksman, Ultim + PeakPlus, Ultim + Striker and
Accent + Summit) provided good to excellent full season control of horsenettle
across 3 sites in corn in 2000.
A single application of glyphosate provided good control of horsenettle
in Roundup Ready soybeans. In conventional soybeans, FirstRate was the
product of choice.
Summit provided excellent full season control of perennial sowthistle
in corn. Atrazine + COC and Marksman provided very good control, while
the Banvel II and Distinct treatments provided good control of perennial
A sequential application of glyphosate provided excellent control of
perennial sowthistle in Roundup Ready soybeans.
All the postemergence treatments tested (atrazine + COC, Banvel II,
Distinct, Marksman, Shotgun, Pardner + atrazine, PeakPlus and Summit)
provided excellent control of annual sowthistle in corn.
Classic, Reliance STS, Viper, Meridian Plus and glyphosate all provided
excellent control of annual sowthistle in soybeans.
A preemergence application of Converge provided good to excellent full
season control of proso millet in corn. A postemergence application
of Ultim provided excellent control, while Elim or Accent provided good
to excellent proso millet control.
A single application of glyphosate, Venture II, Assure II, Excel Super,
Select or Poast Ultra provided excellent control of proso millet in
Roundup Ready soybeans.
Note that information on herbicide rates used in these tests, and/or
use of adjuvants, etc. are not included above. Also, most of these herbicides
have not been registered for use on these species in corn. Growers interested
in further information should visit Mr. Sikkema's website: www.ridgetownc.on.ca/weeds
Sufficient data has been compiled to date to proceed with applications
for a User Requested Minor Use Label Expansion (URMULE) for each of
the following weed species/control options:
. Ultim for control of sandbur in corn
. Distinct for control of perennial sowthistle in corn
. Distinct for control of prickly lettuce in corn
. Distinct for control of Canada thistle in corn
. Banvel II for control of Canada fleabane in corn.
OCPA will be assisting in these label expansion requests.
Control of horsetail now appears on the Ultimax and Striker product
labels as the result of a previous request based on Mr. Sikkema's research.