Weed Management
Control of Difficult to Manage Weeds in Ontario Corn
By Peter Sikkema, University of Guelph, Ridgetown College
Ken Hough, OCPA Director of Research and Market Development

Over 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.

A 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).

Spreading Atriplex

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 + Marksman).

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.

Swamp Smartweed

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.

Hemp

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.

Canada Fleabane

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 Link corn.

FirstRate and Classic provided the best postemergence control of Canada fleabane in conventional soybeans while glyphosate provided excellent control in Roundup Ready soybeans.

Crab Grass

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 corn.

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.

Sandbur

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.

Bur Cucumber

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 and Reliance.

Field Horsetail

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 Eastern Canada.

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.

Horsenettle

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.

Perennial Sowthistle

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 sowthistle.

A sequential application of glyphosate provided excellent control of perennial sowthistle in Roundup Ready soybeans.

Annual Sowthistle

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.

Proso Millet

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 or you can connect from gocorn.net.

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.