Control Options for Group II Resistant Weeds in Soybeans

• Eastern Black Nightshade
• Common Cocklebur
• Pigweed Species
• Common Ragweed
• Green Foxtail
• Common Lamb's-Quarters
• Suspicious that you may have resistant weeds?

In Ontario there are now several different weed species resistant to group II herbicides. Over the past 3 years, field research trials have looked at alternative control options in conventional (non-Roundup Ready) soybean. The following tables provide a list of different herbicide programs with the rates that each program was tested at, average weed control and the range in weed control amongst the field trials. The intent of these tables is to provide a list of effective herbicides for managing group II resistant weeds. Always refer to OMAFRA Publication 75 - Guide to Weed Control and the manufacturer's product label for specific information on any of the herbicide treatments shown in the six tables.

Eastern Black Nightshade

Weed Staging: Weeds had not emerged for the pre-emergent (pre) treatments and were at the 4-leaf stage at the time of application for the post-emergent treatments.

A summary of 2 trials conducted by Peter Sikkema in 2002 and 2003.

Bottom Line: There are numerous products that will provide over 80% control of eastern black nightshade. In general, the level of control is proportional to the herbicide program's cost. Applications of Lorox L, Dual II Magnum followed by Blazer or Reflex provided the best and most consistent control of eastern black nightshade, but are also the most expensive.

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Common Cocklebur

Weed Staging: Weeds were between the cotyledon and 5-leaf stage at the time of application.

One field trial conducted by Peter Sikkema in 2001.

Bottom Line: Both Blazer and Basagran Forté provide good control of common cocklebur.

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Pigweed Species

Weed Staging: Weeds had not emerged for the pre-emergent (pre) treatments and were up to the 10-leaf at the time of application for the post-emergent treatments.

One field trial conducted by Peter Sikkema in 2002.

Bottom Line: The post-emergent herbicides have tended to provide inconsistent control of pigweed. Therefore, a soil applied herbicide program should be used to target group II resistant pigweed.

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Common Ragweed

Weed Staging: Weeds had not emerged for the pre-emergent (pre) treatments and were up to 10-leaf stage at the time of application for the post-emergent treatments.

A summary of 4 trials conducted by Peter Sikkema in 2002 and 2003.

Bottom Line: Although not incredibly common, populations of triazine resistant common ragweed do exist. Therefore, products that contain metribuzin (i.e. Boundary, Sencor) will not control common ragweed. Furthermore, it would appear that when metribuzin is applied to triazine resistant common ragweed, "rescue" treatments with Blazer or Reflex have been ineffective. This may be because the weeds have been "hardened off" by the initial metribuzin application. If you are confident that triazine resistance is not an issue, then Boundary alone or Boundary followed by Reflex will provide good control of common ragweed. If common ragweed is both resistant to triazine and group II herbicides, then a post-emergent application of Reflex will provide the most consistent level of control.

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Green Foxtail

Weed Staging: Weeds had not emerged for the pre-emergent (pre) treatments and were up to the 4-leaf stage at the time of application for the post-emergent treatments.

A summary of 2 trials conducted by François Tardif in 2002 and 2003.

Bottom Line: Any of the soil applied grass herbicides will provide good control of group II resistant green foxtail. Alternatively post-emergent grass herbicides (i.e. Poast) will also provide good control of green foxtail.

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Common Lamb's-Quarters

Weed Staging: Weeds were at the 4-leaf stage at the time of application for the post-emergent treatments.

A summary of 1 trial conducted by François Tardif in 2002.

Bottom Line: Of the post-emergent products, only Basagran Forté provided adequate control of common lamb's-quarters. However Basagran Forté must be applied to very small common lamb's-quarters (4 leaf or less) and at the high rate to achieve the level of control shown in Table 6. Although not tested, soil applied applications of Sencor or Lorox L should provide good control of common lamb's-quarters, unless the species is also resistant to triazine (i.e. Sencor) herbicides.

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Suspicious that you may have resistant weeds?

If you are seeing:

• A weed species that should have been controlled but is healthy while other susceptible species have been controlled
• A weed control failure even when the correct herbicide rate was used and it was applied at the appropriate weed stage and under favourable environmental conditions

Then stop a potential problem from getting worse. If you see weeds that you suspect are resistant, report them by calling the Agricultural Information Contact Centre at 1-877-424-1300

By taking advantage of this toll-free number, suspicious weed species will be tested for resistance by the University of Guelph. Any information obtained from this service will allow weed researchers to develop control options for resistant weed populations.

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