Resistant Weeds!

The list of herbicide resistant weeds, and counties where they can be found, continues to grow! Since 1997, redroot pigweed, green pigweed, common ragweed and Eastern-black nightshade resistant to Group 2 herbicides (such as Pursuit, Pinnacle, Classic, Ultim, etc) have been positively identified in Ontario soybean fields. Are your fields part of this dilemma?

If you had weed escapes, which should have been controlled by your herbicide program, and you cannot find a good reason for the lack of control, the possibility of herbicide resistance should be explored. The most common sign of resistance is when all the weed species are controlled except for one, a weed that should have been easily controlled by the herbicide program. Resistant weeds often begin in patches or streaks corresponding to the harvest pattern of the combine. Over time, resistant weeds will spread across the entire field.

Before declaring a weed resistant, make sure that other explanations for weed escapes and misses are investigated. Weeds that emerge after application with non-residual herbicides can confuse the diagnosis. Some species are naturally more tolerant to some herbicides. Improper equipment setup, missed adjuvants, poor spray pattern, poor canopy penetration, improper weed stage at time of spraying or weather issues can all lead to misses that can be misdiagnosed as weed resistance.

The following guidelines will help manage and/or prevent weed resistance:

• Use crop rotation. Rotate crops and herbicides.
• Avoid the using the same herbicide or herbicides from the same grouping in the same field, in consecutive years.
• Avoid using herbicides with the same mode of action more than once per season.
• Use tank mixes where 2 or more products give effective control against the target weed, and the products are from different mode of action groupings.
• Keep accurate records of crop rotations and weed control programs used in all your fields, including weather conditions at time of application. Map the location of resistant weeds so they can be found in future years.
• Use clean seed and clean equipment when moving field to field.
• Scout your fields after herbicide application to detect weed escapes or weed shifts. If a potentially resistant weed or weed population is detected, use available control methods to avoid weed seed production in the field. Rescue sprays must be from a different herbicide group.

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