Weed Escapes or Potential Resistant Weeds?

Prepared by Nasir Shaikh, MAFRI Weed Specialist

The 2013 season created situations where weeds got ahead of the crop and field conditions were not always favorable at the proper time for application. The result was weed escapes due to missed applications, advanced size/development, or other unexplainable reasons.  It is the unexplainable escapes that cause concerns specifically when Herbicide Resistance has developed.

In cases where there are weed survivors, information on time of application, weather conditions, weed species and staging becomes important.  A buckwheat plant or redroot pigweed plant that has escaped the herbicide – is it resistant or was the plant just too big and developed to be completely uncontrolled?

Knowing the limitations of the products being used (i.e. weather, water quality, etc) and the limit on the size or developmental stage of the weed can assist in determining an escape and potential resistance issues.  A weed is considered ‘tolerant’ when effective herbicides fail to control the weed in subsequent years.  The developed resistant populations tend to be in patches or patterns across the field or fields.  Looking for the patterns and keeping notes on escapes is important in determining if there is an issue. This can be accomplished by scouting the field’s post-herbicide application to determine effectiveness. 

In cases where Herbicide Resistance is suspected, plants can be submitted to the following labs to be tested. 

Ag-Quest, Inc. Box 144, #10 South Railway Street, Minto, Manitoba, CANADA, R0K 1M0. Phone: (204) 776-2284; Fax: (204) 776-2250

Herbicide resistance testing services at Ag-Quest include:

  • soil bioassay for group 3 and group 8 herbicides in wild oat and green foxtail
  • petri-dish bioassay for selected group 1 herbicides in wild oat and green foxtail
  • Whole-plant bioassay for group 1, 2, 8 and 25 herbicides in many major weeds of western Canada and US


Crop Protection Laboratory   346 McDonald Street, Regina SK S4N 6P6.  Phone: (306) 787-8130; Fax: (306) 787-8803.  E-mail [email protected]

Herbicide resistance testing at Crop Protection Laboratory include:

  • Green foxtail (group 3) $100
  • Green foxtail (group 1) $100
  • Wild oats (group 1) $100
  • Any other herbicide resistance tests $100


University of Guelph   Attn: Peter J. Smith, University of Guelph, Plant Agriculture Department, Crop Science Building, Guelph, ON., N1G 2W1.  Phone: 519-824-4120 x 58372; Fax: 519-763-8933.  E-mail: [email protected]

 Herbicide resistance testing at this Laboratory include: Group 1,2,3,6,7,9 and 22. And the test is free!

Have a follow-up question?

Are Herbicides to Control Dandelions Still Effective After a Fall Frost?

Fall is a great time to work on controlling perennials such as Canada thistle and dandelions. As temperatures get cooler, the nutrients start moving down into the roots and if a herbicide can be taken up and translocated with those nutrients, it can equal better control.

A frost event though, can kill some weeds or can damage leaf tissue which will reduce herbicide uptake and therefore reduce the level of weed control. Within the next few days after the frost, you need to assess the target weeds in the areas that you want to obtain control – are the weeds still growing?  How much leaf tissue has been damaged?

If the plant leaves are still shiny green with minimal leaf tissue damage (i.e. not blackened/brown or brittle) there still may be the window to control the perennial, biennial and winter annual weeds with glyphosate.  If you do spray – spray in the afternoon when temperatures are warm and sunny, this will help with herbicide uptake.   Use rates appropriate to the stage and time of year – fall applications of glyphosate are recommended at a higher rate than when controlling pre-harvest.

One last thing – look at the forecast for the next week following the application.  If the temperatures look like they are going to continually be below freezing each night, it may be too late to make the application to get the economic control you are looking for.

Have a follow-up question?