Glyphosate resistant weeds in Ontario: Spreading through pollen?

Glyphosate resistant (GR) weeds have been a hot topic lately! Since the massive adoption of glyphosate tolerant soybean, corn and canola in the late 1990's, in-crop use of glyphosate has increased and integrative weed management strategies have decreased. This combination has created cropping systems with high selection pressure for GR traits.

In Ontario, there are four confirmed GR weed species:

  • 2008 Giant Ragweed (Ambrosia trifida)
  • 2011 Canada Fleabane (Conyza canadensis)
  • 2012 Common Ragweed (Ambrosia artemisiifolia)
  • 2014 Tall Waterhemp (Amaranthus tuberculatus)

Although currently not an issue in Canada, GR Palmer amaranth has been reeking-havoc for cotton producers in the United States. An article by Sosnoskie et al. (2012) describes that within three years of confirmed resistance, GR Palmer amaranth had infested 50% of the total cotton producing area (2 million ha) in the United States.

How did this happen so quickly?

Palmer amaranth is a dioecious annual, which reproduces through cross pollination. This means that pollen travels from a male flower to a female flower on separate plants. Sosnoskie's study revealed that GR Palmer amaranth was able to spread resistance through airborne pollen to female plants 300m away (almost 3 football fields!). The female plants receiving that pollen produced 21% offspring with glyphosate resistance.

Why should Ontario growers care?

Of the four GR weed species confirmed in Ontario, three are cross-pollinating species (giant ragweed, common ragweed and tall waterhemp). This means that a similar risk of GR spread could occur here in Ontario. Research out of the University of Missouri confirmed that GR common ragweed could successfully spread pollen to female plants up to 91m away (Dierking, 2011).

What can growers do to supress the spread of GR weeds through pollen dispersal?

  • Scout before and after a herbicide application
  • In addition to a burn-down herbicide, apply another with residual activity
  • Control 100% of weeds prior to flowering
  • Implement Integrated Weed Management strategies throughout the season
    • Use herbicides with multiple modes of action within the same year
    • Cultural control methods (competitive varieties, planting date)
    • Mechanical control methods (tillage)
    • Cover crops
    • Crop rotation


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