Manitoba Weed Resistance Update

In 2016, Hugh Beckie, Scott Shirriff and Julia Leeson of Agriculture Canada led a field survey of Manitoba to determine the incidence of weed resistance.  In late summer of that year, 150 randomly-selected fields were surveyed throughout Manitoba.  Before harvest, mature seeds from all residual weed species were sampled and subsequently plants were tested with Group 1 or 2 herbicides.  Survey results indicated:

• 68% of fields had a herbicide-resistant (HR) weed, this compared with 48% in 2008 and 32% in 2002
• 78% of fields had Group 1 resistant wild oats
• 43% of fields had Group 2 resistant wild oats
• 42% of fields had Group 1 and 2 multiple-resistant wild oats

This trend is being driven by a lack of new herbicide chemistries in the last 30 years, so growers use the same tools to manage their weed issues.  The inevitable consequence is a rise in weed resistance incidence.  Growers are also advised to be wary of HR broadleaf weeds.  Resistant populations of kochia, cleavers and chickweed to Group 2 herbicides are common.  Another weed of concern is glyphosate-resistant kochia, with Manitoba reporting its first few cases.  In the U.S. Midwest and southern states, herbicide resistant issues with the pigweed complex (Palmer amaranth, waterhemp, redroot and smooth pigweed) are causing major production challenges.  Hugh Beckie states, “It’s probably the number one global focal point for resistance now.”

Hugh Beckie has developed a “Top 10” list of the best management practices (BMPs) to address weed resistance.  Adopting these BMPs will improve weed control and slow the rate of weed resistance development:

• Crop diversity (rotation)
• Use competitive crops and practices that promote competitiveness
• Scout fields before and after herbicide applications, know your enemy
• Use multiple modes of action through herbicide mixtures and sequences
• Herbicide group rotation
• Rotate in-crop herbicide modes of action (selective mechanisms) in wheat
• Weed sanitation – border control, slow down the dispersal of HR weeds
• Site-specific (patch) weed management
• Strategic tillage – if, where, or when needed
• Accurate record-keeping of herbicides used and weeds present, invaluable reference

Please contact your local Shur-Gro, Munro, or South East Seeds agronomist to help plan an effective program to combat weed resistance.