Weed resistance has been an issue in crop production for many years, but is coming to the forefront as increasing numbers of weeds are no longer responding to herbicides.
“Most producers are aware of the issue, but unless it affects them directly, don’t see it as a huge issue,” said provincial crop specialist Harry Brook. “For example, resistance to glyphosate, commonly known as Roundup, is found in other parts of the world and Canada. We also now have glyphosate resistance in kochia in southern Alberta and it continues to spread.
“These should serve as a wake-up call to producers to the importance of rotating different herbicide groups when treating problem weeds. Failure to take this problem seriously will eventually result in the loss of our most popular weed control products.”
Wild oats is currently resistant to the most number of herbicide groups of any weed — and the problem was evident a decade ago.
“In 2007, 39 per cent of fields had some Group 1-resistant wild oats. The majority of herbicides used for wild oat control are in this group. If wild oats is resistant to a single herbicide in a chemical group, it’s pretty well resistant to all the herbicides that use that particular mode of action. Also in 2007, 12 per cent of fields had Group 2-resistant wild oats in them and eight per cent had wild oats resistant to both Group 1 and 2. Soil-applied wild oat control is in Group 8, which is older chemistry and 15 per cent of fields had wild oats resistant to this as well, despite the fact it hasn’t been used much for the last 20 years.”
Cleavers, kochia, chickweed, spiny annual sow thistle, hemp nettle, green foxtail, wild mustard, smartweed, Russian thistle, and stinkweed have all developed resistances to Group 2 herbicides, said Brook.
“That group contain the sulfonylureas, the ‘imis’ and florasulam. Back in 2007 it was estimated that 40 per cent of annually cropped land had some herbicide-resistance issues. It is undoubtedly much higher now.”
There are a few ways to detect a herbicide-resistance issue, said Brook.
“Investigate areas in the field where weed control didn’t occur. Rule out other factors that might have affected herbicide performance including misapplication, spray misses, unfavourable weather conditions, misapplication of herbicide at wrong leaf stage, or late weed flushes. Other warning signs include other weeds listed on herbicide being controlled adequately, patchy control with no reasonable explanation, a history of herbicide failure in the same area, lack of signs of herbicide injury on plants, and, finally, a history of using the same herbicide group on the land year after year.”
When using the same herbicide or products using the same mode of action, producers are actually helping those plants that are either not affected or affected less by the active ingredient than other plants, said Brook.
“By killing off susceptible plants you are actually setting the stage for the resistant ones to thrive as all their competition is killed off.”
Herbicides that have one specific mode of action are most likely to develop resistant weeds.
“Group 1 and Group 2 herbicides fall into this category,” said Brook. “However, the most important reason for having resistance show up is due to repeated use of the same chemical. Short crop rotations and a lack of crop variety has set up the conditions to encourage weed resistance to emerge.”
Canada has reported resistance issues in weeds to at least six different herbicide groups.
“If we ignore the risk of developing resistances, the day may come when we might lose some of our best herbicide tools from the weed management tool box. Pay attention. Scout your fields. Keep field records. Use a good crop and herbicide group rotation to keep this problem at bay.
“The consequences of not doing so are not cheap or pretty.”
For more information, contact the Alberta Ag-Info Centre at 310-FARM (3276).