Researchers say herbicide resistance is common to the vast majority of grain land on the Prairies.
Since the mid-1990s, monitoring by provincial and federal agronomy staff has turned up weeds that are cross-resistant to Group 1 and Group 2 herbicides.
From 1996 to 2006, 20 percent of wild oat samples screened were resistant to ACCase inhibitor Group 1 herbicides and/or ALS inhibitors.
Hugh Becky of Agriculture Canada in Saskatoon authored a recently published scientific paper that outlines the level of herbicide resistance across the Prairies.
He and colleagues from the provincial agriculture departments and the Universities of Alberta and Saskatchewan now estimate herbicide-resistant weeds are present on more than 12.5 million acres of prairie farmland.
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“Of 1,076 wild oat seed samples submitted by growers for testing between 1996 and 2006, 725 were Group 1 resistant, 34 were Group 2 and 55 were resistant to both,” he said.
Researchers across the Prairies also found that about 25 percent of samples tested between 1996-2006 showed Group 2 herbicide resistance in kochia, wild mustard, field penny grass, common chickweed and hemp nettle.
Clark Brenzil of Saskatchewan’s Ministry of Agriculture was a contributing author to the publication.
“It’s those two modes of action that are the most prone to select for resistance. They are most widely used and are traditionally herbicides that farmers turn to for low cost control of problem weeds,” he said.
Brenzil and Beckie say because most grain producers aren’t aware of the resistance that lurks in their fields they aren’t managing the problem.
Brenzil said by the time many producers realize there is a problem, the herbicide resistant weed population has reached economically damaging levels.
“With higher grain prices the (economic) impact of these weeds has grown dramatically over the past two years,” he said.
Even if one in a million weeds is naturally resistant to a couple of different groups of herbicides, the selection pressure applied by repeated use of the two herbicide types will quickly produce economically damaging populations.
Beckie said traditional crop production methods, including highly efficient chaff and straw spreading behind the combine, and failure to clean machines thoroughly between fields adds to the spread of herbicide resistant weed genetics.
Researchers say the economic impact is greater than most producers are aware.
Part of the herbicide monitoring program is a farmer questionnaire. Twelve percent of producers, of 463 participating, said they were aware or suspected there were herbicide tolerant weeds in their fields.
“Obviously this isn’t the case. Realistically most producers have the (herbicide tolerant weeds), but they haven’t reached the point where the problem is significant enough to be noticed, or the weed populations are being blamed on something else,” he said.
Agrologists say often producers observe weeds in the field and blame equipment errors or other mechanical problems.
“It’s a sprayer miss, or the weeds came in a flush after I sprayed (in-crop). This year, dry conditions resulted in cereal germination that was spread over a month. Herbicide tolerant weeds are easily camouflaged when spray is delayed,” said Beckie.
Brenzil said in the survey, the area of infestation in many fields was small and spread out across the land.
They observed that in Group 1 resistant wild oats the patches of resistant plants covered about half an acre on average. For Group 2 resistant wild oats it was a quarter of an acre.
“That’s not very big and could be ignored or blamed on something else,” he said.
But when added together a bigger problem emerges. The survey estimated that nine million acres of prairie farmland contains Group 1 or 2 herbicide tolerant wild oats.
“And wild oats do a lot economic damage,” said Brenzil.
While some weeds such as kochia have a half-life in the seedbed of only a few months, others, such as wild oats can wait for five to nine years to germinate.
Mustard can stay dormant for 20 years waiting to come back, while pigweed will last up to 30 years.
In long-lived species that select for herbicide resistant traits, correcting a tolerance issue can be difficult.
“For kochia it is simpler. Put it into hay for three seasons and the problem is solved. Not so easy in mustard or wild oats,” said Brenzil.
“It means that assuming you have (herbicide tolerant) weeds in your field is the best way. From that point forward you can take a proactive approach to something that is inevitable,” he said.
By combining cultural management practices with herbicide rotations producers won’t eliminate the problem, however they can reduce serious economic consequences, say agrologists.
Brenzil said herbicide tolerance often appears to come out of nowhere, when in fact it likely has been building for a decade.
A new round of Prairie herbicide tolerant weed field surveys began with Alberta last year, is being completed in Manitoba this fall and begins in Saskatchewan next spring. Along with grower submitted samples, these will be added to the accumulated data to paint a more accurate picture of the issue by 2010.
