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Herbicide resistance continues to spread

A weed resistance survey in Alberta in 2007 is confirming what many researchers, agronomists and farmers suspected. Herbicide resistance is getting worse, and in some crops, the herbicide choices are limited. 

If growers find kochia, expect it to be Group 2 resistant, as shown by the healthy kochia plants that have been sprayed with a Group 2 herbicide, compared to the dead susceptible kochia populations. (Photo courtesy of Agriculture and Agri-Food Canada, Saskatoon.)

“Of the top 10 weeds in southern Alberta, seven have resistance to a herbicide,” says weed scientist Dr. Hugh Beckie with Agriculture and Agri-Food Canada at Saskatoon. “One hundred of 300 fields (33 percent) surveyed in late summer had a herbicide-resistant weed, compared with 18 percent of fields six years previous.”

The survey is part of a five-year project from 2007 through 2012 called ‘Trends in herbicide-resistant weed occurrences across the Prairies’, supported by the Alberta Crop Industry Development Fund and many CropLife Canada companies. As part of the project, 300 randomly selected fields in Alberta were survey in 2007, 300 in Manitoba in 2008, and 400 in Saskatchewan in 2009. The Alberta survey is the first to be analysed and released. In addition to the field survey in late summer in Alberta, another 100 fields were surveyed in the early spring of 2007 for kochia and Russian thistle resistance. The last Alberta weed resistance survey was conducted in 2001.

Group 1 wild oat resistance growing
Wild oat and green foxtail are the second and ninth most abundant weeds in Alberta. Seed samples collected in 2007 were screened in the greenhouse with seven Group 1 herbicides, and three Group 2 herbicides. Of the 179 fields where wild oats were sampled, 43 percent had resistance; of those, 39 percent had Group 1 resistant wild oats, up sharply since 2001 when 11 percent of the fields had Group 1 resistance. “That’s quite a jump over 2001, and based on that trend, by 2010 likely one-half the fields with wild oats will be resistant to Group 1 herbicides,” says Beckie. 

Two-thirds of the Group 1 resistant wild oats were found in the Parkland region (Black and Gray soils), although they were found throughout Alberta. The resistance also showed broad cross-resistance to the three chemical classes found in the Group 1 group: the FOP (aryloxyphenoxy-propionates), DIM (cyclohexanediones) and DEN (phenylpyrazolin) classes. “There wasn’t a significant difference in the extent of resistance among the various Group 1 classes. This is quite a change from the 2001 survey where we saw more FOP than DIM resistance,” explains Beckie. “As time goes on, we will likely continue to see more broad cross-resistance among Group 1 products.” 

Somewhat surprisingly, Group 2 resistant wild oats had a frequency of 12 percent of the populations in 2007, which was similar to 2001. Again, two-thirds of the resistance was found in the Parkland region. 

Beckie explains that the difference in frequency of resistance between Group 1 and Group 2 is related to herbicide use. Seventy percent of the surveyed fields were cereals. Group 1 herbicides are used much more frequently in cereals than Group 2 wild oat herbicides. For example, in wheat crops since about 2005, Group 1 products were used 87 percent of the time for wild oat control, compared to only 12 percent for Group 2. In pulses, which comprised less than five percent of surveyed fields, Group 2 grassy herbicides were used 67 percent of the time with Group 1 the other 33 percent of the time. “Given the substantial acreage cropped to cereals in Alberta, the heavy reliance on Group 1 herbicides and less frequent use of Group 2 wild oat herbicides largely explains the resistance results,” says Beckie. 

A growing concern for cereal producers is Group 1 + 2 dual-resistant wild oats. Fifteen fields (eight percent) had this dual resistance compared to only six fields (three percent) in 2001. The most likely reason for the dual-resistant wild oat was the frequent use of a Group 1 herbicide that resulted in resistance, followed by the use of a Group 2 herbicide. This is a big concern for cereal producers because there are limited options for control of Group1 + 2 resistant wild oats. 

Green foxtail, a minor weed in Alberta, also had relatively minor resistance, with two Group 1 cases found in the survey. There were not any Group-resistant green foxtail populations found.

Expect kochia to be Group 2 resistant
Kochia and Russian thistle are ranked 20th and 24th in relative abundance in Alberta, but in southern Alberta, Russian thistle is ranked second and kochia third. The survey used different methods to determine herbicide resistance. In collaboration with Agricore United, an early spring survey was conducted, seedlings collected, and then grown out to maturity in greenhouses. Seeds were harvested, later planted, and then sprayed with the Group 2 herbicides to test for resistance. Ninety-five kochia fields and 14 Russian thistle fields, mostly from southern Alberta, were selected on the basis of known occurrence of those weeds and sampled in late April or early May. 

Almost 90 percent of the kochia samples had resistance to the Group 2 herbicides. “When you consider that we have gone from complete susceptibility to Group 2 herbicides to almost complete resistance in 20 years, that’s astounding,” says Beckie. 

The Alberta result is similar to an earlier 2004 survey in Manitoba where 90 percent of the 112 fields had Group 2 resistant kochia. Beckie explains that kochia is well suited to developing and spreading resistance, since it produces a large amount of seed per plant, and has three different herbicide target site mutations that can confer resistance. The most common mutation confers broad cross-resistance across Group 2 chemistries. 

Group 2 resistant kochia is also able to germinate at lower temperatures than susceptible kochia, especially at soil temperatures lower than five degrees C. Essentially, when spraying with a Group 2 herbicide, farmers are selecting for hardier populations that are also more adapted to more northerly areas, perhaps one of the reasons that kochia is spreading northward (the other being that kochia does well in direct seeding). “If you have kochia, assume it is Group 2 resistant,” says Beckie. “There are some good options in cereals for Group 2 resistant kochia control, but pulse growers will be increasingly challenged with Group 2 resistance since they rely on these products for broadleaf weed control.”

Only one-in-14 fields had a Russian thistle population that was Group 2 resistant. “It is a bit of a mystery why resistance has taken divergent paths between these two tumbleweeds that are biologically and ecologically similar,” says Beckie.

Group 2 resistance was documented in 40 percent of the 30 fields with chickweed (17 percent in 2001), all 11 fields with spiny annual sow-thistle (67 percent of fields in 2001), 17 percent of 30 fields with cleavers (not reported in 2001), and one field with wild buckwheat, the first global report. Resistance is steadily increasing in chickweed and spiny annual sow-thistle, and most recently, cleavers.

However, the good news is that weed resistance to other herbicide groups remains low or nil. “These herbicides with different modes of action will be needed more and more to manage Group 1 or 2 resistance.” concludes Beckie.