Varied crop rotations the best tool in fighting weed resistance, says study
Farmers who use a diversity of crops such as cereals, oilseeds, pulses, specialty, perennial forage and fallow in their rotation strategy reduce their risk of resistance. "Cropping system diversity is clearly identified as the foundation of proactive weed resistance management," says Hugh Beckie.
Posted: September 1, 2009
Diversified crop rotations are the most effective tool producers have against weed resistance, according to new research. At the same time, producers relying on cereal-based rotations may be putting their crops at risk.
According to the results of a project set up to identify farm management practices that influence the risk of herbicide resistance in weeds in the Prairies, farmers who grew three or more crop types in their rotation over a six-year period were at least risk of weed resistance. The risk of weed resistance was greatest in reduced-tillage systems due to greater herbicide use, faster weed seed bank turnover or both. Of these, fields with cereal-based rotations were at greatest risk.
"Even two crop types in the rotation, not including forages or fallow (summer or green manure), are still insufficient to significantly reduce the risk of weed resistance," says project spokesperson Hugh Beckie with the Agriculture and Agri-Food Canada (AAFC) research centre in Saskatoon, Saskatchewan.
Meanwhile, the researchers found that other things producers are doing to reduce weed resistance may not be effective. "Farmers had indicated a high level of adoption of herbicide group rotation, tank-mixing broadleaf weed herbicides, and scouting fields after in-crop herbicide application. Surprisingly, none of these practices affected the risk of weed resistance, suggesting that the rigor or effectiveness of their implementation is less than required."
The take-home message, says Beckie, is that farmers who use a diversity of crops such as cereals, oilseeds, pulses, specialty, perennial forage and fallow in their rotation strategy reduce their risk of resistance. "Cropping system diversity is clearly identified as the foundation of proactive weed resistance management," he says.
Before embarking on the project, the team reviewed what it knew about resistance based on previous research. In a field and farmer survey in Saskatchewan in 1996, resistance to Group 1 herbicides in wild oat was linked to frequent use of those herbicides. Weed sanitation, such as cleaning equipment when moving between fields, tarping grain trucks or applying composted versus fresh manure reduced the spread of resistant seeds.
Also, in a survey in southern Alberta in the late 1990s, Group 1 and Group 2 resistance in wild oat was linked to a lack of crop rotation diversity. Group 2 resistant wild oat was also associated with reduced-tillage systems and recent use of herbicides with that mode of action.
The new project included 370 of 800 fields surveyed for resistant weeds from 2001 to 2003. One-quarter of the 370 fields had herbicide-resistant weeds including wild oat, green foxtail, kochia, chickweed, spiny sowthistle and redroot pigweed populations.
Four farm management practices were studied for their relationship to weed resistance. They included weed management, crop rotation, tillage intensity and farm size and type.
Weed management. Farmers who identified wild oat as most troublesome were most likely to have resistant populations than those who did not. This suggests that farmers – most of whom are unaware of resistance in their fields – are not avoiding herbicides to which the weed is resistant, says Beckie.
Crop rotation. The risk of weed resistance was greatest in cereal-based rotations. However, resistance was reduced when perennial forage crops or fallow (summer or green manure) were included at least one year in a six-year rotation. Those farmers who grew three or more crop types – such as cereal, oilseed, pulse, specialty, perennial forage and fallow – in their rotation had a reduced risk of weed resistance.
Tillage intensity. Weed resistance risk was greatest in reduced-tillage systems and particularly low soil disturbance zero-tillage. Greater herbicide use in these systems compared to conventional tillage had been documented in the 2003 Saskatchewan weed survey. Half of the fields in this study were included in that survey.
"In addition, faster weed seed bank turnover in reduced versus conventional tillage systems favours the evolution of herbicide resistance in weeds," says Beckie.
Farm size and type. Producers with farm sizes greater than 1,000 acres had an increased risk of weed resistance than smaller farms. According to Statistics Canada, the average farm size in Manitoba, Alberta and Saskatchewan in 2006 was 1,000, 1,055 and 1,450 acres, respectively. "The reason for this association is unclear, but greater time pressures for producers with large farms might be negatively impacting the adoption of integrated weed management practices," says Beckie.
Farmers with hogs as their main commodity were also at increased risk of weed resistance. "There may be a linkage between weed resistance and application of hog manure, and presumably associated weed seeds, onto land, although no association was detected between weed resistance occurrence and farms with cattle as the main commodity."
The research team included Hugh Beckie, Julia Leeson and Gordon Thomas with AAFC Saskatoon, Linda Hall with the University of Alberta in Edmonton, and Clark Brenzil with the Saskatchewan Ministry of Agriculture in Regina.
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