Tank mixing two herbicides with different modes of action targeting the same weed is a good way to delay the development of herbicide-resistent weeds, a study by weed scientists at the University of Illinois and United States Department of Agriculture concludes.
“We don’t say that mixing is the end-all solution,” study co-author Pat Tranel of the University of Illinois said in a news release May 21. “What we saw from this study (is) if success for farmers is measured by lack of resistance or lower frequency, then successful farmers use multiple herbicides per application.”
The study, which focused on glyphosate-resistant waterhemp, collected weed management data from 105 central Illinois grain farms, including almost 500 site-years of herbicide application records. The data from 2004 to 2010 helped the researchers identify relationships between past herbicide use and current glyphosate resistance.
The study’s conclusions are similar to those reached in research papers published by Hugh Beckie, an Agriculture and Agri-Food research scientist in Saskatoon. However, Beckie notes the University of Illinois study is based on one weed — waterhemp — and one herbicide — glyphosate. One needs to be cautious applying the results to other herbicide-resistant weeds or cropping systems, he said.
The University of Illinois study concludes rotating herbicides annually “actually increased the frequency of resistance.” However, that was in fields where glyphosate had been used in more than 75 per cent of the seasons studied and where fewer, different modes of action (MOA) were used each year.
“When using an average of 2.5 MOAs per application (through a tank mix), you are 83 times less likely to have resistance compared to if you used only 1.5 MOAs per application,” Tranel said. “That’s pretty amazing that adding one additional mode of action in your tank reduces your chances of resistance by that much.”
Beckie also sees value in rotating modes of actions. Tank mixing different modes of action is harder to do with some weeds such as wild oats and green foxtail because there are fewer options, he said.
“That’s why we encourage farmers to do both (tank mix and rotate herbicide MOAs),” he said. “To delay resistance, both mixing partners need to have activity on that weed and usually when farmers tank mix they want to broaden their spectrum of weed control.”
The University of Illinois study, like Beckie’s research from 10 years ago, concludes what farmers do in their fields affects how quickly herbicide-resistant weeds multiply.
“The good thing is, not only does management matter, it’s what you do in your own field that matters,” Tranel said. “If you’re doing the right thing it will stay at a small frequency.”
Some herbicides, such as those in Group 1 and 2 result in quicker resistance than others. A study Beckie published in 2009 showed 85 per cent of pennycress (stinkweed) developed Group 2 resistance after just four applications.
But another Beckie study revealed rotating crops can slow the development of resistant weeds, especially when fall-seeded and perennial crops are included.
“Our crop rotations in Western Canada desperately need fall-seeded crops,” he said. “All of our studies have found a statistical correlation between diverse crop rotation and less resistance. That has always come out on top.
“That’s the bottom line I always tell growers: ‘Don’t grow canola year after year or don’t grow soybeans for 10 years in a field.’
“Perennials help to clean up the seed bank, especially wild oats.”
Beckie has also found reduced tillage brings on herbicide-resistant weeds quicker than conventional. No till is worse for two reasons — more herbicide use and quicker seed bank replacement.
“When that seed bank is turning over fast, then you get resistance happening quicker than say, when they’re buried and they’re dormant and more staggered germination over time,” he said.
Nevertheless, Beckie says farmers shouldn’t abandon no till because it provides so many other benefits.
Herbicide-resistant weeds were first found in Manitoba in 1989 by the late Ian Morrison.
“We’re getting into almost 30 years now of resistance. If they don’t have resistance to some key weeds, we can talk about delaying it. But in many respects these mixtures and rotations are good for managing resistance even if they have it.”