Rotate, rotate, rotate.
Not long ago that was the mantra of weed specialists.
The best defence against herbicide resistant weeds was a robust crop rotation and also a rotation of herbicides with various modes of action, or MOAs.
But rotating herbicides is less valuable than we thought.
Weed specialists are now talking about the superior benefits of tank mixed herbicides in preventing resistant weeds.
This may be true, but we worry that over reliance on tank mixes will raise public relations issues. Many consumers already fear the impact of farm chemicals on human health and the environment.
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And activists ratchet up the rhetoric, painting pictures of marauding super weeds. Anti-GMOers see solutions such as tank mixes and crops with stacked herbicide tolerances as evils propagated by profit-hungry corporations.
It is easy to imagine the furor and fear sparked by social media mavens demonizing farmers for their use of “heavier doses of poisonous chemicals,” in “popular food products that children eat.”
Farmers don’t spray herbicides negligently and they aren’t enthusiastic about using more because they are costly.
But they are interested in preserving the value of existing herbicides and one way to do that is to use a tank mix.
Research looking at five years of data on 105 Illinois grain farms, focusing on the factors that contributed to glyphosate resistance found that herbicide rotation alone was ineffective in stopping the development of resistant weeds, particularly in systems that often used glyphosate.
But using a herbicide mix, so that the weeds face two MOAs at the same time, was highly effective.
The study’s conclusions were similar to research conduced by Hugh Beckie of Agriculture Canada in Saskatoon and Xavier Reboud, a French plant scientist, that also found herbicide mixes more effective than herbicide rotations.
This is important information for farmers who might develop a false sense of security thinking that herbicide rotations were enough to avoid resistant weeds.
Farmers must resist the siren song of prescription or recipe farming that can get them on a pesticide treadmill that is not sustainable and that opens the door to consumer mistrust and potentially, government regulation.
The strongest science supports integrated weed management, a systems approach designed to keep weeds off balance and less able to adapt to herbicides.
Integrated management still puts a lot of emphasis on rotation, of crops and seeding dates, as well as seeding rates to crowd out weeds, use of clean seed, adequate fertility, inclusion of spring and winter-seeded crops and perennial forages.
Tank mix herbicides will be part of this system, but are not adequate alone.
An integrated management system attuned to the dynamic ecology of modern, efficient farms is a compelling story that will ring true with most consumers.
