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Farmers have been battling herbicide-resistant weeds for generations. A common practice for most of that time has been to rotate between different herbicides every season. But despite farmers' best efforts, herbicide resistance has grown through the years, with some weed populations showing resistance to not one but four or five different herbicides. A new study from the University of Illinois explains why herbicide rotation doesn't work.
"If you were to ask farmers what is the one thing you can do to delay resistance evolution, they'll say rotate herbicides. This study shows that's not true," says Pat Tranel, Ainsworth Professor in the Department of Crop Sciences at U of I.
Herbicide - Resistance - Results - Mutations - Weeds
Herbicide resistance results from random genetic mutations that keep weeds from being harmed by a particular herbicide. When farmers continually spray the same herbicide year after year, those with the mutation, referred to as a resistance allele, survive and reproduce. Over time, the proportion of plants with the resistance allele grows.
Conventional thinking says that any defense trait—in this case, herbicide resistance—should come at a cost to the plant. It might be well protected against the herbicide, but it might not grow as tall, or flower as early. When the trait reduces a plant's reproductive output, that's known as a fitness cost.
Fitness - Cost - Resistance - Years - Herbicides
A fitness cost to herbicide resistance should be apparent in years when alternative herbicides are used. "If plants have glyphosate resistance, but they're sprayed with 2,4-D, for example, the majority of those plants will die because they're not resistant to 2,4-D. But no herbicide kills 100 percent of the weeds, resistant or not," Tranel says. "You have to think about the small percentage that live.
"If there's a high fitness cost to the glyphosate resistance allele, most of the surviving plants will be small or will flower late and they won't produce many seeds....
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