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Herbicide Resistance
by LuAnn Rolling, District Conservationist
The first synthetic herbicide was discovered in the 1940s. Scientists discovered new modes of action every few years through the 1980s. That changed with the introduction of glyphosate-resistant crops.  Roundup, paired with glyphosate-resistant crops, was so effective it overwhelmed most competition, contributing to a lapse in herbicide research.

According to the EPA, about 280 million pounds of glyphosate are used in the U.S. annually - three times more than all other pesticides.  Unfortunately the excessive use of this and other herbicides has led to crops “evolving” to be resistant to chemicals.

Resistance is part of a weed’s basic biological struggle for survival. When a field of weeds is sprayed again and again with a single herbicide, that herbicide kills weaker individual plants, but some naturally strong weeds have random traits that allow them to survive the herbicide. These survivors reproduce, passing on resistant traits to the next generations.

The Washington Grain Commission estimates herbicide-resistant weeds across all crops increase growers’ costs on average 30% to 40% per acre. In U.S. corn and soybeans alone, researchers estimate uncontrolled weeds amount to more than $43 billion in annual losses.  Growers now grapple with so-called “superweeds” present on hundreds of millions of acres across the U.S., according Stratus Ag Research

Gina Nichols, a Ph.D. candidate at Iowa State University who researches weed control, says that with the way we use herbicides, even if we introduce a new one, it’s just a matter of time before resistance evolves.  She firmly believes we should be pursuing integrated weed management.  Nichols said she likes to think of weed suppressing methods such as crop rotation and cover cropping as non-chemical modes of action.  The farmers Nichols works with say they’ve seen herbicide-resistant weeds decrease when using cover crops.

Nichols advocates a multi-strategy approach. Her research shows cover crops reduce the biomass, or size, of weeds but not the number. Crop rotation, in contrast, reduces the number of weeds but not the biomass. So cover crops and rotations, she said, are complementary.

“Shaking up the planting interval”, leaving more time between diverse crop rotations, can reduce weeds by 50%, her research shows. And including a forage in the crop rotation can reduce weeds by 80%.

As producers look at 2021 cropping decisions, and herbicide programs, I would like to encourage them to consider a third crop in the rotation and cover crops.  We have a special program through the Allamakee Soil & Water Conservation District where we can assist producers with planting a small grain followed by a diverse summer cover crop.  We also have cost sharing for cover crop after harvest and for inter-seeding cover crop into corn at the V2 - V5 stage.

By approaching weed resistance with cover crops and breaking up rotations we can also greatly improve soil health which will lead to other input reductions and cost savings.

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