Researchers tackle plains super weeds
FORT COLLINS, Colo. — While tumbleweeds have long settled into the romantic imagery of the plains, the rolling symbol of the Wild West remains far from home.
On Colorado’s eastern plains, increasingly resilient strains of kochia, one of the various plants behind tumbleweeds, have created a headache for both researchers and farmers.
Free from the competition found in its natural environment, the European and Asian transplant has found an easy home in the West — and its roots on the plains continue to dig deeper.
At Colorado State University in Fort Collins, thousands of kochia samples collected on the Front Range grow in greenhouses run by the Department of Bioagricultural Sciences and Pest Management.
Here, researchers hope to find genetic clues on how to manage a species that now resists the industry’s most effective herbicides, including glyphosate, known commercially as Roundup.
“This has all come on at the same time for Colorado, Kansas, Nebraska and Wyoming,” said Eric Westra, a Ph.D. candidate who works with the CSU Weed Lab.
Westra shared a survey map of kochia resistance in Colorado, where its westward spread can be seen with concerning clarity.
While instances of herbicide-resistant kochia remained mostly isolated to far eastern Colorado in 2011, as of 2013, dozens of new cases had already reached across the Front Range and up to the foothills.
Of 62 samples collected in 2011, 9.7 percent had shown resistance to glyphosate. In a survey of 26 samples in 2014, the resistance rate had grown to 46 percent.
CSU professor Scott Nissen said the growth observed in Colorado follows trends already observed in the Midwest and other major corn-growing regions where Roundup-ready seeds have become a blessing and a burden for growers.
The advent of Roundup for crop control has brought a long list of benefits for farmers, Nissen said, including savings in fuel and water, as well as a reduction in soil erosion. Under a chemical fallow system, farmers cut out labor-intensive tillage and increase planting capacity by instead leaving crop residue on the surface and then spraying for weed control.
While the system has proven easy and effective, Nissen said reliance on a single control system has created an ideal environment for the strongest strains of kochia.
The ability to withstand Roundup has always existed in kochia’s gene pool, Westra said, clarifying that so-called “super weeds” are not a creation of herbicides. Rather, these resilient strains have thrived in the absence of competition.
“There can sometimes be a misconception that herbicide use is creating these individuals or doing something to the genes,” Westra said. “A lot of the resistance is naturally occurring in these populations, but with really low initial frequency.”
As weaker strains succumb to herbicide applications, stronger strains are then left with more room to leave their seed and continue rolling across the plains.
“We’d like to be able to tell growers that an option is to increase the rate (of spraying) but then at some point, that is not going to be effective,” Nissen said.
Researchers have also encountered kochia strains that survive sprayings of another common herbicide, known as dicamba. In the most extreme cases, kochia has demonstrated the ability to withstand both dicamba and glyphosate.
Neither herbicide has indicated a greater correlation with resistance, Nissen said. Different populations, however, show distinct reactions. The key to control comes down to implementing variation.
Nissen pointed to dependence on manual weed control on rice farms in the Philippines to demonstrate that no one method can provide a sure solution.
The sole use of de-weeding by hand has resulted in the surge of barnyard grass species that look indistinguishable from rice plants until they are ready to drop their seed.
Fellow CSU weed researcher and assistant professor Todd Gaines said when growers first suspect herbicide resistance has developed on their operations, they should act quickly by implementing alternative control methods. An integrated managed system can include steps as simple as pulling weeds by hand and ensuring fields are as clean of weeds and seeds as possible before planting crops.
He also cited a promising method developed by researchers in Australia that have utilized a special combine to sort out and destroy kochia seeds from the soil.
As herbicide resistance spreads, Gaines said the issue will need to be addressed not only in Colorado, but worldwide.
“We have some of these issues here regionally but really herbicide resistance has been an issue nationwide,” Gaines said.
In 2017, CSU will help bring researchers together in Denver for the second Global Herbicide Resistance Challenge, held previously in Perth, Western Australia.
The conference will seek to gather scientific resources and encourage collaboration as agriculturists tackle difficult management questions.
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