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The Threat Is Real

By United Soybean Board
Herbicide Resistance Palmer Amaranth
Herbicide resistance is growing; make sure your management practices keep pace
Herbicide-resistant weeds are not new, but the affliction is growing. The first reported instance of weeds becoming resistant was in the 1960s.
Today, according to the International Survey of Herbicide Resistant Weeds, there are 464 unique cases of herbicide-resistant weeds globally among 249 species. Weeds have developed resistance to 22 herbicide sites of action and to 159 different herbicides. Herbicide-resistant weeds have been reported in 86 crops in 66 countries.
University of Arkansas weed-resistance expert Jason Norsworthy, Ph.D., says the rate of glyphosate resistance continues to grow and spread. By 2020, almost all U.S. row-crop acreage, more than 164 million acres, will be affected by glyphosate-resistant weeds.
Farmer survey information showed that, in 2010, about 7 percent of farmers dealt with resistant waterhemp. By 2014, over 28 percent of farmers contended with resistant marestail. Similar resistance growth patterns have been observed for ragweed, kochia and the feared Palmer amaranth.
A statement by American Soybean Association director Eric Maupin, who farms in Tennessee, puts the threat of herbicide resistance in a chilling perspective: “We are quickly approaching the day when we will be unable to grow soybeans in parts of the U.S. because there are no effective herbicide options.”
Glyphosate reliance
Norsworthy likens the discovery of glyphosate to that of penicillin, which was an extremely important development with tremendous effectiveness. Overreliance on that treatment led to reduced effectiveness.
“Resistance develops when we plant single-trait seed, use a single herbicide and maybe spray once,” Norsworthy says. “If you lack diversity, it (resistance) will develop. Simplicity has a cost. If we think another simple solution is coming, we’re wrong because there is nothing.”
“We got here because we were doing the same thing over and over because it was effective and easy,” adds crop consultant Greg LaPlante. “The easy button is gone.”
Growers are seeing more resistance in marestail, waterhemp, kochia and ragweed. Glyphosate-resistant Palmer amaranth is present in 30 states.
Game changer
LaPlante says that resistance can become a problem in fields quite quickly. A single weed that escapes can produce thousands of seeds. The next year, if treatment methods don’t change, the population of weeds with resistance to that method can grow, but still not be enough to cause alarm for farmers. Dozens of plants producing thousands of seeds is a major problem waiting to happen. By the third year, if practices don’t change, the number of weeds growing in the field that have resistance can be overwhelming.
To compound issues with glyphosate resistance, Norsworthy says that farmers and scientists are now seeing weeds with two-, three- and even four-way resistance. “Stacked resistance is a game changer.”
Glyphosate resistance has resulted in increased governmental regulation as evidenced by the new Enlist Duo label which contains herbicide-resistance language. On the label, the Environmental Protection Agency (EPA) says that herbicide failures must be monitored and reported. The label mandates which nozzles must be used and stipulates tank-mixture requirements. Product registration is conditional and expires in 4 or 5 years, based on the number of acres planted.
A delicate balance also exists between resistance management and the required buffer areas. Some herbicides require buffers between application areas and susceptible vegetation or waterways. Those unsprayed areas can be problematic because buffer and border control is an important component of weed management.
Develop a plan
While glyphosate resistance is growing, weed control is still possible, even if it may be more complicated. Experts say that successful management involves knowing what weeds are present in the field and developing a plan to treat them. That plan needs to include multiple sites of action.
Experts advise farmers to start clean and to stay clean, which means having no weeds at planting and then overlaying residual herbicides. Norsworthy says that a good rule is to apply residual herbicides every 2-3 weeks through canopy formation using multiple effective modes of action. He also says that a quick crop canopy is an effective method for weed control.
Norsworthy also advocates flagging the technology. Flags marking which herbicide-tolerant seed varieties are planted in certain areas of the field can help to ensure that correct modes of action are used on those crops, reducing the chances of inadvertently spraying fields with the wrong herbicide.
The weed-management process is becoming more complex, and growers may also incur additional costs. In the long run, it is still cheaper than losing entire fields to out-of-control weeds.
“It’s most expensive when fields are overrun and farmers take a large yield hit,” says LaPlante. “The cost of a herbicide program is much higher to address a problem than it is to control weeds in the first place.”
Multiple solutions
Weed management involves more than herbicides. Norsworthy encourages farmers to integrate both chemical and non-chemical solutions. 
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