“It’s a serious problem and one that will continue to grow,” said Paul Meints, research program manager for Minnesota Soybean.

Weeds that won’t succumb to mainstream herbicides are a rising concern nationally, especially in cotton, corn and soybean country, and the largest agribusinesses are racing to propose solutions. In Minnesota alone, growers plant nearly 16 million acres of corn and soybeans each year.

Goplen, a University of Minnesota graduate student, is testing whether crop rotation and other non-herbicide methods can make a difference in keeping weeds under control. He records the number of giant ragweeds as they come up, collects and counts seeds that fall from mature plants, and even sifts seeds in the soil to map hot spots in the seedbank where seeds are waiting to sprout next year.

In states such as Arkansas, Tennessee and Georgia, the primary menace is a different weed.

Thousands of acres of soybeans and cotton had to be mowed down in recent years because the herbicide-resistant Palmer amaranth had overrun the fields.

Meints said the Palmer weed has reached southern Iowa but is not yet in Minnesota, where farmers grow more than 7 million acres of soybeans and about 8.5 million acres of corn. No Minnesota farmers have lost entire crops to herbicide-resistant weeds, he said, but some have experienced yield losses.

University researchers and grower associations have pushed hard the past couple of years to let farmers know that using the same herbicide year after year is a bad idea, Meints said, and that herbicides and crops need to be rotated more frequently to lessen the chances of runaway superweeds on their fields.

‘The big hammer’

Weeds that can tolerate herbi­cides are nothing new, said Jeff Gunsolus, University of Minnesota Extension agronomy professor and weed specialist. Weeds are able to adapt to different environments, he said, so it shouldn’t be surprising that they can also adapt to certain herbicides.

He said only a small number of weeds within a species, perhaps one in a billion, have the genetic makeup that enables them to survive a particular ­herbicide application. But that single weed can produce 10,000 to 300,000 offspring seeds, depending on the species, that also will be resistant to the herbi­cide. Those that sprout the next year or remain dormant in the soil for a longer period also won’t be killed, he said, unless the farmer applies a different herbicide that’s effective against them.

Changing herbicides annually or using multiple herbicides was a standard practice in the 1970s and 1980s, and agronomists referred to the chemicals as tools to control weeds and get the most yield from crops.

But in 1996 came the tool that some nicknamed “the big hammer”: Monsanto introduced Roundup Ready, a seed and herbi­cide combination that allowed farmers to plant genetically engineered soybeans that would not be harmed by the herbicide glyphosate, sold under the trade name Roundup.

Farmers could apply Roundup and kill nearly everything in sight, except the soybean plants, whose seeds were genetically altered to tolerate the herbicide. Corn, cotton and sugar beets were soon modified to tolerate it as well.

The system was revolutionary for farmers because it simplified the issue of what herbicide to use and reduced time and money spent to grow crops. Gone was the need to till the soil, apply multiple herbicides and use cultivators to turn under weeds or hire laborers to yank them.

Roundup Ready seeds now account for about 90 percent of soybeans and 85 percent of corn planted in the United States each year. “Initially when the technology first came out, you literally could go into the fields and kill weeds that were 18 inches tall and they would all die,” Gunsolus said. “It was just like penicillin was in the 1950s. It was a miracle.”

Losing effectiveness

But overreliance on Roundup accelerated the spread of weeds resistant to glyphosate. After the first few years of remarkably clean fields, farmers began to notice that they needed to apply Roundup earlier in the year, when weeds were no more than 3 or 4 inches tall. Then, some fields began to need two or three applications a year for effective weed control.

Weeds that were resistant to glyphosate survived, flowered and seeded. In fields that used the herbicide year after year, the weed populations skyrocketed.

“We fell into the habit of this particular scheme of producing our crops, and one of the consequences is the development of these resistant [weed] populations,” Meints said. “By developing these fantastic systems using a single or only a couple of herbi­cides, we also accelerated the [ability] for those herbicide-resistant weeds to flourish.”

He said the main glyphosate-resistant weeds of concern in Minnesota are kochia, a tumbleweed-type plant found mainly in northwestern Minnesota’s wheat and sugar beet fields, common waterhemp in southern and western cropland of the state, and giant ragweed, which appears almost everywhere.

No one knows how many farms in Minnesota are battling ­glyphosate-resistant superweeds or how severe the problem is. “Whenever there’s something new that’s a problem, farmers don’t want to talk about it because they think it reflects poorly on them,” Gunsolus said.

Brian Hicks, who grows corn, soybeans and hay on about 2,500 acres near Tracy in southwestern Minnesota, said it’s clear to him that controlling weeds is no simple matter.

“We still do use Roundup, but we also use other chemistry to make sure that the weeds that we’re targeting will die,” he said. “It’s easy to drive around the countryside and see guys and gals that haven’t been proactive and they do have some [weed] issues that are getting worse all the time.”

Combo herbicides

Large agribusinesses are pushing hard to develop a successor to Roundup.

Dow AgroSciences, a subsidiary of Dow Chemical, has spent the past decade developing Enlist Duo, a herbicide containing a new form of the 2,4-D weedkiller and glyphosate. Company spokesman Garry Hamlin said the combo herbicide has “less drift potential, less volatility, less odor and better handling characteristics.” It would be used with new corn and soybean seeds genetically engineered to tolerate the chemicals. The products need to be approved by two federal agencies and have been opposed by environmental and other groups.

Monsanto proposed the Roundup Ready Xtend Crop system, which combines the herbicide dicamba with glyphosate to be used with newly engineered corn and soybeans.

And Syngenta Seeds Inc. and Bayer CropScience are proposing a soybean genetically engineered to tolerate exposure to the herbicides glufosinate and mesotrione.

Critics of the products say new chemical combinations will harm the environment and public health and may lead to more resistant weeds.

Ben Lilliston, vice president of programs for the Minneapolis-based Institute for Agriculture and Trade Policy, said that combining herbicides into new mixes “keeps us on an increasingly toxic treadmill.”

Gunsolus said herbicides are important tools with great economic benefits but added, “We also need to be looking at non-herbicide solutions and maybe looking at some old ideas in new ways.”

His suggestion list: better targeting of weeds, cultivation and rotation with crops such as alfalfa or winter wheat that could disrupt weed resistance. Some of those traditional practices may be less expensive and time-consuming than farmers think, he said, especially when precision agriculture tools are added to the mix.

Goplen, the weed watcher, also envisions farmers again using many little hammers instead of one big hammer in the battle against weeds.

“You might delay your planting date a week or two, and that will help eliminate 10 or 20 percent of your weeds, or you might till the field an extra time in the spring,” he said. “It’s not going to kill all the weeds, but it’ll take care of some of them, and then you use various herbicides that will take care of any weeds that do escape. The idea is to do a whole bunch of little things to combat the problem because one big thing isn’t going to do it anymore.”