On a research plot near the Rochester, Minn., airport, Jared Goplen has watched weeds for the past three summers. His specialty is giant ragweed, one of more than a dozen species of "superweeds" that resist the most widely used herbicides. Superweeds can take over cropland, reduce yields and wipe out farmers' profits. Even consumers can face a secondary effect in the form of higher food prices.

"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.

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."

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.