On the Dennis Potter farm in southwest Minnesota, long rows of soybeans stretch to the horizon. Between most rows is bare dirt. But in one patch near the farm's driveway, Potter points to weeds covering the ground between the soybean rows.
"This is water hemp that's kind of mutant because it was damaged by the Roundup," says Potter. "It's still alive, it'll still grow seed for next year and we have to try to hit it with Roundup again and kill it."
Some call the common farm weeds a super weed. That's because they're resistant to weed-killing herbicides, such as Monsanto's bio-engineered RoundUp Ready herbicides. The spotlight has recently been on the company, which admits resistance is a growing problem, though one Monsanto official said it can be controlled.
The weed has a thick root and one slender sprout that seems too small for its big underground structure. That illustrates the herbicide resistance problem. The Roundup herbicide, known as glyphosate, killed the above ground part of the plant but the root lived on. Later, it sent up a new shoot.
"It's part of what happens frequently now," Potter said. "We're developing, I think, plants that can tolerate Roundup to a certain degree."
The super weeds are a concern because if farmers can't control them it could reduce crop yield. If yields suffer, farmers could lose money. If the weed problem becomes severe enough, it could even affect the nation's crop production.
The problem started more than a decade ago when Monsanto launched its Roundup line. The company was able to place a gene into soybean, corn and cotton plants that makes them immune to glyphosate weed-killers.
Before Roundup, farmers largely destroyed weeds by plowing them under, or by walking their fields and spraying a weed treatment on the pests. With Roundup, southwest Minnesota farmer Potter said he can spray herbicide on his fields without worrying that the weed-killer will harm his soybeans.
"We were hoping this was the miracle herbicide that was going to kill everything," he said.
But the weeds had a way to fight back. Even though it may have looked like the herbicide was killing everything, it wasn't. Certain weeds, maybe only one in a field, had a slightly different genetic makeup from their parents -- something that allowed the plant to survive glyphosate. The offspring of the weed also contained this immunity. So the problem grew.
It grew so big that the company that originated the Roundup system is concerned.
"We view resistance very seriously," said John Soteres, Monsanto's lead scientist on herbicide resistance.
Soteres said farmers must adopt new practices to combat weeds that survive the glyphosate herbicide.
"All situations of glyphosate-resistant weeds, there are options for the grower today to manage those," he said.
Soteres recommends farmers use multiple herbicides on their crops. The idea is that any glyphosate-resistant weeds will be killed by the new weed treatment, one which they don't have immunity to. But the strategy of using an ever-larger number of herbicides, though, worries some farmers -- especially those who don't use any weed killers at all. They say all those herbicides actually harm crop production.
Some studies show glyphosate herbicides prevent Roundup-treated crops from absorbing some nutrients from the soil, said Dan Specht, an organic farmer from northeast Iowa.
"Roundup-treated crops just aren't quite as healthy as the non-Roundup crops," he said.
Monsanto said there's no evidence to support that finding.
But in southwest Minnesota, Potter is paying attention to the weed resistance problem in his fields. He plans to spray weeds that survived the initial glyphosate treatment a second time.
"Hopefully we get them in a stage where we can kill them instead of just slow them down," he said.
Potter said he's had to spray as many as three times to kill all the weeds. It's the sort of situation more Minnesota farmers could face if herbicide resistant weeds continue to spread.