With the first of a new generation of genetically engineered crops ready to hit the market, the battle lines are being drawn.

Food safety activists have promised to fight the crops—corn and soybeans designed to tolerate multiple herbicides—in court. They and many scientists argue that these crops will harm environmental and possibly human health. The companies that make them say they're providing a much-needed tool to fight the growing scourge of herbicide-resistant weeds.

Lawsuits aside, these crops and others like them may force a showdown between conflicting approaches to farming: one that depends on chemicals to fight weeds, and another that embraces ecology's lessons.

"We are on the brink of a crisis situation," said Neil Harker, a weed ecologist with Agriculture and Agri-Food Canada, the Canadian equivalent of the US Department of Agriculture. "I do consider right now to be a watershed, direction-defining moment for agriculture."

On September 17, the USDA officially approved Dow's new corn and soy varieties. These can withstand both glyphosate, the world's most widely-used herbicide, and a popular but comparatively lesser-used herbicide called 2,4-D. The U.S. Environmental Protection Agency is expected to approve Dow's proprietary formulation shortly, allowing the system to enter commercial use next year.

Regulatory review of Dow's crops, marketed as the Enlist Weed Control System, took several years. Critics have argued that dramatically increased 2,4-D use poses a threat to people using the herbicides, and also to the environment. Moreover, the problem the crops are solving—so-called superweeds that tolerate glyphosate, better known by its trade name Roundup—was created by glyphosate's indiscriminate use.

Most corn, soy and other field crops grown in the United States are genetically engineered to tolerate Roundup, a trait developed by agrotech giant Monsanto in the early 1990s. Glyphosate use exploded: rather than spraying herbicides on a weed-by-weed basis or pulling them by hand, farmers could use the herbicide on entire fields.

"This was an economically rational decision. It just wasn't a biologically rational decision," said herbicide resistance specialist Stephen Powles at a recent Weed Science Society of America meeting. It favored the evolution of superweeds, which now pose an enormous agricultural threat.

Superweeds now infest an estimated 70 million acres of U.S. farmland, causing roughly $1 billion in damage. The problem is growing fast, and farmers have scrambled for solutions. Dow and other large agrotech companies, including Monsanto, Pioneer and Syngenta, have responded by engineering plants to withstand combinations of herbicides rather than glyphosate alone.

Enlist is the first of these crops, and it could set an important precedent. Yet many scientists say simply using more, different herbicides will hasten the evolution of ever-more-resistant superweeds, putting agriculture on what some scientists have called an herbicide treadmill: more herbicides and more resistance, over and over.

The EPA ad USDA have largely downplayed that possibility, with the EPA's plan to manage the evolution of resistance to Enlist consisting largely of Dow-led monitoring efforts. According to the EPA, that's a model for regulating future multiple herbicide-resistant crops.

"In our view, it's pathetically weak," said science policy analyst Bill Freese at the Center for Food Safety, an advocacy group that has pledged to "pursue all available legal options" in fighting Enlist's approval. "There is not a single meaningful" requirement for preventing superweed evolution in the EPA's plan, said Freese.

The Center's legal strategy will likely focus on the USDA, said Freese. The agency is required by law to consider the spread of so-called "noxious weeds" in its decisions, and the Enlist system will almost certainly hasten their spread. A similar strategy was used in the Center's lawsuits over Monsanto's glyphosate-resistant alfalfa and sugar beets.

Those crops were ultimately approved, however, with lawsuits only delaying their market arrival for a few years. And the legal approach doesn't address what could happen if Enlist and other such crops are rejected: farmers may turn, as many already are, to simply using more herbicides on existing crop varieties. They'll just spray the herbicides directly onto weeds, rather than across entire fields in one convenient swoop.

There are alternatives, Harker and other weed scientists say. They've simply been neglected—and not just by farmers and companies, but by scientists.

"It is clear to most weed scientists who are involved in herbicide research, and even those who are not, that the best way to reduce selection pressure for herbicide resistance is to minimize herbicide use," Harker wrote in a 2012 paper in the journal Weed Science. "However, the 'solutions' that have emerged ... have usually involved more herbicide use."

In a review of scientific papers published between 1994 and 2012, Harker found that studies on herbicide-based weed control methods dramatically outnumbered those on other methods. Known as integrated weed management, or IWM, these methods usually involve growing multiple crops in rotations designed to slow the spread of weeds.

Diversity is the key: while weeds inevitably evolve in response to pressure, mixing the type and timing of those pressures keeps resistance from accumulating. "Herbicides alone are not sustainable. Diversity is the only way forward," Powles said at the Weed Science Society meeting.

For IWM to go mainstream, scientists must play a crucial role in making it as profitable and productive as herbicide-dependent industrial agriculture. They'll fine-tune techniques like seeding rates, harvest times, weed seed destruction and the use of perennial plants, said Harker, and work with economists to show that the methods are viable at large scales.

Transforming agricultural practices won't be easy. Farms in the United States have steadily expanded while employing fewer people, making homogeneous, chemically-intensive approaches more convenient. Yet IWM already has a long track record of success.

Many of its practices were used and refined by generations of farmers before glyphosate's ubiquity. Weed ecologist Adam Davis of the USDA, who with agronomist Matt Liebman of Iowa State University has pioneered the modern study of IWM, said much of their work involved preserving knowledge that was previously maintained in oral traditions.

"The practices I was eager to see adopted 20 years ago are now being sought out by growers," said Davis. He and Liebman have demonstrated that, with a few modern tweaks, those integrated methods can produce industrial-scale yields with comparatively small chemical input, at competitive costs.

Herbicides are still a small but useful part of their system, applied judiciously when absolutely necessary. Which leads to another another criticism of the new, multiple herbicide-resistant crops: they're making existing herbicides, particularly glyphosate, less useful.

Glyphosate is popular for good reason. Compared to most other herbicides, including those making a return with the newly-engineered crops, it's effective and fairly safe. In his talk, Powles likened it to penicillin, the wonder-drug antibiotic crippled by overuse. "Glyphosate is the world's greatest herbicide. It's a once-in-a-hundred-year discovery," he said, and the new crops could render it obsolete.

Powles isn't very optimistic that modern farming will cure itself of what he calls its herbicide-only syndrome. "There will likely be a glyphosate train wreck before change occurs in the United States," he said. But Adam Davis struck a different note.

Especially in the southern United States, where farmers have been hit especially hard by superweeds, many farmers are already seeking out information on integrated weed management, said Davis. "They're not doing this because it's groovy or green," he said. "They're doing it because they're running out of chemical options. The weeds have forced their hands, and will increasingly do so across the globe."