The latest in a new generation of genetically engineered crops is poised to enter widespread use—and critics think they’ll cause more problems than they solve.

Proponents of the new cotton and soybean varieties, engineered by Monsanto to tolerate spraying with multiple herbicides, say they’re a much-needed tool. “These weed management solutions will provide farmers with more consistent, flexible control of tough-to-manage broadleaf weeds,” said Monsanto in a press release issued after the US Department of Agriculture approved the crops for use last month.

But others think the benefits will at best be short-lived. Weeds may soon become resistant to the new herbicide mixtures, resulting in new generations of ever-more-intractable weeds that will need to be controlled with yet more herbicides.

The new crops now await commercial deployment pending an ongoing review by the Environmental Protection Agency. If approved, it will “demonstrate once again that biotechnology in agriculture is all about increasing pesticide use and dependence,” said Bill Freese, a science policy analyst at the Center for Food Safety, an advocacy group that opposes the crops. In a critique of the USDA’s evaluation, Freese warned of “an era of much increased use of and dependence on pesticides.”

The cotton—technically known as MON 88701, or Bollgard II(r) XtendFlex(tm)Cotton—can survive exposure to three herbicides: dicamba, glufosinate and glyphosate. The soybeans—MON 88708, or Roundup Ready 2 XtendTM Soybeans—withstand dicamba and glyphosate. The crops’ resistance means that farmers can spray entire fields of these crops with the herbicides, rather than laboriously targeting individual weeds.

Existing versions of Monsanto’s cotton and soybeans are resistant only to glyphosate, better known by its trade name of Roundup. Developed in the late 1990s, these so-called Roundup Ready varieties soon became hugely popular, now accounting for some 75 percent of all US cotton and 90 percent of soybeans.

In certain ways, the Roundup Ready trait was quite beneficial. Compared to many other herbicides, glyphosate is powerful and relatively non-toxic. It was convenient to use, and saved farmers time and money. Indeed, many farmers came to rely almost exclusively on glyphosate and the Roundup Ready system for weed control—and that, say scientists, was a recipe for trouble.

Over-reliance turned America’s agricultural landscape into an evolutionary crucible of accelerated selection for any genetic mutation that helped weeds survive glyphosate. The resulting plants, often called “superweeds,” proliferated dramatically, and now infest at least 61 million acres of US farmland, an area roughly equivalent to the size of Michigan.

Their spread is sending farmers scrambling for solutions. The industry’s response has focused on making crops withstand more herbicides: first Dow’s Enlist corn and soybean varieties, designed for spraying with both glyphosate and the 2,4-D herbicide, which were federally approved last fall, and now Monsanto’s Xtend cotton and soybeans.

In reviewing genetically engineered crops, the USDA is tasked with determining whether they are “plant pests,” causing direct or indirect risks to agricultural crops and other plants. The Xtend cotton and soy varieties, said the USDA, are not: According to the agency’s final environmental impact statement, they are “unlikely to pose a plant pest risk.”

Indeed, the USDA argued that Xtend cotton and soy could have several benefits. Without them, asserted the agency, farmers fighting superweeds will increasingly practice tillage: weed-control by churning soil. That generates erosion as well as air, water and greenhouse gas pollution. Xtend mixes might also replace herbicides other than glyphosate, thus reducing their use.

The herbicide savings, however, are unclear. Though non-glyphosate herbicide applications have increased dramatically in recent years, they’re still relatively low-volume—especially in comparison to projected dicamba use in the Xtend system.

Monsanto expects Xtend will eventually account for half of all US cotton planting and 40 percent of soybeans. Dicamba application would increase 14-fold on cotton in comparison to current use rates, and 88-fold on soybeans. For the latter crop, Freese estimates that farmers will use an additional 20 million pounds of herbicides every year.

‘I’m not against the use of those tools. I’m against their poor use.’

Those figures may even be conservative: as weeds gradually become harder to kill with dicamba and glufosinate, farmers could respond by using more of those herbicides. That would fuel even more resistance, and eventually turning some farmers back to soil-degrading mechanical tilling.

Erosion control is one of the last quarter-century’s unsung agricultural achievements, said Freese. Soil conservation programs enacted in the 1985 Farm Bill encouraged farmers to take erosion-prone land out of production and also to practice conservation tillage, in which crop residue remains on fields all year. As a result, annual farmland erosion slowed from 3 billion tons in 1982 to 1.9 billion tons in 1997.

Over the next decade, during Roundup Ready’s rise, soil erosion declined only minimally and now threatens to rise. That’s nominally what Xtend and other multiple herbicide-resistant crops are supposed to forestall, but “by generating still more herbicide-intractable, multiple herbicide-resistant weeds,” they’ll make tillage and soil erosion more common, predicts Freese.

In its impact statement, the USDA minimized the potential problems of weeds evolving in response to widespread glyphosate, dicamba and glufosinate use. They say resistance will likely be monitored by Monsanto, though critics like Freese note that previous company-led monitoring efforts worked poorly. And eventually resistance will emerge, wrote the USDA, but the problem isn’t herbicides in general. It’s over-reliance on just a few.

This is true, say weed scientists. A variety of weed control tactics, from different herbicide types to crop rotations and so-called cover crops that interrupt weed population cycles, are needed to keep weed evolution in check. As plants adjust to one pressure, another knocks them back.

Yet whether farmers actually will use Xtend as one of many weed-control strategies, or simply recapitulate the mistakes made with Roundup Ready crops, is a nagging question.

“If we treat these systems similar to how we treated Roundup Ready systems in the late 1990s, they will be a short-lived and temporary solution to the resistance problem,” said weed scientist Jason Norsworthy of the University of Arkansas.

Norsworthy believes Roundup’s lesson about the dangers of over-reliance was learned. “We are going to approach this with a different mindset than we did,” he said. “If you look back at what industry recommended at the time, it was: ‘Let’s rely on one herbicide, and not look at anything else!’ There’s not a company out there promoting that now. Companies and academia are strong proponents of multi-tactic resistance.”

Monsanto spokesperson Danielle Stuart pointed to the company’s involvement with the United Soybean Board’s Take Action program, which promotes balanced weed management. She noted that the company also promotes the use of so-called residual herbicides—which remain environmentally active for weeks—in addition to Roundup and eventually Xtend mixes.

According to agroecologist Bruce Maxwell of Montana State University, adding residual herbicides to the equation is potentially disastrous. Apart from environmental impacts, it increases the chances of weeds evolving all-purpose resistance. “You’re going to select the mechanisms that render all herbicides useless,” said Maxwell. “We’re already seeing more and more of these arise.”

USDA spokesperson Lyndsay Cole said the agency “recommends that farmers diversity their weed control efforts,” and noted several new programs that promote multi-tactic weed management. Freese said these are an important step, but for now a relatively small one.

Herbicide resistance specialist Stephen Powles of the University of Western Australia is also wary. Some farmers are now taking a more balanced approach, said Powles, but he fears many will over-rely on multiple herbicide-resistant crops. They’re an economically safe choice, and also a deeply engrained habit of mind.

“US growers and industry have an herbicide-only syndrome when controlling weeds in row crop agriculture,” Powles said. Changes to that mentality, Powles suspects, “will only occur after widespread herbicide failure.”

If that’s to be avoided, people need to recognize “the realities of resistance evolution threatening the ongoing efficacy of herbicide technologies,” said Powles. One very good reason: the need to keep herbicides useful. Powles called them a precious technology.

“Eventually we may find that the most sustainable kinds of systems might include some of those tools,” said Bruce Maxwell of herbicide-resistant crop traits. “I’m not against the use of those tools. I’m against their poor use. I want as big a tool chest as possible—and having all of them be effective, using them in a wise way, is ultimately where we want to go.”