New York (AFP) - The rapid growth of Monsanto's new GMO seeds resistant to the controversial herbicide dicamba has revived worries about the company's stranglehold over farming during a period of industry consolidation.

Long a producer of dicamba, Monsanto last year introduced genetically-modified cotton and soybean seeds that can resist the weed killer.

The products took off, amassing more than 20 percent of US soybean fields and 50 percent of US cotton fields in just two years, according to Monsanto data.

The seeds are popular because they boost yield on farms, and some consumers also use dicamba in their fields to get rid of weeds that have become resistant to other herbicides.

However, dicamba is controversial in the US farm belt amid complaints that neighboring crops have been damaged by the herbicide.

Now some farmers say they are being forced to use the new GMO seeds to guard against dicamba.

Nathan Reed, a farmer in Marianna, Arkansas, whose crops were damaged by dicamba from fields more than two miles away, worries about his business.

"We use overwhelmingly non GMOs, not because we are anti-GMO but because we found some niche markets," Reed said at a public meeting last month. "We are in the business of making money, just like Monsanto is."

"It is going to put that ability at risk for us," he said.

Farming states Missouri, Minnesota and North Dakota have imposed restrictions on dicamba, though they permit farmers to use the herbicide one or two times at the start of the season.

But in a victory for Monsanto, an Arkansas legislative panel on Tuesday blocked the state's agricultural department from enacting proposed restrictions.

- Dominant position? -

University of Wisconsin professor Kyle Stiegert said Monsanto's approach to dicamba is part of a larger pattern of increasing dominance by a few players.

"Monsanto has been an aggressive business entity in dominating the seeds industry for some time now," said Stiegert, who teaches agricultural and applied economics. "I would see the dicamba situation as just another step in that direction."

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Stiegert said Monsanto has grown by acquiring smaller seed technology companies over the last 20 years.

Now Monsanto itself is in the process of being acquired by German rival Bayer, creating a behemoth that many consumer advocates fear.

"They can decide which of those varieties get the technology that they want to put on board, and farmers don't dictate which technologies are available to which variety," he said.

"We are letting large corporation dictate what will ultimately be available to the entire global food system. this is a big problem."

Monsanto disputes this.

"There are a number of companies that make genetically-modified seeds," said Scott Partridge, Monsanto's vice president of global strategy. "They can choose not to use our products if they wish, but we want to make sure that they have that choice."

Partridge said Monsanto's success in penetrating the market is because of the effectiveness of the product, and the issues raised with dicamba are due to misuse of the product.

Monsanto is targeting greater than 50 percent of the US soybean market in 2019.

Critics warn that the dicamba controversy resembles that of another prominent Monsanto product, the controversial herbicide glyphosate, marketed as Roundup. It took off in the 1990s after Monsanto introduced genetically-modified seeds that could resist the herbicide, known as "Roundup ready."

But in recent years, issues have emerged.

"Farmers have been struggling with the emergence and spread of Roundup-resistant 'superweeds,'" said Marcia Ishii-Eiteman, senior scientist at Pesticide Action Network.

"Rather than clean up its act, Monsanto has redoubled its efforts to follow the same recipe for disaster, racing against rivals like Dow AgroScience to introduce new lines of transgenic seeds engineered to resist even more chemical herbicides."

A 2015 World Health Organization study said Roundup "probably" causes cancer, although later studies have disagreed.