A farmer drives a tractor equipped with a GPS guidance system and other terminals in England. Bayer cited the growth in such digitally assisted farming as a key reason for its bid for Monsanto, which provides analytics used by growers. (Luke MacGregor/Bloomberg News)

Seed and chemical giants Bayer and Monsanto said Wednesday that they will merge to become one of the world’s biggest agriculture giants, a $66 billion mega-deal that could reshape the future of farming and enhance their influence over the planet’s food supply.

Bayer, the German firm better known for pharmaceuticals such as Aleve and Alka-Seltzer, said it will spearhead the largest all-cash buyout in history in hopes of taking over St. Louis-based Monsanto, the world’s largest supplier of genetically modified seeds.

The merger marks one of the most prominent signs yet of the broadening acceptance of genetically modified foods, a bogeyman for environmental activists that has nevertheless redefined the capabilities for crops in the United States and worldwide.

The deal would also further strengthen the companies’ grips on vital seeds, pesticides and farm technologies, a concerning turn that critics said could raise prices, reduce choice and stifle innovations needed to feed a growing world.

“These companies make the case that they need to get bigger to help them respond to climate change, changing diets, growing populations . . . but so much of their research focuses on the big commodity crops that make the most money,” said Pat Mooney, executive director of the ETC Group, a Canada-based environmental advocacy group.

The German drug and chemicals company is paying a hefty premium for the American seeds maker. The deal will likely face close regulatory scrutiny. (Reuters)

“They’re just so narrowly focused,” Mooney said, “that there’s a general feeling they can’t get us to the innovations we need.”

The deal is likely to draw intense scrutiny from antitrust regulators, who will assess whether the merger would unfairly lead to higher prices and fewer choices for farmers’ most important building blocks. The new company would preside over roughly one-quarter of the world’s seed and pesticide supplies.

Justice Department investigators have in recent years launched investigations into “possible anticompetitive practices” in the Monsanto-led U.S. seed industry, although a formal investigation was closed in 2012 without pursuing charges.

Regulatory crackdowns have dashed several high-profile mega-mergers this year, including a $160 billion deal between pharmaceutical giants Pfizer and Allergan.

The Bayer deal — the largest corporate mega-merger in a year full of them — is also likely to meet congressional resistance. Calling the merger “a threat to all Americans,” Sen. Bernie Sanders (I-Vt.) on Wednesday urged regulators to block the deal and reopen an investigation into Monsanto’s massive influence over the seed and chemical markets.

“These mergers boost the profits of huge corporations and leave Americans paying even higher prices,” Sanders said in a statement.

Genetically modified seeds dominate U.S. farming and are used in the growing of more than 90 percent of corn, cotton and soybean crops. But their use remains a major driver of environmental protests in Europe and has led to political action at home. In July, President Obama signed into law a bill requiring food companies to label products that include genetically modified organisms, or GMOs.

With its rapid development and fierce protection of genetically engineered seeds and pesticides, Monsanto has been made into an arch-villain for some environmental and consumer activists, who worry that the chemical underpinnings of “Frankenseeds” threaten human health and allow laboratories to play God.

The advanced seeds have also provided for a range of benefits, including stronger pest resistance, streamlined weed control and more efficient harvests around the world.

Years of research have revealed no proof that genetically engineered crops pose risks to human health. A National Academies of Sciences expert panel concluded in May that there was “no substantiated evidence” that the crops had endangered our bodies or hurt the globe.

“Asking whether GMOs are safe is like asking whether toys are safe,” said Rick Amasino, a University of Wisconsin professor of biochemistry who participated in the report. “Most people would recognize that as too broad a question. You can’t lump all chemicals into one bin.”

“Farmers don’t have to use these things. They have a choice,” Amasino added. “Industry is giving farmers something that, I suspect, farmers want.”

Dana Perls, a senior campaigner on food and technology with Friends of the Earth, said agricultural juggernauts such as Monsanto have crafted institutional blocks that prevent small farmers from freely choosing how to grow their crops.

“This further consolidation over our food system removes even more power from people to control their agriculture, and to choose what we can and can’t consume,” Perls said. The deal, she added, would “make it even more difficult to make sure that what comes onto the market is safe for people and the environment.”

The deal would be the largest German takeover yet of an American firm. Known in the United States largely for its health-care products, Bayer would emerge from the deal further focused on its business in agriculture chemicals, crop supplies, and compounds that kill bugs and weeds.

Bayer first made a $62 billion offer for Monsanto in May and has increased its bid over months of negotiations. The all-cash deal is valued at about $128 a share and is larger than the previous record, the $60 billion merger between brewers Anheuser-Busch and InBev in 2008.

Sales in 2015 at Bayer, which has 117,000 employees, totaled roughly $51 billion, about 30 percent of which came from its crop division. Sales at Monsanto, which employs 20,000 and also develops such products as the weed-killing herbicide Roundup, totaled $15 billion last year.

The companies portrayed the merger as a landmark agreement that would help them invest more in researching and developing chemicals for the global harvesting of vegetables, corn and other crops. The deal, they added, would also help them save $1.5 billion through cost-cutting, added purchasing power and other “synergies” within three years.

“The whole agricultural industry around the world is basically going through a transformation. It’s the last big industry in the world to be digitized,” said Robb Fraley, Monsanto’s chief technology officer. “This allows us to make more investments, have more capabilities and build better products for farmers, that they can use to grow crops with higher yields . . . and farm better, farm smarter.”

Both companies said that they would seek antitrust approval in 30 global jurisdictions, possibly including emerging markets for seeds and pesticides such as China, India and Brazil. Bayer has committed to paying a $2 billion antitrust breakup fee if the deal falls apart.

Asked whether they were worried about regulatory challenges from a new U.S. administration, Monsanto chief executive Hugh Grant said on a call Wednesday that the companies were “much more focused on the innovation horizon than the political horizon.”

David Balto, a former Federal Trade Commission policy director who works with farmers and consumer groups, said there was a strong chance that the Justice Department regulators would crack down on the deal.

“Antitrust cops are learning they’re cops,” Balto said. The companies “have chosen to do a deal in the year of merging dangerously. They are in for a tough time.”

A growing Bayer-Monsanto giant could also block out smaller competitors, said Matthew Crisp, the chief executive of Benson Hill Biosystems, an agricultural technology firm.

“Large companies’ model of innovation . . . has served as a barrier to entry for smaller companies interested in developing more choice for farmers,” Crisp said.

Monsanto’s St. Louis headquarters will become the companies’ commercial headquarters for North America, but it’s unclear how the mega-deal might affect jobs there. Grant said on a call Wednesday that the merger would help the city become a “global center” for the seed business, adding, “This is good news for St. Louis.”

Shares of Bayer and Monsanto climbed less than 1 percent on Wednesday, a reflection of investors’ timidity over a potential antitrust block. Jim Nelson, a portfolio manager at Euro Pacific Asset Management, said that “some Bayer shareholders want the company to focus on the pharmaceutical business, and not go off into Monsanto’s signature GMO-seed business, which they view as a risk.”

The $100 billion global market for seeds and pesticides has grown increasingly competitive, as farmers duel for crop and market share on a planet whose population is expected to grow more than 30 percent by 2050, to 9.7 billion.

Tensions have escalated further because global crop prices have fallen for three years in a row, squeezing profits and forcing the seed and agriculture industries to cut costs and trim their workforces. Monsanto said last year that it would lay off 12 percent of its employees, or 2,600 jobs.

Rival seed and chemical giants, including Dow Chemical, DuPont and Syngenta, have launched their own mega-deals in recent months as part of an accelerating race to consolidation.

Diana Moss, president of the American Antitrust Institute, said that rapid tightening could weaken the inventiveness of an industry focused on advancing the world’s food supply.

“It is vitally important to have competing, head-to-head, research-and-development programs in this market,” Moss said. But with this deal, “we would go from six competitors in agricultural biotechnology down to four. That would have significantly harmful effects on the pace of innovation.”