WASHINGTON — The German chemical company Bayer said it will take over US seed giant Monsanto to become one of the world’s biggest agriculture conglomerates.
The $66 billion deal — the largest corporate mega-merger in a year full of them — could reshape the development of seeds and pesticides necessary to fueling the planet’s food supply.
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, making it the weightiest all-cash buyout in history, beating the $60 billion deal between brewers Anheuser-Busch and InBev in 2008.
Bayer in the United States is known largely for its pharmaceuticals, with scientists who developed modern Aspirin and Alka-Seltzer. But the deal would pivot the 117,000-employee company more toward its farm-targeting business in agriculture chemicals, crop supplies, and compounds that kill bugs and weeds.
Monsanto is the world’s largest supplier of genetically modified seeds, which now dominate American farming but are still a major source of environmental protests in Europe and elsewhere. The 20,000-employee company also develops Roundup, the weed-killing herbicide.
Bayer’s sales totaled 46.3 billion euros last year, or roughly $51 billion, about 30 percent of which came from its crop division. Monsanto’s sales totaled $15 billion last year.
The deal is a sign of just how influential the industry of genetically modified seeds has become around the world. Decried as unsafe and chemically tarnished ‘‘Frankenseeds’’ by some environmental activists, they have also allowed for greater harvest efficiency, stronger pest resistance, and more widespread crop availability around the world.
A National Academies of Sciences panel of experts said in May that there was no ‘‘substantiated’’ evidence that genetically engineered crops had triggered human health concerns or hurt the environment, though they added that more research was needed.
The companies portrayed the deal as a landmark that would help them invest more in seeds, pesticides, and technology for the global harvesting of fruits, vegetables, corn, cotton, soybeans, and other crops.
‘‘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 it 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.’’
The deal is likely to warrant intense scrutiny from American and German antitrust regulators, who will assess whether the merger would unfairly lead to higher prices for farmers worldwide. The new company would preside over roughly a quarter of the world’s seed and pesticide supplies.
Regulatory crackdowns have busted several high-profile mega-mergers this year, including a $150 billion deal between pharmaceutical giants Pfizer and Allergan.
Justice Department investigators have in recent years launched probes into ‘‘possible anticompetitive practices’’ in America’s Monsanto-led seed industry, though a formal investigation was closed in 2012 without pursuing charges.
Bayer and Monsanto said on a Wednesday call that they would be seeking antitrust approval in 30 global jurisdictions. Bayer has committed to paying a $2 billion antitrust breakup fee if the deal falls apart.
Liam Condon, the head of Bayer’s crop science division, said the company was ‘‘pretty confident’’ that the deal would be approved by regulators because both companies have ‘‘highly complementary’’ product lines and geographic offerings.
‘‘We have very, very little overlap,’’ Condon said. ‘‘The whole strategic rationale of the deal is built around encouraging more innovation.’’
David Balto, a former Federal Trade Commission policy director who now does legal work on mergers for farmers and consumer groups, said there was a strong chance the deal would be opposed by the Department of Justice.
Regulators, he said, will take a sharp look at the potential for a deal that would lead to few benefits for consumers and limit innovation.
‘‘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.’’
The companies said the deal, the largest German takeover of a US firm, would help them save $1.5 billion through cost-cutting, added purchase power and other ‘‘synergies’’ within three years.
Monsanto’s current headquarters, in St. Louis, would become the companies’ commercial headquarters for North America.