ST. LOUIS -- Monsanto Co. shareholders Tuesday overwhelmingly approved a $57 billion merger with Bayer AG, a deal that would combine two of the world's biggest agricultural companies.

But the path toward securing regulatory approval may be rough. Critics say the combination would shrink competition in the agricultural market, drive prices higher for farmers and consumers, and escalate damage to the environment. Monsanto and Bayer officials say growers and ranchers stand to benefit, and the combined company will be better suited to address issues such as climate change.

Preliminary results showed 99 percent of all votes cast favored the merger announced in September, St. Louis-based Monsanto said. Shareholders will receive $128 per share in cash at the closing of the merger, which must still receive regulatory approval. Monsanto said the deal is expected to close by the end of 2017.

Monsanto shares declined 7 cents to $104.52.

"This is an important milestone as we work to combine our two complementary companies and deliver on our shared vision for the future of agriculture," Monsanto chairman and CEO Hugh Grant said in a statement.

The Bayer AG corporate logo is seen on display on a building of the German drug and chemicals company May 23 in Berlin.Markus Schreiber ~ Associated Press, file

Bayer, the German medicine and farm-chemical maker, and Monsanto, maker of seeds, herbicides and pesticides among other agricultural products, have faced concern from some government and ag industry leaders who worry the merger will hurt farmers by reducing competition at a time when the agriculture economy has slowed.

The National Farmers Union has said the merger would mean three companies would have more than 80 percent of U.S. corn seed sales and 70 percent of the global pesticide market.

Roger Johnson, president of the National Farmers Union, said in a statement the vote "underscores NFU's concern that these megadeals are being made to benefit the shareholders of multinational corporations at the expense of family farmers, ranchers, consumers and rural economies."

Both the NFU and the Natural Resources Defense Council urged the U.S. Department of Justice to reject the merger.

"Consolidating Monsanto and Bayer would escalate the use of dangerous toxic pesticides and create a bad deal for farmers, bees, consumers and the planet," Rebecca Riley, senior attorney for NRDC, said in a statement.

Top officials for both companies say the merger will be a boost for farmers and the environment.

"By bringing together our expertise and our resources to drive this shared vision, we can do even more together to benefit growers around the world and to help address broad global challenges like climate change and food scarcity," Grant said.

Werner Baumann, CEO of Bayer, said the acquisition of Monsanto "is driven by our strong belief that this combination can help address the growing challenges facing farmers and the overall agriculture industry today and in the future. Together, Bayer and Monsanto will be able to offer the new, innovative solutions that our customers need."

The merger calls for Bayer to pay $57 billion to Monsanto shareholders and assume $9 billion in Monsanto debt.

Bayer sells crop-protection chemicals used to kill weeds, insects and plant fungal diseases and also makes popular pharmaceutical products such as Bayer aspirin, Claritin allergy medicine and Alka Seltzer. Bayer also owns Dr. Scholl's foot products and Coppertone sunscreen.

Monsanto sells seeds for fruits, vegetables, corn, soybeans, cotton and other crops, plus Roundup weed killer. The company is a leading producer of genetically-modified seeds engineered to resist drought and herbicides, among other things. Protests against Monsanto by opponents of genetically-modified organisms, or GMOs, are common.

Plans call for the combined company's seeds and North American business to be headquartered at Monsanto's St. Louis base. Executives for both companies have not said whether the Monsanto name will change.