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

Preliminary results showed that 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.

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.

But top officials for both companies say the merger will be a boost for farmers.