Now it's official: Monsanto is going to be swallowed up by a rival in the biggest takeover announced so far this year.

To clinch a deal, Germany's Bayer pumped even more money into its bid for the U.S. seed giant, valuing it at $66 billion, including debt.

It's the third time Bayer (BAYRY) has cranked up its offer, and this time it was enough to win the backing of Monsanto's board. The takeover would create a vast conglomerate spanning pharmaceuticals, health products and pesticides.

Related: The year's biggest takeover bid just got even bigger

Announced Wednesday, the bid that sealed the deal is for $128 per Monsanto (MON) share, valuing the U.S. company's equity at about $56 billion. Bayer will also be taking on about $10 billion in debt.

The big obstacle the companies now face is getting the merger past regulators around the globe.

Monsanto shares in the U.S. were flat in premarket trading at around $106. That's way below the offer price, suggesting investors doubt the deal will go through.

If regulators do kill the transaction, Bayer has agreed to pay $2 billion to Monsanto.

Bayer, which is strongest in Asia and Europe, stands to gain from Monsanto's expertise in agriculture and seeds. It would also benefit from the U.S. company's big presence in North America.

Bayer has said combining the companies would generate synergies of $1.5 billion over three years.

The deal is the latest mega-merger aimed at reshaping the agribusiness and chemical sectors.

Another huge tie-up -- the planned $130 billion alliance between Dow and DuPont -- is under investigation by Europe's top antitrust watchdog.

Related: DowDuPont's massive merger at risk over antitrust concerns

In February, a giant state-owned Chinese chemical company agreed to pay $43 billion for seed and pesticide firm Syngenta. That deal cleared a major regulatory hurdle in the U.S. last month, but it still has a series of antitrust reviews around the world to get through.

The purchase of Syngenta by ChemChina was the biggest deal announced worldwide this year before Bayer and Monsanto reached agreement. Monsanto had in the past tried unsuccessfully to buy Syngenta.

-- Alanna Petroff contributed to this article.