If executed, this deal is going to be the biggest this year. 

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

To close the deal, Germany's Bayer flowed 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 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.

Announced Wednesday, the bid that sealed the deal is for $128 per Monsanto 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.