(Reuters) - German drug and crop chemical maker Bayer clinched a $66 billion takeover of U.S. seeds company Monsanto on Wednesday, ending months of wrangling with a third sweetened offer that marks the largest all-cash deal on record.

The $128-a-share deal, up from Bayer's previous offer of $127.50 a share, has emerged as the signature deal in a consolidation race that has roiled the agribusiness sector in recent years, due to shifting weather patterns, intense competition in grain exports and a souring global farm economy.

"Bayer’s competitors are merging, so not doing this deal would mean having a competitive disadvantage," said fund manager Markus Manns of Union Investment, one of Bayer’s top 12 investors.

Grain prices are hovering near their lowest levels in years amid a global supply glut, and farm incomes have plunged.

But the proposed merger will likely face an intense and lengthy regulatory process in the United States, Canada, Brazil, the European Union and elsewhere. Hugh Grant, Monsanto's chief executive, said Wednesday the companies will need to file in about 30 jurisdictions for the merger.