Monsanto Co. dropped its roughly $46 billion bid for Syngenta AG after the Swiss agribusiness giant rejected a sweetened offer, dashing—at least for now—chances of a union that would have reshaped the global market for seeds and pesticides.

The surprise disclosure on Wednesday capped four months of aggressive pursuit by St. Louis-based Monsanto, which Syngenta had consistently rebuffed even as some of its shareholders pressured the Swiss company to negotiate.

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