In a statement, Syngenta confirmed April reports that Monsanto had made an offer for a merger that would have formed a world market leader in both seeds and crop chemicals.
"The Board of Syngenta confirms that it has received an unsolicited proposal from Monsanto to acquire the company at a price of 449 Swiss francs (USD 486) per Syngenta share with approximately 45 per cent in cash," said the statement.
"The offer fundamentally undervalues Syngenta's prospects and underestimates the significant execution risks, including regulatory and public scrutiny at multiple levels in many countries," it said.
The US firm had timed the offer with a slackening in Syngenta's performance but the Swiss firm's chief said the business outlook was good.
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"Syngenta is the world leader in crop protection, the number three in seeds and the first company to introduce integrated solutions for growers. Monsanto's proposal does not reflect the outstanding growth prospects of Syngenta's integrated strategy," chairman Michel Demare said.
"While Syngenta's valuation is currently affected by short term currency and commodity price movements, the business outlook is strong, with emerging markets accounting for over 50 per cent of our sales," he added.
Syngenta was formed in 2000 by the merger of Novartis Agribusiness and Zeneca Agrochemicals.