The written proposal to Monsanto offered $122 a share in cash, Leverkusen-based Bayer said yesterday.
Its shares dropped as much as 4.1%, extending losses since the potential deal was first revealed.
Monsanto shares posted muted gains, rising 6% to $107.67, signaling investors remain sceptical about the deal.
“I don’t think they will get it at $122 because I don’t think Monsanto will accept that price,” said Andrea Williams at Royal London Asset Management.
Buying St Louis-based Monsanto would allow Bayer to tap growing demand at a time when farmers must boost productivity to feed an estimated 10bn people globally by 2050.
Bayer chief executive Werner Baumann is hoping that rationale will win over sceptical investors — and overcome a public backlash in Germany against Monsanto’s genetically modified seeds — as he seeks to pull off the biggest corporate takeover ever by a German company after less than a month at the helm.
The offer represents a 37% premium to the May 9 closing price.
A similar deal earlier this year suggests Bayer would need to offer $145 a share for Monsanto, according to an analyst.
The stock plunged by the most in seven years last week when it confirmed having made an offer, without disclosing the financial details.
“What we saw last week was an uneducated reaction in the media and the press because we did not communicate the details of our proposal,” Mr Baumann said yesterday.
“We are utterly convinced of the rationale of the proposal,” he said.