When Bayer’s top executives greet investors and analysts for a “meet management” day in Cologne next Tuesday, those expecting a mutual love-in may be sorely disappointed.

The dust will barely have settled on Bayer’s $66bn blockbuster takeover of Monsanto, which was unveiled on Wednesday. Investors were not particularly enthusiastic when Bayer made its first offer for the US company four months ago, and they remain unconvinced: the German group’s shares are down 8 per cent since May 11, when media reports of a bid first surfaced.

Back then, the sense of disappointment was greatest among investors who saw Bayer primarily as a pharmaceutical and healthcare company and then watched dumbfounded as it announced plans to become the world’s largest supplier of seeds and crop sprays to farmers.

“People only interested in Bayer’s pharma and healthcare business just won’t buy this agrochemicals story,” says Markus Manns, portfolio manager at Union Investment and a Bayer shareholder.

In countless interviews on Wednesday, Werner Baumann, Bayer’s chief executive, said the Monsanto deal was about nothing less than saving the world. By improving agricultural yields, the combined company would help meet one of the “greatest challenges of our time” — the task of feeding a world population that is expected to grow by 3bn people by 2050. The enlarged company would be a one-stop shop offering seeds, crop sprays, digital solutions and advice to farmers across the globe.

But the deal has faced a wall of scepticism, and not just from disgruntled pharma investors. Many wonder how Bayer’s balance sheet will be able to cope with the strain of such a pricey, all-cash deal, even if it gains all the required regulatory approvals. Environmentalists warn of the damage to Bayer’s brand. And some analysts wonder whether Bayer might have bought a pig in a poke.

Bayer’s net debt is expected to increase to a hefty four times earnings before interest, tax, depreciation and amortisation on completion of the Monsanto deal, and Fitch said on Thursday that the German company’s A rating would fall by at least two notches, although it would remain investment grade.

The strongest reaction, however, was from the green lobby, for whom Monsanto is evil personified. In Germany, the US company is synonymous with genetically-modified seeds and crops — “Frankenfoods”, in green parlance — and a herbicide, glyphosate, which some studies suggest could cause cancer. Bayer may ultimately seek to defuse this issue by dropping the Monsanto brand.

That might not be enough, though, to persuade the environmentalists. Their mood was summed up by Christian Meyer, the Green agricultural minister of North-Rhine Westphalia, where Bayer is based. Bayer-Monsanto, he said, could dictate “what farmers plant and what is served on our plates”. He added he would use “all our political and legal options” to block the deal.

Some wonder about the effect on Bayer’s otherwise squeaky-clean brand. “Anyone interested in sustainability is going to be a lot more sceptical about Bayer than they were before,” says Torben Bo Hansen, head of Philipp und Keuntje, a German advertising agency. “And in Germany at least, that’s a not insignificant share of the population.”

Other risks abound, not least the challenge of combining two very different corporate cultures. Sceptics have cited the ill-fated merger of Daimler-Benz and Chrysler, which showcased many of the perils of integrating US and continental European companies. The tie-up ended in 2007 when Cerberus, the private equity group, bought Chrysler for a fraction of the $35bn it had cost Daimler in 1998.

For some, the deal makes sense, despite the execution risks. “Our standalone forecast for Bayer has them pumping out 7-8 per cent earnings growth per year,” says Peter Verdult, analyst at Citi. “That’s not very attractive for the sector, and that’s why it trades at a discount. But if you put in Monsanto, you get 11-12 earnings growth per year over the next five years.”

However, others are more pessimistic. Analysts at Bernstein Research suggest Bayer’s $66bn valuation of Monsanto, including debt, represents a “roughly 60 per cent premium to the true value of [Monsanto’s] business”.

They note that demand for Monsanto’s GM seeds may have peaked, with genetically-modified corn, soyabean and cotton already planted on more than 90 per cent of US acreage. Meanwhile the products in Monsanto’s pipeline “do not provide the sort of revolutionary value that the first generation of GM products unlocked”.

The Bernstein analysts also highlight how margins on glyphosate, the herbicide Monsanto pioneered and still one of its big sellers, “have been crushed by an oversupply out of China”.

Mr Baumann’s missionary zeal is unquenched. “This combination is going to create a global leader in agriculture,” he said on Wednesday. The deal represented the kind of “revolutionary approach to agriculture that will be needed to sustainably feed the world”.

For Friends of the Earth the choice of words is darker. The tie-up is, it says, “a marriage made in hell”.