"The Monsanto acquisition was and is a good idea," Werner Baumann told newspaper Frankfurter Allgemeine Sonntagszeitung, when asked if he would have changed his mind about buying the US group if he could.

Bayer bought Monsanto for $63 billion but the deal has turned out to be plagued with other massive costs.

Just two months after the acquisition was completed, Monsanto lost a case to a school groundskeeper suffering from terminal non-Hodgkin's lymphoma, who had sued the company over the glyphosate weedkillers Roundup and Ranger Pro.

Monsanto was initially ordered to pay $289 million to Johnson, before the damages were reduced to $78.5 million.

Bayer has filed an appeal.

The company suffered a new set back this month as a US jury ruled that Roundup was a "substantial factor" in another case brought by an amateur gardener who was suffering from cancer.

It now faces a total of 11,200 US cases over Roundup and its active ingredient glyphosate, a herbicide key to Monsanto's business model that has come in for intense scrutiny around the world.

Werner insisted the acquisition of Monsanto was carried out after careful due diligence.

Bayer has also pointed to findings from regulators around the world, especially in advanced economies like the US, Europe and Canada, and reams of scientific studies as proof of the safety of its product.

"Regulatory authorities around the world consider glyphosate-based herbicides as safe when used as directed," the group has argued, highlighting "800 rigorous studies" of glyphosate's effects.

The World Health Organization's International Agency for Research on Cancer found in 2015 that glyphosate is "probably carcinogenic," although the European Food Safety Authority and the European Chemicals Agency have not issued similar judgments.

Since the Monsanto takeover was completed, Bayer's stock has shed almost 40 percent of its value.