A vote to ratify the board's actions features prominently at every German AGM. It has no bearing on management's liability, but is seen as a key gauge of shareholder sentiment.

Bayer's management, led by CEO Werner Baumann, could see an embarrassing plunge in approval ratings, down from 97 percent at last year's annual general meeting, which was held shortly before the Monsanto takeover was closed in June.

Asset management firm Deka, among Bayer's largest German investors, said over the weekend it would vote against ratifying management's action, having voiced sharp criticism earlier this month.

Also this month, shareholder advisory firms Institutional Shareholder Services (ISS) and Glass Lewis recommended investors not to give the executive board their seal of approval.

Singapore state investor Temasek and Norway's oil fund, Bayer's next two biggest shareholders after BlackRock, both declined to comment on their voting intentions.

Major Frankfurt-based asset managers DWS and Union Investment also would not disclose their plans.

Approval ratings of more than 95 percent have long been the norm at German AGMs but international investors have become more critical over recent years.