GrainCorp has all but waved the white flag in its takeover bid for feed miller Ridley Corporation - it has not raised its script-based offer in spite of a declining GrainCorp share price lowering the monetary value of the takeover bid.

GrainCorp managing director, Mark Irwin, has admitted the ratio on which the bid was based was potentially unattractive to Ridley shareholders and admitted there was uncertainty over whether the deal would get through.

If the terms of the bid are not satisfied by August 26, the deal will lapse.

Mr Irwin said he believed his company's share price, which has slipped from $12.51 on May 15 to $7.95 today was undervalued.

It has seen the value of the offer for Ridley slide from $1.39 per Ridley share to 88 cents per share.

The GrainCorp offer is well below Ridley's current share price of $1.08 and the lower GrainCorp share price means the total deal is worth more than $200 million less.

Mr Irwin said that the current predicament was temporary and a result of the recent overall market slump and that the deal could still offer value for Ridley shareholders.

"We believe the fundamental rationale for the offer remains sound and that the synergies described in the bidder's statement can be achieved," he said.