Deputy Prime Minister Warren Truss says Australia must control its food security and selling GrainCorp would give control of ports and handling facilities to a US firm.

Mr Truss says both sides of politics want agriculture to be a key part of the nation's future, but it would be very difficult for Australia to make decisions for the sector to prosper without owning any of the supply chain.

"It's very important for Australia to maintain control of its own food security that we have control of our own destiny in what is an important pillar of our national economy," he told ABC television.

The government is considering whether to approve a $3.4 billion offer for GrainCorp, Australia's largest grains handler, from US grain giant Archer Daniels Midland.

The Australian Competition and Consumer Commission approved the deal in June.

Last week, Treasurer Joe Hockey said the government needed more time to consider all relevant issues and advice from the Foreign Investment Review Board before making a decision. That will now be made on December 17.

The Nationals oppose the sale, which would place most of Australia's grain infrastructure in foreign hands.

Mr Truss said this meant decisions on the Australian grains industry would ultimately be made in foreign boardrooms.

"On top of that, GrainCorp is the largest listed agribusiness in Australia and if we lose this business to foreign ownership, then we will lose the potential to have an international-standard agribusiness trading around the world," he said.

Mr Truss said he was confident Mr Hockey was aware of the issues in this deal.

"Certainly the Nationals' views on the issue are well known," he said.

"People feel very strongly about the issue. It is certainly an important one. But it's not just Nationals, there are Liberals also."