Grain handling group Viterra has announced it will no longer be using rail freight in the South Australian Mallee region.

The decision has been pending for over 12 months, after the company extended its existing agreement with freighter Genesee & Wyoming Australia.

Viterra will instead use road transport on the Mallee and Karoonda Highways, roads the South Australian Opposition has described as in a state of disrepair.

Viterra group commercial manager Andrew Hannon said the change would not affect prices at the farm gate and would not increase waiting times for transport during harvest.

He said the rail lines would no longer be used by the company from August.

"Based on our estimates and modelling, we've forecast approximately 3,000 trucks annually on the road," he said.

"The decision to move to road is not an impact on the pricing decision for our transport in the region.

"This will have no impact on our services at harvest. Those sites in the Mallee region will continue to operate the same as they have previously and road transport during that harvest period is completely unaffected.

"We'll be able to provide strong links by road to the port infrastructure from the Mallee region.

"Much of the Mallee road will truncate at Tailem Bend and the task from Tailem Bend will continue to be completed by the important rail service that operates down the main line."