BULK handler GrainCorp has rejected calls to trace the contamination of two grain silos with genetically modified canola back to grain growers.

The Victorian Farmers Federation has called for the investigation of Round-Up Ready canola in 500-tonne grade samples at Dunolly and Lillimur.

GrainCorp spokesman David Ginns said the samples were 1 per cent GM and were binned in 2000-tonne stacks, so the "actual adventitious presence level of the stack is theoretically 0.25 per cent."

The adventitious presence standard accepted by the industry is 0.9 per cent, so the grain can be marketed as non-GM.

Victorian Farmers Federation grains group president Russell Amery said variety contamination was a wider issue than just canola.

He said the issue should be investigated to find out what happened and why.

"It's not just an issue of making sure GM is separated from non-GM," Mr Amery said.

"There are problems if malting barley is mixed with feed barley."

He said variety contamination could cause problems with marketing the grain and malting or milling it.

"We want to know whether it was a mistake by growers or whether it was a mistake in the testing stand with someone tipping the grain into the wrong running sample," Mr Amery said.

"It could be that simple and it might not be the grower's fault."

Mr Amery said the problem needed to be understood so it would not be repeated in future.

But GrainCorp said it was not necessary to trace back the sample as the level of GM in the stack was only 0.25 per cent.

Mr Ginns said the level at which the detections had come in clearly indicated that the incident had not been a deliberate action on behalf of growers.

"It would appear there has been unintended mixing of varieties, possibly from field bins, chaser bins or silos on the farm," Mr Ginns said.

"The detection was at such low levels and grain at both sites complies with industry quality standards. So there is no need to pursue any farmers involved."