South Australia's Agriculture Minister has been criticised for attending a protest against genetically modified (GM) crops.

Minister Leon Bignell joined a crowd of about 200 people at the 'March Against Monsanto' rally on the steps of Parliament House in Adelaide.

Although the protest targeted the American biotechnology company Monsanto, it called for a global boycott of GM produce.

South Australia is the only mainland state in Australian that bans commercially grown GM crops.

Mr Bignell denies he was protesting at the rally, instead saying his attendance was in celebration of the state-wide ban.

One of the state's biggest industry groups, Grain Producers South Australia (GPSA), says the minister's attendance at the rally shows his bias on the issue.

GPSA chief executive Darren Arney says farmers should have the right to choose.

"South Australian grain growers are falling behind their counterparts in other states.

"They must have the freedom to choose to grow whatever crops will deliver the best return for their business.

"It should be an on-farm decision made by growers, not a decision made for them as part of the political process."

The minister rejects similar criticism from Senator Sean Edwards and Federal Agriculture Minister Barnaby Joyce.

"As a Member of Parliament, I've got a right to stand up for the 22,000 people in my electorate, but also for people more generally.

"In eight years in politics, I've never had any person in South Australia who is a consumer come to me and say 'can we have more GM food?'."

Mr Bignell says food and wine from a 'clean environment' is a growing market and South Australia will be seen in the future as the state that made 'the right choice'.

"People don't want GM food - the people who want GM are the big poison companies," he said.

"We've got a premium price on our premium food and wine from our clean environment and that's something we want to maintain."

Independent South Australian Senator Nick Xenophon also spoke at the rally, labelling Monsanto 'a corporate cannibal'.

Monsanto's Adam Blight says the company is disappointed the minister joined the rally.

"Monsanto is a company whose products are used by tens of thousands of farmers across Australia each year, including in South Australia, so we were disappointed to see a minister marching against a company that has been working in Australian agriculture for more than 60 years."