Traffic moved slowly in central Winnipeg on Friday as farmers, union members and other supporters of the Canadian Wheat Board marched on the board's downtown offices.

They demonstrated in support of the CWB and against the federal government's move to take away the board's monopoly over wheat and barley marketing.

About 150 farmers gathered at the Red River Exhibition grounds at the western edge of the Manitoba capital before moving downtown in grain trucks and farm equipment to the CWB headquarters on Main Street.

Earlier this week the CWB filed a lawsuit against the federal government over the legislation that aims to open up marketing of western Canadian wheat and barley. The lawsuit, filed in the Federal Court of Canada, argues Ottawa was legally obliged to consult western Canadian farmers prior to introducing Bill C-18.

Rally organizer Bob Roehle said the majority of farmers support the status quo and single desk selling via the CWB.

Roehle contends the federal government has broken the law by not consulting farmers, adding the rally isn't just about the CWB it's about the future of Canadian democracy.

"This is the hill to die on. This is it. And if we don't get the ear of Parliament and the Harper government, the wheat board is gone."

Agriculture Minister Gerry Ritz maintains that western Canadian farmers shouldn't be forced to sell their grain through the wheat board. The Western Canadian Wheat Growers Association agrees, saying it will sue the Canadian Wheat Board and eight of its directors for alleged misuse of farmers’ money.

Another group, Friends of the Canadian Wheat Board wants the courts to review the federal legislation.

Ritz was unmoved by the latest demonstration. Supporters of the board have also held rallies in Saskatchewan and Alberta.

"Not only does the government have the right to amend legislation, we also have the responsibility to fulfil our long-standing promise to give Western Canadian farmers marketing freedom," Ritz said Friday.

NDP and Liberal MPs say the wheat board is important for keeping prices fair for farmers, and fear large agribusinesses will have an upper hand in dealing with farmers if the wheat board loses its monopoly.

Bill C-18 is now before a House of Commons committee for review before the legislation is sent back to the House for a final vote.

Costs difficult to determine

Meanwhile, there is disagreement about how much it will cost to transform the wheat board.

One report, prepared by the board, said it will cost between $300 and $500 million to wind up the agency and start a new grain company.

The board included expenses relating to staff pensions and severance packages as well as the cost of ending long-term contracts to suppliers and other service providers.

But federal officials told CBC News Friday some of those costs may be avoided.

"Those costs are considerably higher than what the actual transition costs will be, mainly because we’re not winding up the CWB, we’re establishing a voluntary CWB that will be an ongoing concern," said Greg Meredith, an assistant deputy minister with Agriculture Canada.

Meredith said if the legislation is passed, the government plans to hire an independent accounting firm to determine what transition costs should be paid.