The Harper government's legislation to strip the Canadian Wheat Board of its monopoly over Prairie wheat and barley sales received royal assent to become law late Thursday.

The bill received third reading in the Senate Thursday afternoon and Conservative majority approved it by a vote of 51-33 after the government side invoked closure to speed the process.

It was granted royal assent at Rideau Hall by Gov. Gen. David Johnston at 6:17 p.m. ET, and notice of royal assent was read in the upper chamber shortly after 6:40 p.m. ET.

The government wanted the law in place by Christmas so farmers can plan for the next crop year.

The legislation, which ends a seven-decade monopoly by the wheat board, passed despite the fact it is being challenged in the courts.

The marketing agency's board of directors wants the Manitoba Court of Queen's Bench to declare the law invalid. The directors say the government can't alter the board's mandate without a plebiscite among grain farmers.

A Federal Court judge ruled earlier this month that the bill violates the existing Canadian Wheat Board Act, which requires the plebiscite.

Justice Douglas Campbell made it clear, however, that his ruling was simply a statement on the government's actions. He did not order the government to stop the bill and said he wouldn't interfere in the legislative process.

The government is appealing that ruling, saying Parliament has the right to change its own legislation.

Open market could start Aug. 1

The Western Canadian Wheat Growers Association, which has long fought the wheat board's monopoly, wants the board of directors to abandon their court fight so that farmers can prepare for an open market as of Aug. 1 — the start of the next crop year.

The Harper Conservatives have wanted to strip the wheat board of its monopoly for years, but were stymied in the past because they couldn't muster the needed Parliamentary majority.

The government says the bill is simply a matter of justice. They say it gives western farmers the freedom to sell their product as they see fit.

Opponents say the bill will leave farmers at the mercies of the railways and the big, international grain companies.

The board was created after the Great Depression to help Prairie farmers get better prices and greater market stability. It now sells about 21 million tonnes of grain to customers in 70 countries.

Board supporters say the monopoly stops producers from undercutting each other in pursuit of sales.

Opponents say they want the freedom to seek the best deal on the open market. They point out that producers of other grains and wheat farmers in other parts of Canada already have that freedom.

with files from CBC News