PIERRE, S.D. - Cattlemen and other groups worried about the risk of mad cow disease have asked a federal judge to stop the U.S. Agriculture Department from relaxing rules that have prevented Canadian cattle from being imported into the United States.

The lawsuit, filed last week in federal court in South Dakota, seeks to block a U.S. Agriculture Department rule change announced in September that would allow Canadian cattle more than 30 months old into the U.S. market starting Nov. 19.

The change would expose U.S. consumers to a fatal disease linked to eating meat contaminated with mad cow disease, increase the risk that U.S. cattle would be infected with the disease, and could harm the U.S. cattle market, according to the lawsuit.

The lawsuit was filed by R-CALF United Stockgrowers of America, based in Billings, Mont.; the South Dakota Stockgrowers Association; four South Dakota cattle producers; the Center for Food Safety; the Consumer Federation of America; the Creutzfeldt-Jakob Disease Foundation; and the Food & Water Watch.

The U.S. Agriculture Department cannot comment on a pending lawsuit, but the rules promote fair trade consistent with current scientific knowledge, USDA spokeswoman Karen Eggert said Monday.

"We are very confident in taking this step in allowing additional commodities from Canada while at the same time protecting the beef supply here in the United States, protecting animal health in the United States," Eggert said.

A number of measures, including inspections at slaughterhouses, protect human health, Eggert said.

The discovery of an Alberta cow with mad cow disease in May 2003 caused the United States to close the border to cattle imports from Canada.

The border between the world's largest trade partners reopened for Canadian beef from younger cattle within months of the original ban. Live cattle under the age of 30 months have been allowed to move across the border since July 2005.

Eating meat products contaminated with mad cow disease, known scientifically as bovine spongiform encephalopathy, or BSE, has been linked to more than 150 human deaths, mostly in Britain, from variant Creutzfeldt-Jakob disease.

The U.S. has had three cases of mad cow disease. The first, in December 2003 in Washington state, was in a cow that had been imported from Canada. The second, in 2005, was in a Texas-born cow. The third was confirmed last year in an Alabama cow. There have been 10 cases of mad cow disease in Canada.

The lawsuit argues that the USDA should be prevented from relaxing rules on cattle imports because the new rule is arbitrary and capricious in that it is inconsistent with facts. The USDA also exceeded its legal authority and failed to follow proper procedures in adopting the rule, the lawsuit said.

The relaxation of restrictions on cattle imports from Canada "will expose U.S. consumers to increased risk of an invariably fatal disease associated with consumption of BSE-contaminated meat, will increase the risk of invariably fatal BSE infection in cattle in the United States, and will expose U.S. cattle producers to severe economic hardship," according to the lawsuit.

R-CALF, which represents more than 12,000 U.S. cattle producers, lost a similar lawsuit in Montana after the 9th U.S. Circuit Court of Appeals ruled that the U.S. agriculture secretary had a firm basis for determining the resumption of Canadian imports would not significantly increase the risk of BSE to the American population.