WASHINGTON — The Department of Agriculture began to allow the importation of older Canadian cattle into the U.S. yesterday for the first time in more than four years, marking a final rollback of trade restrictions imposed after the 2003 discovery of mad cow disease in Canada.

The change, which drew criticism from some cattle and consumer groups, means that cattle up to 8 years old may enter the U.S. for sale, slaughter or breeding. Initially, the U.S. Department of Agriculture in 2003 had halted all Canadian cattle imports. But in 2005 it started allowing younger Canadian cattle into the U.S.

Canada has discovered nine more mad cow cases since the first diseased cow was isolated in May 2003. The first U.S. case was discovered in December that year in Washington state, and involved a cow that was born in Canada. Two other U.S. cases, one in Texas and one in Alabama, have since been discovered.

Some scientists believe that mad cow disease, formally known as bovine spongiform encephalopathy, or BSE, presents itself in older cattle.

The USDA's chief veterinary officer, John Clifford, said that a department analysis shows the risk of mad cow disease due to import to be minimal.

"We've evaluated the potential risk, and we consider the risk to be extremely small," Clifford said. "There are very much interlocking safeguards in both Canada and the U.S."

The new import provision was praised by some U.S. cattle groups as the normalization of trade after several years of mad cow scares.

"We look at this rule as finalization of a process that started some time ago," said Jay Truitt, vice president of government affairs for the National Cattlemen's Beef Association, which favors the new rule. "At some point or another we had to get to this."

Other cattle groups, though, opposed the change, as did consumer groups that have long opposed relaxed import rules.

"We think it's a bad idea," said Michael Hansen, a mad cow disease expert with the Consumers Union, which publishes Consumer Reports magazine. "It doesn't make sense on the science. The prevalence of (mad cow) in Canada is 30 times the level it is in the U.S."

The disease is believed to be a variant of Creutzfeldt-Jakob disease in humans, which attacks and destroys the brain and nervous system. No one has been recorded as contracting those symptoms after eating beef in the U.S. But in England, which suffered a serious mad cow outbreak in the 1980s, more than 100 people died from the disease.

Scientists suspect that mad cow disease can be transmitted to cows through feed that contains animal bone and tissue that is ground into feed for protein. Canada and the U.S. have placed restrictions on just what animal parts may be used to make cattle feed.

The USDA's new Canada import rule makes it legal to import cattle born after March 1, 1999, the date when restrictions were put on cattle feed in Canada. However, five of the 10 cases of mad cow discovered in Canada involved cattle that were born after that feed ban date, said Hansen of Consumers Union.

"That doesn't suggest that the feed ban is working very well," he said.

Yesterday's rule change follows a lengthy dispute over Canadian cattle imports, and concerns by ranchers and consumer advocates about mad cow disease in the U.S. and the effect that cheaper Canadian cattle will have on the U.S. market.

Right now, Truitt said, U.S. cattle are priced at about $93 per each hundred pounds, compared with a price of about $70.50 per hundred weight for Canadian imports.