Washington, D.C. ? The Department of Agriculture began to allow the importation of older Canadian cattle into the U.S. on Monday 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.”