WASHINGTON - The Bush administration said Thursday it will seek to increase cattle and beef imports from Canada despite questions about Canada's safeguards against mad cow disease.

Canada discovered five new cases of mad cow disease in 2006. One case in particular was disturbing, because the cow was born years after Canada adopted safeguards to keep the disease from spreading.

The United States banned Canadian cattle and beef after Canada found its first case of mad cow disease in May 2003. Later that year, an imported Canadian cow in Washington state became the first U.S. case of mad cow disease.

Canadian imports of beef resumed swiftly, but a court battle with a Western ranchers' group kept the border from reopening to live cattle until July 2005.

Still, beef and cattle imports have been restricted to animals younger than 30 months, because older animals carry a higher risk of having mad cow disease.

The Agriculture Department is proposing to allow imports of beef and cattle from Canadian cattle 30 months and older. Live animals who are imported for breeding and slaughter in the U.S. -- rather than slaughtered in Canada -- must be born on or after March 1, 1999.

The plan will go through 60 days of public comment until March 12. Department officials said they will "take into account'' all comments before proceeding, possibly in the summer.

"This proposal would continue to protect against BSE in the United States while taking the next step forward in our efforts to implement science-based trade relations with countries that have appropriate safeguards in place to prevent BSE,'' said Agriculture Secretary Mike Johanns.

He said U.S. officials conducted a risk assessment and determined that the older cattle would be safe for consumers.

"Our approach is consistent with science-based international guidelines,'' Johanns said.

The department waited to relax the trade rules while investigators tried to solve the mystery of the cow that got infected years after Canada put its safeguards in place.

The safeguards bar the use of cattle remains in cattle feed. This is the primary firewall against the disease, because the only known way for cattle to get infected is by eating feed containing diseased cattle tissue.

The practice was largely outlawed in Canada -- and the United States -- in 1997. But the cow was born in 2002.

Canadian officials blamed the infection on cross-contamination, either when the feed was mixed or when it was transported, because cattle remains have been allowed in food for other livestock and pets. Canada announced last year it will ban cattle tissues known to carry the disease from feed for all livestock and pets.

The U.S. feed ban is less strict than Canada's, drawing criticism from companies like McDonald's, the No. 1 U.S. hamburger seller, and food and agribusiness giant Cargill Inc., one of the world's largest privately held companies.

Officials have proposed tightening the ban, but not as much as Canada has. The National Cattlemen's Beef Association says further restrictions are unnecessary because this country has lower risks.

However, the western ranchers' group that sued, R-CALF USA, says older cattle from Canada present too great a risk. The magnitude of Canada's mad cow epidemic is still unfolding, said Bill Bullard, the group's CEO.

"USDA's proposal to allow over 30-month cattle and beef is premature, and we are asking Congress to intervene to stop it,'' Bullard said.

The U.S. imports roughly 12 per cent of its beef. In 2005, Canada accounted for nearly a quarter of those imports, shipping about C$1.3 billion worth of beef and veal, an estimated 368 million kilograms, to U.S. markets. Imports dropped off in 2006; the most recent data shows about C$876 million in Canadian shipments through October.

The medical name for mad cow disease is bovine spongiform encephalopathy, or BSE. The brain-wasting disorder infected more than 180,000 cows and was blamed for more than 150 human deaths during a European outbreak that peaked in 1993.

In humans, eating meat contaminated with BSE is linked to variant Creutzfeldt-Jakob Disease, a rare and deadly nerve disease.

The U.S. has found three cases of BSE, two of them in native-born animals in Texas and Alabama. Canada has found eight cases in all.