Angry with Japan for refusing to lift a mad cow-related ban on U.S. beef, senators retaliated Tuesday by voting to retain a ban on Japanese beef.

Once the biggest customer of American beef, importing more than $1.5 billion's worth in 2003, Japan has refused to allow the purchase of U.S. beef since the first U.S. case of mad cow disease was confirmed in December 2003. Last fall, Japan agreed to lift the ban but still hasn't done so.

In June, U.S. authorities confirmed a second domestic case in a Texas-born cow. Japan, in contrast, has found 20 cases of mad cow disease (search). U.S. regulators proposed last month to partially lift the ban on Japanese beef.

Japan's stalling is "just unfair," said Sen. Ben Nelson (search), D-Neb.

"There have been two cases of mad cow disease in the United States, one from Canada," Nelson said in a Senate speech. "Statistically, it's nonexistent, in terms of the millions of head of cattle that are sent to slaughter every year."

Senators adopted, on a 72-26 vote, an amendment by Nelson prohibiting importation of Japanese beef until Japan lifts its ban on U.S. beef. They adopted a similar, nonbinding measure by Sen. Wayne Allard (search), R-Colo., on a voice vote.

The Senate vote came during debate on an annual spending bill for agriculture, food and drug programs. Senators also were debating emergency food assistance for victims of Hurricane Katrina (search).

Japan's food safety panel issued a draft report last week saying U.S. cows faced higher risk of exposure to mad cow disease than Japanese cows because of insufficient feed controls in the 1980s and 1990s, according to Kyodo News agency.

Ground-up cattle remains — leftovers from slaughtering operations — were used as protein in cattle feed until 1997, when a mad cow outbreak in Britain prompted the U.S. to ban the feed industry from using cattle remains in cattle feed.

The U.S. bans beef from all countries with confirmed cases of mad cow disease, including Japan. The exception is Canada, which resumed limited beef shipments in 2003 and live cattle shipments earlier this year.

Mad cow disease is also known as bovine spongiform encephalopathy, or BSE. Eating infected beef is believed to cause variant Creutzfeldt-Jakob disease, a fatal brain disorder that has killed more than 150 people, mostly in Britain in the 1990s.