Seeking to close a gap in the nation's defense against mad cow disease (search), the Bush administration on Tuesday proposed to eliminate cattle parts from feed for all animals, including chickens, pigs and pets.

The government already bans cattle remains from being used in cattle feed. The proposal from the Food and Drug Administration "will make an already small risk even smaller," acting FDA Commissioner Andrew von Eschenbach said.

However, the proposal falls short of what FDA had promised 19 months ago. FDA originally said it would prohibit the use of blood, restaurant plate waste and poultry litter, all potential pathways for mad cow disease, in cattle feed.

The proposal is designed to eliminate the need for banning chicken litter, because chickens would no longer be fed at-risk cattle parts. It does not apply to cattle blood, often fed to calves as a milk replacer, or to restaurant leftovers.

The feed rules are important because the only way cattle are known to get mad cow disease is from eating feed containing contaminated cattle remains.

Ground-up cattle remains — leftovers from slaughtering operations — were used as protein in cattle feed until 1997, when Britain's outbreak prompted the U.S. to ban the use of those remains in cattle feed. The ban applies specifically to tissues that can carry mad cow disease, including brains and spinal cords.

FDA's new proposal bans using in cattle feed any of those tissues, called specified risk materials, from cows older than 30 months. The age cutoff is specified because infection levels are believed to rise as cattle grow older.

Consumer groups criticized the ruling for not closing all the gaps in the feed ban.

"There is no question that we should not be feeding the remains of any mammals to food animals, and by not closing this dangerous loophole, we are exposing the American public to unnecessary risk," said Michael Hansen, a biologist for Consumers Union.

The new FDA proposal would ban from cattle feed:

— Brains and spinal cords of cattle 30 months and older.

— Brains and spinal cords of all cattle not approved for human consumption.

— Entire carcasses of cattle not approved for human consumption, if brains and spinal cords have not been removed.

— Tallow derived from at-risk tissues, if it contains more than .15 percent impurities.

— Mechanically separated beef from at-risk materials identified in the new proposal.

Mad cow disease is the common name for bovine spongiform encephalopathy (search), or BSE. A rare but fatal form of the disease in humans, variant Creutzfeldt-Jakob disease (search), is linked to eating meat products contaminated with BSE and was blamed for about 150 deaths, most of them in Britain.