USDA started work on a final rule in early 2004 but has not said when any regulation will be ready. Agriculture Secretary Mike Johanns and House Agriculture Committee Chairman Bob Goodlatte have suggested some downer cattle are safe to eat.

"I think the risk of them (USDA) backing off is very high," said Caroline Smith DeWaal, food safety director for the consumer group Center for Science in the Public Interest. "It is important for them to move forward and finalize the rule."

Both US cases of mad cow were found in downer cattle, a nearly 8-year-old Holstein dairy cow in Washington state and a 12-year-old crossbreed Brahma beef cow in Texas.

The second US case of mad cow was confirmed last month.

Often elderly, downer cattle are considered at higher risk of brain-destroying mad cow disease. Among the symptoms of mad cow are staggering, falling and inability to walk. Last year, there were about 450,000 downers among 95 million cattle on 800,000 farms, ranches and feedlots, USDA says.

Two animal welfare groups, the Humane Society of the United States and Farm Sanctuary, said on Friday they formally petitioned USDA to issue a permanent ban on downers.

"Downed cattle are far more likely to have BSE (mad cow) than ambulatory livestock," said HSUS President Wayne Pacelle in a statement.

"The USDA has delayed making the downer ban permanent, perhaps in response to pressure from a few key members of Congress and some in the cattle industry, but the hedging and the delays must stop."

Some 23,000 comments from the public were filed on the downer ban and two related proposed regulations, said USDA meat safety spokesman Steve Cohen. They were being reviewed along with information from a special, stepped-up testing program for high-risk cattle.

"There's no date certain when there will be any type of decision regarding finalization," said Cohen.

The two major US cattle groups support a total ban.

"This added firewall ... is an illustration of the protection consumers have," said Gary Weber of the National Cattlemen's Beef Association.

Janet Riley of the American Meat Institute, a trade group for meatpackers, said, "At this point, we are not opposing the ban." As USDA acquires more knowledge about mad cow, Riley said, it would be appropriate to review mad cow safeguards.

Although the Humane Society said 95 percent of the comments supported the proposed USDA ban on downers, skeptics say it is an economic drain on ranchers and cattle feeders.

A young 1,100-pound steer is worth around $1,000 if slaughtered for steak and ground beef. If it stumbles and breaks a leg, it may bring less than $200 as a "downer" used for pet food. Mad cow generally is found in older cattle.

"An enormous waste of resources," was Goodlatte's assessment of the ban in late June. The Virginia Republican said the ban unfairly cheapened the value of young cattle.

In mid-April, Johanns cited a similar situation, of young, otherwise healthy cattle that break a leg during shipment. "There is no risk of BSE (mad cow) whatsoever," he said.

"Why are we doing this? I'm going to thoughtfully consider those arguments," he said.

Rep. Gary Ackerman, a New York Democrat, said in late June he would file a bill to ban slaughter of downed cattle, sheep, hogs, horses and mules for use as food.