Democrats on a House Appropriations subcommittee criticized both ideas as wrong-headed. Participation in the traceback system should be mandatory, they said, and the testing program is a valuable tool.

USDA officials said the testing program was designed only to gauge the prevalence of mad cow disease. Public health is guarded by rules that that protect cattle feed from contamination and require meatpackers to remove the brains and spinal cords from older cattle, they say.

"By any stretch of the imagination, we have proven we have a very low incidence," said acting Undersecretary Chuck Lambert. He said USDA would shift to a "maintenance" program that did not need to test as many cattle.

"What we're devising and considering now ... is consistent with international standards," added Ron DeHaven, head of the Animal and Plant Health Inspection Service.

Always fatal in cattle, mad cow disease is believed to spread through feed that contains parts of infected cattle.

People can contract a human version of mad cow by eating meats from infected animals. No US-origin cases are known.

"It just seems to me that (scaling back) doesn't make much sense," said Connecticut Rep. Rosa DeLauro, the Democratic leader on the panel. "I had hoped the third case would have an effect on that."

Since June 2004, USDA has tested more than 650,000 head, mostly older and higher-risk cattle, for mad cow disease through its so-called expanded surveillance program. The program initially was expected to run 12-18 months.

Lambert and DeHaven said there was no decision yet on the scale of the revised system. USDA requested $17 million for tests in fiscal 2007, enough for 40,000 head. DeHaven told reporters that $17 million was a budgetary place-holder.

"It's pretty expensive," DeHaven told reporters, to run the surveillance program, which tests 6,000-7,000 head a week. USDA allotted $105.5 million in fiscal 2004 and 2005 for the tests.

When the first US case of mad cow was discovered in December 2003, the Bush administration embraced a nationwide traceback system as one of its prime responses to the disease.

Since mid-2005, USDA has revised plans for the system, first to rely on private databases to compile data on the whereabouts of food animals. More recently, USDA has said "no actions have been initiated" to require participation.

"It is our expectation that market forces are going to drive this to a mandatory program," said DeHaven. Retailers and meat processors often pay a premium to farmers and ranchers who can verify their animal's age, treatment and feed rations.

Democrat Allen Boyd of Florida joined DeLauro in skeptically asking how a voluntary system could assure the total compliance necessary for a reliable traceback system.

"It's probably not going to work very well for cattlemen or consumers," said Boyd.

About 205,000 of the 1 million US livestock producers have asked for "premises" ID numbers. USDA said last week it was ready to issue ID numbers for animals. Lambert said USDA hoped soon to complete a set of rules for the private databases that would compile information on animal movement.