WASHINGTON (AP)--Government investigators say testing is too slow at times to prevent cattle from eating feed that might be contaminated, just one flaw they cited in a program to help stop bovine spongiform encephalopathy from spreading.

Feed safeguards are the most important firewall against BSE, said Sen. Tom Harkin, D-IA, who sought the report the Government Accountability Office issued Nov. 9.

"If FDA's testing program is not catching violations, and catching them in time, that needs to be corrected immediately," Harkin said.

FDA disputed the findings, arguing that the report unfairly focused on a small component of broad government efforts to stop BSE.

The only way BSE is known to spread is through feed containing certain tissue from infected animals. Adding animal protein to feed is commonly done to speed growth, but the United States has banned cattle protein in cattle feed since 1997.

Run by the Food and Drug Administration, the feed testing program is a small part of the government's campaign to keep BSE out of the food chain for animals and people.

The program has many weaknesses, according to GAO, the investigative arm of Congress.

In half the feed samples analyzed, FDA took more than a month to determine whether banned cattle protein was present. Cattle feed is eaten quickly after it's manufactured, and the feed may have been consumed before tests are finished, GAO said. The report examined 989 samples analyzed from August 2003 through June.

Investigators said the agency required no documentation of its reviews and that FDA officials were lax in overseeing the testing program.

The FDA said a timeframe for testing is not critical to minimizing cattle's exposure to potentially contaminated feed.

"FDA relies on its overall inspection program, rather than analytical methods alone, to enforce the feed ban," agency officials wrote in a response included in the GAO report. FDA also inspects feed mills and other firms subject to the feed ban.

FDA officials also said existing tests can't tell the difference between banned cattle protein and protein that is allowed. The government does allow blood and restaurant plate waste in cattle feed.