Cattlemen, politicians and residents of this small farming community gathered Saturday for a rally in support of the state's cattle industry as more cows linked to the nation's first mad cow case were headed for slaughter.

Agriculture officials said they would begin removing cows Saturday from a group of 129 culled from the herd at the Mabton (search) dairy farm that was the last home of the Holstein known to have had the disease.

Investigators have determined that nine of the farm's cows came from the same Canadian farm where the sick Holstein was born. They believe it's possible the other 120 animals also came from that farm and could have been exposed to the same feed source, said Dr. Ron DeHaven, chief veterinarian for the U.S. Department of Agriculture (search).

Scientists believe contaminated feed is the main transmitter of mad cow disease.

USDA spokesman Nolan Lemon said the animals would be taken to a vacant slaughterhouse in Wilbur, about 60 miles west of Spokane. The plan was to remove and kill about 10 on Saturday, 10 more on Monday, then 20 to 30 a day through the week, he said.

"The importance is not on speed. It's not a question of the disease spreading to any of those animals," Lemon said.

This past week, authorities in Washington state killed a herd of 449 calves, including an offspring of the infected cow.

The decision to kill animals from a second quarantined herd with links to the infected cow came as Japanese agriculture and health officials were in Washington, D.C., to discuss safety procedures for American beef. Japan is the largest importer of U.S. beef.

U.S. agriculture officials announced the nation's first case of mad cow disease on Dec. 23. The diseased cow had been slaughtered Dec. 9.

Mad cow disease (search), or bovine spongiform encephalopathy, eats holes in the brains of cattle and is incurable. Humans can develop a brain-wasting illness, variant Creutzfeldt-Jakob disease, by consuming contaminated beef products. Officials have said, however, that the meat supply is safe.

The only other case of the disease in North America was discovered in an Angus cow from Alberta in May.

Officials have said they believe both animals probably were infected as calves because they were born before August 1997, the year that the United States and Canada banned cattle feed that contained parts of cattle, sheep or other cud-chewing animals.

The 129 cows being killed beginning on Saturday came from a herd of 4,000 dairy cows. Investigators were able to rule out killing most of the herd because records showed the animals were born at the Washington farm or arrived there from other locations, DeHaven said.

The killed cows will be tested for mad cow disease. If the tests are negative, they will be buried in a landfill. DeHaven said that if some test positive for mad cow, the carcasses will be incinerated or destroyed with acid.

Tests weren't conducted on the herd of calves killed earlier because the animals were too young for the disease to have yet appeared. BSE has an incubation period of several years.

The government prohibits the sale of the meat from the herds as a precaution aimed at reassuring Americans and international trading partners about the safety of the food supply.

Two cows from the Canadian farm were traced to a Mattawa, Wash., farm, which remains under a state hold order similar to a quarantine.