DENVER - Some of the nation's largest meatpacking companies are planning to pare production or shut down entire plants Monday in part to accommodate workers' requests for time off to participate in pro-immigration reform rallies and demonstrations, company officials said.

Greeley, Colo.-based Swift & Co. will shut down four of its five beef processing plants - including the one in Hyrum, Utah - and two of its three pork processing plants, spokesman Sean McHugh said in a statement Friday.

Tyson Foods, headquartered in Springdale, Ark., will close about a dozen beef and pork plants, the company said in a statement.

And Cargill Inc., based in Minneapolis, will shift production from Monday to Saturday at five beef plants, and shut down pork plants in Iowa and Illinois.

Omaha, Neb.-based ConAgra Foods Inc. doesn't plan to shut down any plants or slow production, a spokesman said, but will honor employees' requests for time off if possible.

Advocacy groups are encouraging immigrants around the country to leave work or school on Monday to demonstrate in support of immigration reforms that include a temporary worker program and in opposition to a bill approved by the U.S. House to build a fence along the U.S.-Mexico border and make it a felony to be in the country illegally.

Decisions to shut down meatpacking plants or shift production were made based on expected shortages of workers and other business conditions, company officials said.

Tyson chose to shut down production at 10 beef and pork plants "because of factors such as market conditions that permit scheduling changes and the potential shortage of workers," said spokesman Gary Mickelson. "We are asking workers not to take any unauthorized time off and instead seek pre-approval from their supervisor or participate during non-work hours."

Tyson, Swift and Cargill are the top three beef-producing companies. A spokesman for the fourth largest, Kansas City-based National Beef Packing Co. LLP, did not immediately return a call late Friday afternoon.

"It was very clear to us that immigration reform is a very emotional issue," Cargill Meat Solutions spokesman Mark Klein told the Fort Morgan (Colo.) Times.

The company's Fort Morgan plant can process up to 4,500 head of cattle a day, and about 70 percent of its 1,900 employees are Latino. Klein did not return several calls from The Associated Press.

Swift, whose Greeley beef plant employs about 1,700 people, said in a statement Friday that its decision to close that plant and five others was based on factors including previously scheduled maintenance, general market conditions and employees' requests for time off. Company spokesman Sean McHugh did not immediately return a call Friday, but said in the statement that Swift will still be able to meet its customers' needs.

ConAgra spokesman Chris Kircher said the company favors immigration reforms that support employees' rights and the company's ability to comply with federal laws to ensure a legal work force.

"We understand there are many illegal immigrants who are working hard to take care of themselves and their families, and any reforms need to take that into account," he said. "We obviously have some employees that this is an important issue and could affect their families, friends and neighbors."

He said the company has encouraged employees to talk with their supervisors about taking time off Monday, but said normal attendance policies will be in place.

Earlier this week, McHugh said Swift supports immigration reforms that "protect employers that comply in good faith with hiring requirements" and that would include border-security and guest-worker measures.