HICAGO, Dec. 20 — The indictment of Tyson Foods Inc., the nation's largest meat processor, on charges that it conspired to smuggle illegal immigrants to work at its plants, is a sign of how dependent the American food and agriculture system has become on foreign-born workers, many of them here illegally.
Because of this heavy reliance, agriculture experts say, a major effort to crack down on the hiring of illegal workers could disrupt the nation's food industry.
"This would really cripple the system," said William Heffernan, professor of rural sociology at the University of Missouri who has studied immigrant labor. "In the communities where these plants are located there isn't an alternative work force. They'd have to raise wages and improve the conditions."
Until 15 or 20 years ago, meatpacking plants in the United States were staffed by highly paid, unionized employees who earned about $18 an hour, adjusted for inflation. Today, the processing and packing plants are largely staffed by low-paid non- union workers from places like Mexico and Guatemala. Many of them start at $6 an hour.
The shift in the economics of the food and agriculture industry has made such jobs unappealing to Americans, but highly enticing to immigrants.
Companies like Tyson, Smithfield Foods and Conagra have profited from paying low wages, pushing production lines faster and hiring workers who are much more willing to endure the hazardous conditions of a meat-processing plant, industry experts say.
"This is certainly not unique to Tyson," Professor Heffernan said. "This has been around for a long time in the meat-processing industry. And employers can take advantage of these people because they can threaten to send them back."
The companies can also benefit from high turnover, which means workers often do not qualify for insurance or vacation time.
But high turnover means the companies face constant pressure to find new workers. That pressure may have been at the heart of the actions Tyson is accused of in the indictment, which was unsealed on Wednesday in Federal District Court in Chattanooga, Tenn.
The government charged the company and six of its employees with conspiring to transport illegal immigrants across the Mexican border and help them get counterfeit work papers for jobs at more than a dozen Tyson poultry plants. The indictment said that, to meet production and profit goals, Tyson officials would contact local smugglers near its plants to get more workers.
Industry experts said it has long been believed that American food companies recruit in Mexico and knowingly hire illegal workers. Some said the companies advertise on the radio in Mexico, distribute leaflets, show videos and hire immigrant smugglers, or "coyotes."
Eric Schlosser, the author of "Fast Food Nation," which chronicled changes in the food industry, said that the recruiting and hiring of illegal workers had been widespread for some time, and that big companies had used it to their benefit.
"For me, I don't care if those workers are from Mars," Mr. Schlosser said in a telephone interview. "It's the way in which using illegals allows them to do all the other practices, like speeding up the production lines, not listening to workers and having a high turnover rate which reduces the power of the workers."
Professor Heffernan, at the University of Missouri, said it was simple: "It's the race to the bottom; it's just the race to the bottom. Companies started breaking the unions, moving the plants to rural areas and hiring immigrants a long time ago."
The same foreign-born workers harvest fruits and vegetables in Florida and California, they milk cows on giant dairy farms in Wisconsin and Oregon and process and pack potatoes in Idaho.
"Immigrant labor, whether it's legal or illegal, is critical," said Keith Esplin, president of the Potato Growers of Idaho. "Most Mexicans here will have papers, but the farmers won't have any idea. There is real good counterfeit stuff out there."
Of course, because of the widespread use of counterfeit documents, no one knows for sure how many of them are working illegally. But industry and government officials say that, for better or worse, foreign- born workers are now one of the most vital elements in the American food and agriculture system.
About one million farm laborers are on the job at any one time, according to the United States Department of Agriculture. And a government study estimated that nearly 40 percent are illegal. A few years ago, the Immigration and Naturalization Service estimated that about 25 percent of meatpacking workers in the Midwest were probably illegal.
"We have enormous segments of agriculture that are critically dependent on hired farm labor," said Keith Collins, the chief economist at the Agriculture Department. "They are needed, particularly for harvesting perishables, like fruits and vegetables in Florida and California."
Asked whether many of the foreign-born migrant laborers were illegal, Mr. Collins said, Absolutely.
"It's in the hundreds of thousands," he said. "No doubt."
The government, though, has had little success in stemming the flow of illegal immigrants to food and agriculture companies. Federal raids on meatpacking plants sent many illegal workers back to their countries. But it outraged food companies, who complained of disruptions. Civil rights officials accused the government of harassing Mexicans and others from Central America. And Midwestern politicians sometimes complained that slowing down the work at meatpacking plants increased the supply of livestock and thereby harmed hog and cattle farmers, who had already been suffering from low prices for their goods.
The big meatpacking companies said today that they would work to ensure that they were not hiring illegal workers. But they have long maintained that it is a difficult thing to achieve. There is a complicated network of smugglers and a huge trade in the trafficking of fraudulent work documents.
Janet Riley of the American Meat Institute, which represents the big meat processors, said it would be hard to root out illegal immigrants because it was difficult to know how many were working at plants. The institute does not even have estimates on foreign-born workers.
"We don't know the number," Ms. Riley said. "But we do know it's significant."