Rivals such as Chipotle Mexican Grill Inc (>> Chipotle Mexican Grill, Inc.) and McDonald's Corp (>> McDonald's Corporation) have announced similar supply-chain shifts, adding pressure on U.S. livestock producers to cut human antibiotics from their beef, hogs and poultry production. Advocacy groups said they were about to present Subway with a petition demanding the company set a timeline for its restaurants to stop serving meat from animals that had been treated with antibiotics.

Subway said customers will be able to start buying chicken raised without antibiotics at its more than 27,000 fast-food restaurants starting in March. The company did not state when antibiotic-free turkey will become available.

By 2018 it expects to shift all chicken and turkey supplies over to antibiotic-free meats. The company said that within six years after that, it will begin serving pork and beef only from animals raised without antibiotics.

"A change like this will take some time, particularly since the supply of beef raised without antibiotics in the U.S. is extremely limited and cattle take significantly longer to raise," said Dennis Clabby, executive vice president of Subway's independent purchasing cooperative. "But, we are working diligently with our suppliers to make it happen."

Subway's was one of the most aggressive moves by the food sector to reduce use of antibiotics in meat production. The Natural Resources Defense Council and U.S. Public Interest Research Group said they and other groups were about to deliver a petition with nearly 300,000 signatures calling for a concrete timeline for the step.

Subway officials could not immediately be reached for comment about the petition.

Public health experts and federal regulators have long been concerned that routine feeding of antibiotics to animals could lead to antibiotic-resistant superbugs, a health hazard for humans.

But finding enough protein raised in the United States without such drugs has proved to be a challenge for food companies.

McDonald's Corp has said it plans to source only chicken raised without antibiotics important to human medicine by 2017 for its U.S. restaurants. Dunkin' Donuts (>> Dunkin Brands Group Inc) will prohibit suppliers from using medically important antibiotics or antimicrobials in healthy animals, but has no timeline.

Many large U.S. fast-food chains still serve meat from farm animals that have been routinely fed antibiotics, consumer groups said in a report released last month.

(Reporting by P.J. Huffstutter; editing by David Gregorio)

By P.J. Huffstutter