Sandwich chain Subway will start serving antibiotic-free chicken and turkey at its U.S. restaurants next year, and within the next nine years will stop selling any meat from animals given antibiotics, the company said Tuesday.

Subway competitors such as Chipotle Mexican Grill Inc and McDonald's Corp have announced similar supply-chain shifts, adding pressure on U.S. livestock producers to cut human antibiotics in their beef and hog units as well as poultry production.

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 of its chicken and turkey supplies over to antibiotic-free meats. In addition, the company said that within six years after that, it will begin selling only pork and beef raised without any 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 is one of the most aggressive moves by the food sector to reduce the use of antibiotics, particularly those deemed crucial to human health by the medical community, in meat production.

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

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 will prohibit suppliers from using medically important antibiotics or antimicrobials in healthy animals, but has not set a 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. The report concluded that many companies have not yet laid out plans to curb the practice. Its grades were based on public statements, survey responses and correspondence with individual chains.

(Reporting By P.J. Huffstutter; Editing by David Gregorio)