McDonald's says it plans to start using chicken raised without antibiotics and milk from cows that are not treated with an artificial growth hormone.

So far, the move will only apply to U.S. locations. A spokesperson for McDonald's Canada said that while the chain will continue to work with its suppliers, regulators and experts to improve its supply chain, "McDonald’s Canada recognizes the leadership of our U.S. colleagues but has made no decision at this time to change our current approach with chicken."

"Given Canada’s sourcing practices and supply chain are different to those in the U.S., we will be evaluating the implications of only sourcing chicken raised without antibiotics," the chain told CBC News.

The U.S. parent says the chicken change will take place within the next two years. It says suppliers will still be able to use a type of antibiotic called ionophores that keep chickens healthy and aren't used in humans. The milk change will take place later this year.

Many cattle, hog and poultry producers give their livestock antibiotics to make them grow faster and ensure they are healthy. The practice has become a public health issue, with officials saying it can lead to germs becoming resistant to drugs so that they're no longer effective in treating a particular illness in humans.

Catching up

Chipotle and Panera already say they serve chicken raised without antibiotics, but the announcement by McDonald's is notable because of its size; the company has more than 14,000 U.S. locations. Chipotle has nearly 1,800 locations, while Panera has almost 1,900 locations.

"This really does move the ball quite a bit," said Gail Hansen, a senior officer with the antibiotic resistance project with The Pew Charitable Trusts. Hansen noted that ionophores, the antibiotics that will be allowed by McDonald's, are not considered medically important for humans.

The announcement comes as McDonald's Corp. struggles to transform its image amid intensifying competition from smaller rivals positioning themselves as more wholesome alternatives.

The company has long battled negative perceptions about its food, but that has become a bigger vulnerability as more people shift toward options they feel are made with ingredients that are higher quality or meet standards on social responsibility.

After seeing customer visits to U.S. stores decline two years in a row, McDonald's had recently hinted changes could be on the way. Franchisees were told of the chicken and milk changes Tuesday night at a "Turnaround Summit" in Las Vegas.

Sales slump

Scott Taylor, a McDonald's franchisee who was at the conference, said ingredients are "becoming more and more important" to customers. And he said the company was suggesting it needs to "be where our consumers want and need us to be."

"You're going to see more stuff like that in the future," Taylor said.

In a statement, chicken supplier Tyson Inc. said it looks forward to working with McDonald's to meet its new standards. Tyson noted it has reduced the use of antibiotics effective on humans by more than 84 per cent since 2011.

Caroline Smith DeWaal, food safety director at the Center for Science in the Public Interest, said McDonald's decision to sell milk produced without rBST was a good step because the growth hormone can cause health problems in dairy cows.

As McDonald's fights to hold onto customers, the company has also made a number of leadership changes, admissions of shortcomings and declarations that changes are in the works.

The pressures reached the top of the company in late January, when the company said CEO Don Thompson would be replaced by Steve Easterbrook, its chief brand officer.

The CEO change officially took effect this week, and Easterbrook was at the franchisee summit in Las Vegas.

Already, McDonald's had been pushing back at critics.

Last year, it launched a campaign inviting people to ask frank, sometimes squeamish questions about its food, such as whether its beef contained worms (the answer was no). McDonald's has also been hammering home the fact that it cracks fresh eggs in stores to make its McMuffins in ad and signs in stores.