For Michigan’s apple growers, McDonald’s Corp. is putting a little more “happy” into the Happy Meal.

Starting this month in some markets, the world’s biggest restaurant chain is including peeled apple slices in all Happy Meals and plans to expand the effort to all 14,000 U.S. locations by the first quarter of 2012. Since 2004, customers had been offered a choice between apple slices and french fries.

McDonald’s was already buying apples from Michigan and other top-producing states and will now buy even more, though it’s unclear how much additional supply the company needs and where it will come from.

“We’re still in the process with suppliers to secure apples,” said Danya Proud, spokeswoman for Oak Brook, Ill.-based McDonald’s.

McDonald’s would likely continue to use Michigan apples, Proud said, though she wasn’t sure how much.

“We will use more,” she said.

A small percentage of McDonald’s apple supplies are purchased from sources outside the U.S., Proud said.

At least one Michigan company, Peterson Farms Inc., has supplied apples to McDonald’s, according to the Michigan Apple Committee. Peterson Farms did not respond to voice and e-mail messages.

McDonald’s and other restaurant chains have been under increasing pressure to reduce fat content and provide healthier menu options for children. In July, McDonald’s announced a commitment to improved nutrition choices that includes adjustments to its Happy Meal — the automatic inclusion of apple slices as well as a low-fat dairy option. The changes will reduce Happy Meals’ calorie count by about 20%, McDonald’s said.

“By adding fruit in every Happy Meal, McDonald’s hopes to address a challenge children face in meeting the recommended daily consumption of produce,” McDonald’s said in the statement. McDonald’s said it is also considering other produce items, including carrot sticks.

The Happy Meals will include four to five apple slices, or about 1.1 ounces total, compared to a 2.4-ounce portion when slices were optional.

McDonald’s sells about 2.5 million Happy Meals a year, according to industry estimates, and the company’s U.S. sales totaled $8.1 billion in 2010.