Mart's push to get its suppliers to give farm animals fewer antibiotics and more room to roam is expected to have a big impact on the food industry, experts say. Though the steps are voluntary, Wal-Mart, which sells more food than any other store, has a history of using its retail muscle to change the way products are made and sold across the retail industry.

Wal-Mart told The Associated Press that it's asking meat producers, eggs suppliers and others to use antibiotics only for disease prevention or treatment, not to fatten their animals, a common industry practice.

The guidelines also aim to get suppliers to stop using pig gestation crates and other housing that doesn't give animals enough space. They're also being asked to avoid painful procedures like de-horning or castration without proper painkillers.

Other major companies, including McDonald's Corp., Nestle and Starbucks Corp., have already pledged to reduce or eliminate the use of gestation crates for pregnant sows and otherwise improve animal treatment.

-- Associated Press