McDonald's Corp. is pushing its pork suppliers to stop confining sows in small pens known as gestation stalls, moving to address concerns raised by animal-welfare advocates -- and catch up with some competitors.

The burger giant on Monday said the pens are "not a sustainable production system" and there are alternatives. The move was announced in conjunction with the Humane Society of the U.S., which praised the decision.

Gestation stalls confine adult female hogs whose offspring are raised and slaughtered for bacon and sausage. The pens are typically about two feet wide, preventing sows from turning around, leaving only enough room to stand up and sit down.

The announcement is the latest in a series of measures McDonald's and other restaurant chains have taken to improve their public image at a time when they are under pressure not only from interest groups but from some consumers to be more environmentally friendly and healthful.

Chipotle Mexican Grill Inc. and some other fast-food chains are touting their animal-friendly supply chains. Chipotle ran a lengthy ad during the Grammy music awards on Sunday criticizing industrial hog-raising practices, including the use of small pens and chemical supplements like antibiotics.

McDonald's on Monday said it has been considering demanding the removal of gestation crates for years.

The National Pork Producers Council, a farmers trade group, said it continues to support the use of gestation stalls, but was also willing to assist in the transition for McDonald's suppliers.

The largest U.S. hog producer, Smithfield Foods Inc., said it has completed a third of its 10-year plan to move all its sows "into group housing systems." Cargill Inc. is also carrying out plans to replace the cages.

There are about 5.8 million breeding sows in the U.S. out of a total swine population of 65.9 million, according to Agriculture Department data. McDonald's says it purchases about 1% of total pork produced in the U.S.

The fast-food chain said it is beginning an assessment with its suppliers of how to reach its goal, and would disclose its next steps in May.

Under pressure from People for the Ethical Treatment of Animals, McDonald's in 2000 got its egg suppliers to stop "forced molting," in which hens near the end of their productive life are deprived of food to jolt them into laying more eggs.

Also that year, McDonald's required its egg suppliers to increase the size of the cages for laying hens.

Other chains have worked closely with animal-rights groups to alter their practices. Burger King Holdings Inc. has pointed to an award it received a few years ago from PETA recognizing improvements it's made in animal welfare.

In May, more than 550 health professionals and organizations called on McDonald's to stop marketing "junk food" to children and to retire Ronald McDonald.