McDonald’s says it will require its U.S. pork suppliers to detail their plans to phase out using sow gestation stalls -– in which pigs cannot turn around -- following similar moves by a number of countries, states and other companies to end a practice that activists say is inhumane and can lead to health problems in the animals.

More than 5.8 million pigs are used for breeding in the U.S. pork industry, with an estimated 60 to 70 percent confined to gestation crates, or sow stalls, during their 112- to 115-day pregnancies. The metal crates are only a little bigger than the sow and are typically placed side-by-side in rows -- often with more than 20 sows to a row, according to a 2011 report by The Humane Society of the United States.

“McDonald’s believes gestation stalls are not a sustainable production system for the future. There are alternatives that we think are better for the welfare of sows,” Dan Gorsky, senior vice president of McDonald’s North America Supply Chain Management, said in a statement dated Friday. “McDonald’s wants to see the end of sow confinement in gestation stalls in our supply chain. We are beginning an assessment with our U.S. suppliers to determine how to build on the work already under way to reach that goal."

The crated animals can “suffer a number of significant welfare problems,” such as an elevated risk of urinary tract infections, weakened bones, overgrown hooves, lameness and behavioral restrictions, said the Humane Society report. Being confined prevents the sows from filling basic psychological needs and engaging in their natural behavior, such as rooting, grazing, wallowing and nest-building.

McDonald's announcement "came after years of dialogue" between the Humane Society and McDonald’s Corp., Wayne Pacelle, the nonprofit’s president and CEO, wrote on his blog.

"This is a bit of an earthquake in the world of the pork industry, with aftershocks that will be felt throughout the entire food retail sector. McDonald’s movement away from gestation crates is the latest acknowledgement from food sellers that extreme confinement practices have to go," he said.

The Humane Society said it has worked to pass laws to ban gestation crates in eight states: Florida, Ohio, Arizona, California, Colorado, Maine, Michigan and Oregon. A number of retailers have taken measures to shift from producers that still use the crates.

Sweden and the United Kingdom ban use of the crates, which are also being phased out in the European Union, New Zealand and Tasmania, according to the Humane Society.

Alternatives to the crates include free-range and pasture-based systems, indoor group housing and “turn-around” stalls, the Humane Society said in its report.

Some animals rights groups were advising a wait-and-see approach to McDonald's plan.

"We think that the company still has a long way to go to stop the suffering of pigs and also chickens, but we’re very encouraged that it’s now at least agreed to look at abandoning the 'iron maidens,’” Lindsay Rajt, a spokeswoman for People for the Ethical Treatment of Animals, told msnbc.com. "We’re just cautioning consumers to ... watch and wait."

Nathan Runkle, the executive director for Mercy for Animals, welcomed the fast-food chain's actions, saying it was a "step in the right direction" that he hoped others would follow, and he also urged the company to make "similar commitments to improve the welfare of other animals raised and killed for its restaurants."

McDonald's will share the results from its assessment with producers and outline its next steps in May, Gorsky said. The company purchases 1 percent of the total pork production in the United States every year.

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The HSUS

Pregnant hogs are seen in small stalls in a photo provided by The Humane Society of the United States to back up its allegations that Smithfield Foods was abusing the animals. The group said the photo was taken at a Smithfield plant in Waverly, Va., in November 2010.

Two years after shelving a pledge to phase out its practice of confining pregnant hogs in small, metal stalls, the world's largest pork producer on Thursday said it was ready to recommit. That was welcome news to the Humane Society of the United States, which had filed a complaint against the practice, and it urged Smithfield Foods' competitors to follow suit.

"(Our customers) want us to do that, and we've heard them loud and clear," Smithfield CEO Larry Pope said in a conference call with investors. "This company is going to do what's in the best interest of the business and the best interest of our customers."

Pregnant pigs are kept in gestation stalls where they stay during their four-month pregnancies. Afterward, they are moved for about three weeks to a stall large enough to nurse their piglets before being artificially inseminated and placed back into the stall for another round of breeding.

By the end of 2011, Smithfield said, 30 percent of its sows will be in group housing rather than in the stalls, and a complete phase-out should be done by 2017 -- the date initially set by the company in 2007 but then shelved in 2009.

Pope said the company "took a two-year holiday" from that conversion in order to deal with the economic downturn.

Humane Society CEO Wayne Pacelle didn't focus on the two-year delay, instead welcoming the move. "We recognize Smithfield's recommitment as progress," he said in a statement, "and urge its competitors such as Tyson, Hormel, Triumph, Prestage, Seaboard, and others to stop lagging behind and follow suit by adopting similar policies."

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Smithfield produces about 17 million market hogs a year at about 460 hog farms in the U.S. It also partners with more than 2,100 independent hog farmers and contract growers in the U.S.

The move comes a month after the Humane Society filed a complaint with the Securities and Exchange Commission, alleging that Smithfield was misleading investors and consumers by suggesting it does not abuse pigs.

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A year ago, the group released photos and video showing about 1,000 large female pigs crammed into gestation stalls at a Smithfield facility in Virginia. The undercover operation also revealed other alleged abuses, including a pig being shot with a stun gun and tossed into a trash bin while still alive and prematurely born piglets falling through gestation stall grates and dying in manure pits.

Msnbc.com's Miguel Llanos and The Associated Press contributed to this report.