(Gary Fandel/Bloomberg/Getty Images)

McDonald's says it has asked its pork suppliers to outline plans that will eventually lead to more humane methods of raising pigs for slaughter.

The fast food corporation has given meat distributors until May to find a way to phase out the use of "gestation crates," cages that are used to confine pregnant sows.

The Humane Society of the United States is praising the move, which was announced jointly by McDonald's Corp. and the animal rights group.

"McDonald's believes gestation stalls are not a sustainable production system for the future," McDonald's announced in a written statement. "There are alternatives that we think are better for the welfare of sows."

In a phone conference, Humane Society of the United States President Wayne Pacelle told reporters he hopes the move will have a cascading effect on industrial agriculture.

"If a company that is very focused on price is committed to ending gestation crates then there's really no excuse for any company, no matter where it is in terms of its marketing, to continue to allow these extreme confinement crates."

Gestation crates, also called sow stalls, are narrow caged pens that hold pregnant sows for much of their adult lives. Once the animal gives birth, it is typically inseminated again to repeat the process for generally three years, when it is then culled. The stalls are too narrow for the animal to turn around. Pigs are known to sometimes coat the area in blood from the animal gnawing on the metal bars.

Studies from the Humane Society of the United States say the conditions can lead to urinary tract infections, lameness, and growth abnormities.

Suppliers that comply with McDonald's order will most likely move their pigs into "group housing" plans, which allow the animals to move more freely and socialize with each other.

Several leading pork distributors have recently announced they would be following similar measures, including Smithfield and Hormel Foods, maker of Spam and Black Label bacon. Eight states have enacted legislation banning gestation crates.

It is unclear how long it will take for McDonald's suppliers to phase out the practice completely. Last November, the fast food chain announced it had pulled business from one of the nation's largest egg farms shortly before an ABC News investigation aired regarding alleged sanitary and inhumane practices at the distributor.