CHICAGO (Reuters) - McDonald's Corp said on Monday it will work with its U.S. pork suppliers to phase out the use of gestation crates, the cramped stalls that millions of mother sows are confined to while they raise piglets.

The fast-food chain said the metal crates were "not a sustainable production system for the future." It said it would work with suppliers to hammer out a timeline for the phase-out and would talk about the planned next steps in May.

"There are alternatives that we think are better for the welfare of sows," Dan Gorsky, senior vice president for supply chain management for McDonald's North American said in a statement.

McDonald's joins a growing list of food producers and retailers, including Smithfield Foods, Hormel, Cargill, Burger King and Wolfgang Puck, that have promised to move away from pork bred from sows confined to the crates, which are typically too narrow to allow the sows to turn around.

But because McDonald's, which uses ham, sausage and bacon in its breakfast menu, is so big, Wayne Pacelle, the president and he chief executive officer of the Humane Society of the United States, predicted the announcement would have a "catalytic" impact on holdouts.

"They're clearly the biggest pork buyer in the fast-food sector and the largest restaurant chain in the world," Pacelle said. "So this will certainly have seismic effect within the pork industry."

The National Pork Board, a trade group representing the pork industry, released a statement defending the stalls as a "conventional" practice. It said that alternatives, including open pens, free-access stalls and pastures, have "welfare advantages and disadvantages that must be considered by an individual farmer."

Sows are often confined in the crates from just before the birth of the piglets until they are weaned months later. Defenders of the practice said that it minimizes the number of tiny piglets crushed by the huge sows in the first days of life. Opponents said it is cruel to keep the mothers so confined for weeks at a time.

The Humane Society said that 70 percent of the pork industry confines its pregnant pigs to gestation crates, which are banned in the European Union and eight U.S. states -- including California, Ohio and Michigan.

McDonald's vow to help put an end to the sow crates is the latest in a string of victories for advocates of animal welfare and food purity.

Last August, McDonald's stopped putting beef trimmings treated with ammonia hydroxide -- a USDA-approved ingredient that critics called "pink slime" -- into its burgers after a number of food activists, including celebrity chef Jamie Oliver, drew attention to the additive.

Also last summer, the Humane Society and U.S. egg producers agreed to work together to essentially double the size of the cages that the 280 million hens involved in U.S. egg production spend their lives in.

Late last month, an amendment to the Egg Products Inspection Act legislating the increased cage size was introduced in the U.S. House of Representatives. The Humane Society and the egg industry issued a release calling the measure "a top legislative priority."

(Editing by Greg McCune)

[Image via]

Mochila insert follows ...