A WTO official said the body received Mexico's official complaint, which starts a 60-day consultation period between Mexican and U.S. authorities. After that, Mexico can ask the WTO to set up an investigative panel. Such trade disputes can result in punitive sanctions, but usually after years of litigation.
The WTO official spoke on customary anonymity and copies of the complaint were not immediately available.
The Office of the U.S. Trade Representative in Washington could not immediately comment.
Canada's government filed its complaint earlier this month, saying it was concerned the U.S. rules were discriminating against Canadian agricultural exporters.
Under country-of-origin labeling, foreign cattle and pigs must be segregated in U.S. feedlots and packing plants, prompting some firms to only deal with American livestock. Foreign animals are also required to have more documentation about where they come from and, in the case of cattle, must have tags that indicate they are free of mad cow disease.
Canadian farm groups say a growing number of meat plants in the U.S. are refusing to accept Canadian cattle and hogs for processing since the Country Of Origin Labeling (COOL) law went into effect on Oct. 1.