MEXICO CITY (Reuters) - Mexico is conducting tests on U.S. rice imports to ensure it is free of genetic material not approved for human consumption, a government official said on Wednesday. Marco Antonio Meraz, who heads a federal biosecurity and GMO commission, said the government was testing for the LL Rice 601 strain, which contaminated the U.S. commercial supply last year. The USA Rice Federation said on Wednesday Mexican officials had stopped rice shipments at the border and were asking for certification that the grain is free of the genetically modified material.