From The USDA Briefing Room - Part 2

Prior to the discovery of BSE in the United States, the largest export market for U.S. beef was Japan, followed by Mexico and the fast-growing South Korean market. Canada, in fourth place, had been gradually declining in importance for several years. These four countries accounted for over 90 percent of U.S. beef exports.
Beef export patterns in 2004 were altered dramatically by the BSE situation. Japan and South Korea (and various other countries) ceased all imports of U.S. beef, while other countries reopened their borders within a matter of months. Beef exports to Mexico rebounded during the year, making it the leading destination for U.S. beef beginning in 2004. Smaller amounts of beef went to Canada, which itself had large supplies of beef following its own trade disruptions related to BSE. Exports to Japan resumed in the second half of 2006, but growth has been slow due to the restriction that U.S. beef to Japan can only come from animals 20 months of age or younger. Exports to South Korea are limited to beef from animals 30 months of age or younger.