From The USDA Briefing Room - Part 3

In recent years, the significant suppliers of U.S. beef imports have been Australia, Canada, and New Zealand. Most of the beef from Australia and New Zealand goes into processed products such as ground beef. The United States also imports a significant portion of its cooked beef from Argentina and Brazil. (U.S. imports from these countries are restricted to cooked products due to disease restrictions.) Their combined share of the U.S. beef market is less than half that of the three largest exporters. In 2004 and 2005, imports from Uruguay jumped significantly, as that country joined the competition for the U.S. manufacturing beef market. U.S. imports from Uruguay have been below 2005 levels, as Uruguay began selling to other countries formerly supplied by Brazil and Argentina, whose beef exports were somewhat reduced due to disease-related problems and internal political and economic issues.
Canada reported the discovery of a case of BSE in May 2003 and since has reported several additional cases. Cattle and beef products from Canada were barred from entry into the United States after the announcement. In August 2003, beef imports from Canada resumed but were restricted to boneless products from cattle under 30 months of age. In November 2007, imports of beef from cattle over 30 months of age resumed with the restriction that imports must be from animals born after Canada's March 1997 feed ban. (BSE is believed to be transmitted by feeding products derived from infected animals, such as meat and bone meal, and the current feed ban prohibits such parts from ruminant feeds.) APHIS provides news and documents on BSE developments.