(Reuters) - The U.S. Treasury said on Monday it plans to borrow $673 billion in the third quarter, less than the May estimate of $821 billion, due to having a higher balance at the beginning of the quarter and lower outlays. The third-quarter estimate assumes an end-Sept. cash balance of $750 billion, the Treasury said in a statement. The Treasury has been cutting issuance since February, when it slashed its borrowing projections for the first and second quarters as part of a plan to reduce its cash balance. The Treasury said it issued $319 billion in net debt in the second quarter, ending the three-month period with a cash balance of $852 billion. It expects to borrow $703 billion in the Oct. to Dec. quarter, assuming an end-of-December cash balance of $800 billion. Reporting By Karen Brettell; Editing by Andrea Ricci Our Standards: The Thomson Reuters Trust Principles.