New York

IN case exiting years of zero interest rates won't be hard enough, Federal Reserve officials have another challenge approaching quickly: when to begin unwinding trillions of dollars of bond purchases that constitute the world's largest fixed-income portfolio.

Less than a year from now, the Fed must decide whether to reinvest US$216 billion of proceeds from Treasury debt maturing in 2016, or shrink its balance sheet by allowing it to expire. By not reinvesting, the Fed would increase the supply of securities available to investors and put upward pressure on yields.