Partly to comply with liquidity cover ratio requirements imposed in the wake of the financial crisis, U.S. banks ramped up their holdings of high-quality liquid assets. But once they got compliant, many of them shifted their asset allocations more to agency MBS and U.S. Treasuries, according to researchers at the Federal Reserve. This could have implications for the U.S. central bank’s massive balance sheet over the long haul, they added. As of Jan. 1, 2015, large banks in the U.S. have needed...