Since the Federal Reserve began rolling out ever more creative steps to unfreeze credit markets, it has sold or pledged a growing portion of its portfolio of Treasurys in order to put loans on its balance sheet to banks and securities dealers backed by mortgage-backed securities and other shunned collateral. This has led some observers to worry that if the Fed continues at such a pace, it could run out of ammunition, forcing it to move to quantitative easing – in essence, buying up assets wholesale and allowing the federal funds rate to fall to zero.