The end of the Federal Reserve's program to buy mortgages backed by Fannie Mae and Freddie Mac could have a ripple effect on the market for U.S. government bonds. Once the Fed stops buying mortgage-backed securities at the end of March, private buyers will need to step in and take over in a market that the government has propped up since the financial crisis reached its peak. But they won't want to buy MBS unless the securities offer a better return than the current rate, so mortgage rates will likely rise.