The Federal Reserve Bank of New York began its latest effort to stimulate the economy and force borrowing rates even lower this week with $3.95 billion in mortgage-backed securities guaranteed by the government. The Federal Open Market Committee said on Sept. 21 that it would begin buying $400 billion in long-term Treasury bonds and reinvest principal payments on the agency MBS it bought previously in the crisis to purchase more. On Monday through Wednesday, the NY Fed bought $2 billion in securities guaranteed by Fannie Mae, another $1.35 billion from Freddie Mac and $600 million from Ginnie Mae programs. The purchases landed in the 3% to 4% coupon range. The NY Fed said it plans to purchase $10 billion in agency MBS this week and next and publish an estimate around the eighth business day of each month after that. "This number is subject to change, should the FOMC choose to alter its guidance to the desk during the monthly period or if market conditions warrant," the NY Fed said. The size of the Fed's agency debt and MBS portfolio totaled $988 billion as of Sept. 21. Write to Jon Prior. Follow him on Twitter @JonAPrior.