* Fed announces $45 bln/month of Treasury buying * Fed adopts numerical thresholds for policy * 30-year bonds underperform, yields at month-high By Karen Brettell and Luciana Lopez NEW YORK, Dec 12 (Reuters) - Treasuries prices fell on Wednesday and 30-year bonds slumped after the Federal Reserve said it would shift more of its purchases to the five-year sector in a new easing program, with expectations the move would boost the economy and thus help riskier assets such as stocks. The Federal Reserve committed to monthly purchases of $45 billion in Treasuries on top of the $40 billion per month in mortgage-backed bonds it started buying in September. It will expand purchases to five-year notes from the current seven-, 10- and 30-year Treasuries. "They are buying less in 7-years through bonds, and buying some of the 5-years, which they weren't doing before. People also think that when the Fed does QE that the economy is going to get better, so it's a steeper yield curve," said Ira Jersey, an interest rate strategist at Credit Suisse in New York. Under Twist, the Fed has been selling shorter-dated Treasuries and using the proceeds to buy longer-dated debt. The program is set to expire at the end of December, and analysts say the Fed has little to no shorter-dated debt to sell. Thirty-year bonds dropped 1-03/32 in price to yield 2.897 percent, up from 2.84 percent late on Tuesday. The yield gap between 5-year notes and 30-year bonds also expanded to 225 basis points from 221 late on Tuesday to the widest since October. The new purchases will extend the average duration of the bonds the Fed holds on its balance sheet, though this is likely to fall back as many of those bonds are repaid. "The new purchases will have an average duration of 9 years, which is longer than the average 8 years of the bonds that the Fed currently holds," said Michael Schumacher, head of global rates strategy at UBS in Stamford, Connecticut. "Over the coming year, as bonds the Fed holds roll down, the duration is likely to fall back to below 8 years," he said. The Fed also took the unprecedented step of saying it would keep interest rates near zero until the jobless rate falls to 6.5 percent, well below its current level, so long as inflation and inflation expectations were contained. The shift to economic thresholds surprised some investors and increased the possibility that rates could increase from near zero before the previous guidance of mid-2015. "The bigger surprise for the bond market was the forward guidance, which changed from a calendar-based one to one based on economic thresholds," said Mike Lorizio, head of Treasuries trading at Manulife Asset Management in Boston. "Many in the market had thought this was a Q1 event, but with the fiscal cliff, they might have decided it was better to do it now," he said. Five-year notes fell 02/32 in price to yield 0.649 percent, from around 0.636 percent late on Tuesday. Benchmark 10-year notes dropped 13/32 to yield 1.699 percent. Treasuries had edged slightly higher earlier on Wednesday following strong demand for an auction of $21 billion of 10-year notes. The notes sold at a high yield of 1.65 percent, around 2 basis points lower than where the notes were trading before the sale. The Treasury on Thursday will sell $13 billion of 30-year bonds and next week it will auction two-, five-and seven-year notes along with five-year Treasury inflation-protected securities. The Treasury on Tuesday sold $32 billion of three-year notes.