Thirty-year bonds were last down 4/32 in price with the yield at 2.47 percent, having hit record lows of 2.35 percent last week.
"It's pretty aggressive, slightly more than the market was looking for,'' said Wilmer Stith, a fixed income portfolio manager at Wilmington Trust in Baltimore.
The bond purchases will further reduce the supply of high-quality debt, which has supported long-dated Treasurys as investors reach for higher yields. That is likely to continue to support bonds even as investors also anticipate improving growth and inflation that should eventually push yields higher.
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"It's bullish for Treasuries,'' said Kim Rupert, managing director of Action Economics in San Francisco. "We've seen record low yields in the periphery in Europe, and that will provide an underpinning for Treasuries for widening spreads.''
The World Economic Forum is well under way, with heads of state and government as well as global business leaders mingling in the Swiss mountain resort of Davos. Chinese Prime Minister Li Keqiang addressed delegates Thursday afternoon on the state of the Chinese economy.
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Reuters contributed to this report.