In a world awash with U.S. government bonds, buyers of the longest-term Treasuries are facing a potential shortage of supply.

Excluding those held by the Federal Reserve, Treasuries due in 10 years or more account for just 5 percent of the $12.1 trillion market for U.S. debt. New rules designed to plug shortfalls at pension funds may now triple their purchases of longer-dated Treasuries, creating $300 billion in extra demand over the next two years that would equal almost half the $642 billion outstanding, Bank of Nova Scotia estimates.

Fewer available bonds, along with a lack of inflation and increased foreign buying, help to explain why longer-term Treasuries are surging this year even as the Fed pares its own bond purchases. The demand has pushed down yields on 30-year government debt by more than a half-percentage point to 3.37 percent, the most since 2000, data compiled by Bloomberg show.