NEW YORK – For the first time, Wall Street’s biggest bond-trading firms hold more U.S. Treasurys than corporate securities, signaling concern the economy’s rebound will be too slow to sustain record demand for riskier assets.

The 21 primary dealers that trade directly with the Federal Reserve held a total of $74.7 billion of Treasurys as of Dec. 28, compared with $61.1 billion of company debt, according to Fed data. The aggregate position in U.S. government bonds has increased from a $38.6 billion bet against the securities in May, while corporate holdings have tumbled 50 percent from $121.8 billion.

The Standard & Poor’s 500 Index rose 15 percent since the start of the fourth quarter, corporate bond sales worldwide are off to a record start and the U.S. jobless rate is the lowest since February 2009. But Fed policymakers say more monetary easing may be needed to ensure growth sticks. JPMorgan Chase economists said this month that they see gross domestic growth slowing to 2 percent this quarter from 3.5 percent in the final three months of 2011.

“Dealers, at least, don’t think the economy’s going to pick up speed in a hurry,” Ira Jersey, an interest-rate strategist at Credit Suisse Group in New York, a primary dealer, said in a Jan. 10 telephone interview. The economy isn’t heading “into recession, but more of the same, meandering growth instead of a significant pickup in growth and inflation,” he said.

The tilt to Treasurys reflects sentiment that the U.S. economic recovery will be held back by sovereign-debt turmoil in Europe, weakness in domestic housing prices and only a partial recovery of jobs lost since 2008. That the economy still needs a boost is underscored by the Fed’s pledge to keep its target interest rates for overnight loans between banks at historic lows of zero to 0.25 percent through mid-2013.

Treasurys outpaced company securities last year, returning 9.8 percent including reinvested interest, compared with 6.8 percent for the Bank of America Merrill Lynch U.S. Corporate & High Yield Master Index. Since the start of the credit crisis in mid-2007, Treasurys have returned 38 percent, just more than the 37 percent for companies.

Yields on company debt average 2.43 percentage points more than Treasurys, compared with a range of about 2.35 percentage points to 3.25 percentage points last year.

The shift is also being influenced by new rules following the financial crisis that limit bets dealers can place while they enjoy government guarantees and access to the Fed’s discount window.

The regulations limit banks’ investments in corporate bonds, while allowing them to hold and trade government securities.

Primary dealers held $28 billion more Treasurys than corporate securities due in more than one year on Dec. 28, the biggest gap in favor of government debt since the Fed began releasing the data in 2001. Wall Street held an average $159.1 billion more in longer-term company holdings than Treasurys during that period.

“A lot of it is stocking up on Treasuries in anticipation of selling them at hopefully reasonable, if not richer, levels,” William O’Donnell, head U.S. government bond strategist at primary dealer Royal Bank of Scotland Group in Stamford, Conn., said by phone Thursday. “Institutional investors are underinvested. Dealers know that on any dip they expect to see buyers, both domestically and overseas.”

Yields reached a 2011 high of 3.77 percent in February before plunging to a record low of 1.67 percent in September.

Traders expect 10-year yields to remain contained, rising to 2.25 percent by year-end. Economists are more bearish, forecasting a climb to 2.59 percent, according to estimates in a Bloomberg survey.