The Globe and Mail reports in its Wednesday edition that the recent drop in U.S. yields has raised speculation that a wave of buying of Treasury securities and derivative products called interest-rate swaps by mortgage portfolio managers and insurance companies was partly responsible for their decline. A Reuters dispatch to The Globe says these purchases are for "convexity" buying that helps offset the effects of mortgage refinancing to take advantage of lower interest rates. They could amplify the decline in Treasury yields that has taken place in recent weeks owing to concerns about economic growth. Convexity buying looks to have started early this month, as yields slid to the lowest since late October, from 15-month highs in February. The benchmark U.S. 10-year yield has not moved much since bottoming around 4.1 per cent on March 4, but the yield fell 18 basis points from March 13 to 4.17 per cent on March 20. "There's always convexity hedging going on," said Gennadiy Goldberg, head of U.S. rates strategy at TD Securities, citing the amount of uncertainty in markets from mid-January to the end of February and early March. "It varies in intensity. But it's a large enough move for folks to position their portfolios."