US corporates took advantage of low borrowing costs to issue over $25bn of investment-grade bonds during the first three days of this week, in a period in which an index tracking yields of high-grade debt in the country fell to the lowest level in its 40-year history.

On Thursday, the Barclays US corporate investment grade index reached 3.096%, the lowest yield since the inception of the index in 1973, according to the bank.