Companies sold $54.3bn in US high-yield debt during the first quarter of the year, according to Dealogic, up from $9.6bn a year ago, as the sharp drop in interest rates made it cheaper to borrow. Including investment-grade debt, bond sales in March rose to their highest level since May. Last week, the growing debt issuance turned swap spreads, a metric of how issuers adjust their interest-rate exposure, negative for the first time on record. And borrowing costs measured by corporate-bond spreads have returned to December 2008 levels, though they are still far higher than they were before the housing bust. Selling debt has become much cheaper for companies as the credit crisis fades into history and investors lay their hopes on the recovery of the economy.