That means funded status has lost 5.3 percent since December 2013's high of 95.2 percent. 

Corporate plans owed their lower liabilities in September to an increase in the Aa corporate discount rate of 20 basis points, bringing it to 4.31 percent over the month. Since plan liabilities are calculated based on the yields of long-term investment-grade bonds, higher bond yields bring lower liabilities. 

"After benefiting from the first monthly decline in liabilities of more than 1 percent since November 2013, pension plans still failed to improve their funded status," said Andrew D. Wozniak, head of fiduciary solutions, ISSG. "Although U.S. large-cap equities outperformed the liabilities over the month, they were the only major equity class to do so. A sustained divergence between U.S. large-cap equity returns and other public equity classes could continue the downward trend in funded status." 

Public defined benefit plans missed targets by 3.5 percent in September, with assets falling 2.9 percent. ISSG said that year-over-year, public plans have met return targets of 7.5 percent. 

September's real return for endowments and foundations was negative 3.6 percent, thanks to a decline in assets of 3.1 percent. Private equity and real estate investment trusts contributed to the losses, making up 15 percent and 8 percent respectively of asset portfolios and falling 5.5 percent and 6.3 percent, respectively, over the course of September. Foundations and endowments are trailing their inflation-plus-spending target by 0.1 percent year over year, according to ISSG.