Milliman reported that the funded status of the 100 largest corporate defined benefit pension plans worsened by $25 billion during April, as measured by the Milliman 100 Pension Funding Index (PFI). The deficit, it said, rose to $411 billion, primarily due to a decrease in the benchmark corporate bond interest rates used to value pension liabilities.

Discount rates, Milliman said, have fallen every month so far this year, with funding ratios following followed suit. In April alone, a decrease of 13 basis points in the monthly discount rate lowered rates to 3.65 percent.

In 2015, the average discount rate actually increased, from 4 percent to 4.4 percent, according to Russell Investments. That was thanks to an increase in the federal funds rate—the first in almost 10 years—but the effect hasn’t been large or lasting.

Nor would it necessarily be so, according to Alan Glickstein, a senior retirement consultant at Towers Watson. As a rule of thumb, Glickstein says that for every 100-basis-point move on the corporate bond rate, pension liabilities fluctuate 15 percent.

Aon Hewitt’s Pension Risk Tracker also indicated that funded status fell during April, to 77.6 percent.

Conning examined the effect of funding status volatility on companies in 2015 by analyzing data from the 20 largest U.S. corporate DB plans. These 20 companies, the report said, accounted for 43 percent of aggregate DB plan assets of the 238 U.S. corporate DB plans with $1 billion or more in assets. With the improvement in funding status in 2015 for these companies, plan sponsors broadly experienced a smaller impact on their balance sheets, long-term financial obligations and contributions than in 2014.

However, a key challenge facing plans in 2016, it said, is “the decrease in discount rates used to calculate both balance sheet and contribution liabilities…. Rates are likely to remain low or perhaps decrease over the short term.” Contributions will therefore get more expensive, putting pressure on companies’ bottom lines. When coupled with volatility, which “reduced assets and asset returns in 2015,” Conning said, “pensions risk management is likely to remain a key focus for many plan sponsors.”