Fixed-income asset allocation can often offer a boost to retirement nest eggs. Jim Barnes, director of fixed income at Bryn Mawr Trust, says people can reduce much of their overall portfolio risk by reducing some of their equities exposure and moving that money to bond funds as they get ready to retire. As investors consider their bond funds, Barnes says retirees should balance their short-term cash needs with their long-term needs. That might mean owning both shorter-duration funds and something longer-term that will produce income later. "What ends up happening is the income you need to generate might not be sufficient to last as long as they need it to," he says. Here are eight of the best bond exchange-traded funds for retirement.

iShares Short Treasury Bond ETF

Barnes says bondholders need to match their cash needs with their investments, so it’s a good idea to cover their near-term money needs with short- to intermediate-term funds. Typically it's one to three years for short term, five to seven for an intermediate one. “You don’t want to have to liquidate a holding prior to maturity,” he says. An example of a short-term bond fund is (SHV), which tracks a market-weighted index of U.S. Treasury debt that limits remaining maturities to 12 months. It has $21.2 billion in assets under management and an inexpensive expense ratio of 0.15%, or $15 for every $10,000 invested annually.

Invesco BulletShares 2021 Corporate Bond ETF

Daniel Milan, a managing partner at Cornerstone Financial Services, says he likes defined-maturity ETFs like Invesco’s BulletShares, which are market-weighted bond indexes. Rather than offering constant exposure to a segment of the bond market, BulletShares ETFs have a specific maturity. With the Fed having cut interest rates last year, Milan says he likes being able to control interest-rate risk with term-based bonds and knowing at maturity investors will be paid par value without having to actively purchase individual bonds. The largest BulletShares ETF by assets is (BSCL), which matures on Dec. 31, 2021. In addition to corporate bonds, there are BullsetShares for municipal, emerging markets and high-yield bonds.

iShares TIPS Bond ETF

Conservative bond investors could use the this ETF with the Treasury’s inflation-protected securities, Milan says, noting he uses this ETF (TIP) for his clients who want capital preservation or conservative growth. This ETF tracks the market-value-weighted Bloomberg Barclays US Treasury Inflation Protected Notes index, which holds a broad and diversified mix of inflation-protected securities with at least one year remaining in maturity. The fund has a duration of 7.39 years. It has $21 billion in assets under management and has much higher trading volume.

First Trust TCW Opportunistic Fixed Income ETF

With interest rates so low, investors may need to look at unconstrained bond funds, which are actively managed bond funds that allow the manager to make investment decisions in different areas of the fixed-income market, depending on market conditions, Milan says. In this space, he uses (FIXD) as one of the ETFs in a core bond position. The fund can hold securities from different issuers, sectors, maturities and credit qualities. Milan says the only constraints it has is to keep the weighted-average duration within one year of the Bloomberg Barclays US Aggregate Index. “It’s important to utilize a very good active manager to manage the duration and credit risk that’s so important now that fixed-income is in such a weird time,” he says.

`PIMCO Active Bond ETF

Marc Pfeffer, chief investment officer at CLS Investments, says for an investor seeking a core fixed-income holding, he chooses (BOND) for its strong yield and diversity. It’s an actively managed ETF, so its expense ratio is higher than a benchmark ETF like AGG, but the 12-month yield of 3.38% more than makes up for the price of 0.73% annually. It holds bonds with a duration of between four and five years, which is short enough that if rates go down, investors are protected, but long enough to have a decent yield. “It’s performed very well and it’s time-tested,” Pfeffer says.

SPDR Blackstone/GSO Senior Loan ETF

For investors willing to take on additional risk for higher yield, Pfeffer chooses (SRLN), an actively managed bond fund that has exposure to domestic and foreign noninvestment-grade, floating-rate senior loans. The fund has a very short duration as it resets every three months. Pfeffer says with the Fed likely keeping interest rates on hold at historically low levels, “you’re picking up a lot of yield by owning the bank loans right now.” With the economy solid, corporate defaults should remain low, too. SRLN’s 12-month yield is 5.4% and the expense ratio is 0.70%.

VanEck High Yield Municipal ETF

For aggressive investors seeking tax-free income, Pfeffer chooses (HYD), which tracks a market-weighted index of high-yield, long-term tax-exempt municipal bonds. It has a duration of 6.97 years, with $3.57 billion in AUM, which makes it a very liquid fund. “Municipality defaults are extremely low,” he says. With the economy still in good shape, he doesn’t see this fund being much riskier than tax-free funds with higher credit ratings. “I think it's worth the extra yield if you're looking for that,” he says. HYD’s 12-month yield is 4.07%.

Invesco Emerging Markets Sovereign Debt ETF

Many investors have little international exposure. (PCY) tracks an equal-weighted index of U.S. dollar-denominated sovereign debt in emerging markets. The holdings are at least three years to maturity, and the effective duration is 9.7 years. “I like this fund a lot,” he says, noting the fund is rebalanced quarterly and has a high 12-month yield of 4.8%. Pfeffer says this fund is suited better for investors who are willing to take on added risk for a higher return.