Deflationary environments can have complex ramifications on financial markets, but investors generally seek out exposure to long-term fixed income securities when CPI begins to slide. That's because the value of a fixed stream of coupon payments increases as prices slide. Another driver behind the long-term bond rally is the scarcity of fixed-income ETF options offering a material current return. The consensus opinion is that the Fed won't start hiking interest rates until the latter part of next year, and perhaps not even until 2012. That means yields on short-term investment grade debt will remain depressed for the foreseeable future, putting investors who rely on their portfolio to provide current returns in a tough position. The Barclays 1-3 Year Treasury Bond Fund ( SHY), for example, has a 30-day SEC yield of 0.40%. The comparable yield for the 7-10 year fund ( IEI) is just 1.46%. But jump to TLT, which has a weighted average maturity of more than 28 years, and that yield climbs to 3.75%. Even if inflation is near zero -- meaning nominal rates equate to real return --these anemic yields don't offer investors much to get excited about. So it's not surprising that many are looking further down the duration curve to pick up a few additional basis points in yield. LONG-TERM BOND ETF OPTIONS Whether you're looking to pick up yield, reduce risk by moving out of equities or deflation-proof a portfolio, long-term bonds can make an intriguing option. There are a number of ETFs offering exposure to this asset class, including the three we have highlighted below: PIMCO 25+ Year Zero Coupon U.S. Treasury Index ETF ( ZROZ): This ETF tracks the Bank of America Merrill Lynch Long Treasury Principal STRIPS Index, a benchmark made up of securities representing the final principal payments of U.S. Treasury bonds with at least 25 years remaining to maturity. iShares Barclays Capital 20+ Year Treasury Bond Fund ( TLT): This ETF offers exposure to long-dated Treasuries, generally offering a higher yield and more interest rate risk than government bonds with a shorter time to maturity. Extended Duration Treasury ETF ( EDV): This Vanguard fund tracks the Barclays Capital U.S. Treasury STRIPS 20-30 Year Equal Par Bond Index, a benchmark made up of STRIPS -- a single coupon or principal payment on a Treasury security that has been stripped into separately tradable components -- with maturities ranging from 20 to 30 years.