As market stock market sell-offs extended from the end of 2015 to the beginning of 2016, many investors fled risky securities and reallocated their assets towards safe haven high-quality bonds, such as U.S. Treasury long-term bonds, which have the highest credit rating and almost non-existent risk of default. With the global economic slowdown becoming more apparent with the Chinese economy cooling and the commodity price rout intensifying, many investors are worrying that their stocks could be overvalued and are pulling their funds from stock funds. While U.S. Treasury bonds can provide refuge during times of stress, they are not immune to losses due to interest rate and inflation risks.

IShares 20+ Year Treasury Bond ETF

The iShares 20+ Year Treasury Bond ETF (NASDAQ: TLT) experienced year-to-date (YTD) capital inflows of $3 billion as of March 4, 2016. The fund commenced operations in July 2002 and tracks the performance of the Barclays U.S. 20+ Year Treasury Bond Index, which is composed of U.S. government bonds with maturity profiles of greater than 20 years. The fund gathered $9.8 billion in assets under management (AUM) and has 32 holdings in its portfolio. As of March 17, 2016, the fund showed a 12-month trailing yield of 2.41% and a 30-day Securities and Exchange Commission (SEC) yield of 2.45%. The fund has an average maturity of 26.62 years and an average duration of 17.7 years, which makes it highly sensitive to interest rate changes. The fund charged an expense ratio of 0.15% and had a four-star overall rating from Morningstar in the long government category.

IShares 10-20 Year Treasury Bond ETF

The iShares 10-20 Year Treasury Bond ETF (NYSEARCA: TLH) witnessed YTD capital inflows of $405 million as of March 4, 2016. The ETF was created in January 2007 to track the performance of the Barclays U.S. 10-20 Year Treasury Bond Index, which is composed of U.S. Treasury bonds with 10 to 20 years of maturity. With $852.6 million in AUM, the fund has 17 holdings. As March 17, 2016, the average maturity of the fund was 13.88 years and had an average duration of 10.25 years. The fund exhibited a 12-month trailing yield of 2.02% and a 30-day SEC yield of 1.9%. The fund charged an expense ratio of 0.15% and received a three-star overall rating from Morningstar.

SPDR Barclays Long Term Treasury ETF

The SPDR Barclays Long Term Treasury ETF (NYSEARCA: TLO) had YTD capital inflows of $91 million as of March 4, 2016. The fund was started in May 2007 to track the investment results of the Barclays Long U.S. Treasury Index, which consists of U.S. Treasury bonds with remaining maturities of over 10 years. The fund accumulated $423.5 million in AUM and has 46 holdings. As of March 17, 2016, the ETF generated a 12-month trailing yield of 2.4% and a 30-day SEC yield of 2.46%. The fund has a long average maturity profile of 25.31 years and an average duration of 17.82 years. Morningstar provided the fund with a four-star overall rating. The ETF had one of the lowest expense ratios among its peers, at 0.1% .

Vanguard Extended Duration Treasury ETF

The Vanguard Extended Duration Treasury ETF (NYSEARCA: EDV) experienced YTD capital inflows of $69 million as of March 4, 2016. The fund was created in December 2007 and seeks to track the investment returns of the Barclays U.S. Treasury STRIPS 20-30 Year Equal Par Bond Index. The benchmark index consists of zero-coupon U.S. Treasury separate trading of registered interest and principal securities (STRIPS) with maturities between 20 and 30 years. The fund had $490.5 million in AUM and 75 bonds in its portfolio. As of March 17, 2016, the fund's portfolio demonstrated an average maturity of 25.2 years and an average duration of 24.8 years. The fund's 30-day SEC yield was at 2.65% and the 12-month trailing yield was 2.78%.

PIMCO 25+ Year Zero Coupon U.S. Treasury Index ETF

The PIMCO 25+ Year Zero Coupon U.S. Treasury Index (NYSEARCA: ZROZ) witnessed YTD capital inflows of $49 million as of March 4, 2016. The fund commenced operations in October 2009 and tracks the investment performance of the BofA Merrill Lynch Long U.S. Treasury Principal STRIPS Index. The benchmark index included U.S. Treasury STRIPS with maturities above 25 years. The fund had about $221 million in AUM and 20 holdings. The fund had an average maturity of 27.99 years and an average duration of 27.99 years. The 12-month trailing yield was 2.63% and the 30-day SEC yield was 2.64%. Morningstar assigned the fund a five-star overall rating. The ETF charged investors an expense ratio of 0.16%.