Commodity ETFs and ETNs

Which of the 6 Agriculture ETFs is Best?
http://seekingalpha.com/article/64802-which-of-the-6-agriculture-etfs-is-best

Commodity ETFs (exchange-traded funds) and ETNs (exchange-traded notes) List
(click on symbol for data and articles)

Broad Based Commodity ETFs and ETNs
GreenHaven Continuous Commodity Index (GCC)
GS Connect S&P GSCI Enhanced Commodity Total Return Strategy Index ETN (GSC)
iShares GSCI Commodity-Indexed Trust ETF (GSG)
iPath Dow Jones-AIG Commodity Index Total Return ETN (DJP)
iPath S&P GSCI Total Return Index ETN (GSP)
PowerShares DB Commodity Index Tracking Fund ETF (DBC)

Agricultural Commodities ETFs
ELEMENTS Linked to the MLCX Biofuels Index ETF (FUE)
ELEMENTS Linked to the MLCX Grains Index ETF (GRU)
ELEMENTS Linked to the Rogers International Commodity Index – Agriculture ETN (RJA)
iPath Dow Jones AIG-Agriculture ETN (JJA)
iPath Dow Jones AIG-Grains ETN (JJG)
iPath Dow Jones-AIG Livestock Total Return Sub-Index ETN (COW)
PowerShares DB Agriculture Fund ETF (DBA)

Gold, Silver and Metals ETFs
iPath DJ-AIG Industrial Metals Total Return Sub-Index (JJM)
iPath DJ-AIG Nickel Total Return Sub-Index (JJN)
iShares COMEX Gold Trust ETF (IAU)
iShares Silver Trust ETF (SLV)
PowerShares DB Gold Fund ETF (DGL)
streetTRACKS Gold Shares ETF (GLD)
PowerShares DB Silver Fund ETF (DBS)
PowerShares DB Precious Metals Fund ETF (DBP)
PowerShares DB Base Metals Fund ETF (DBB)

Oil and Gas ETFs and ETNs
Claymore MACROshares Oil Up Tradeable ETF (UCR)
iPath DJ-AIG Energy Total Return Sub-Index (JJE)
iPath DJ-AIG Natural Gas Total Return Sub-Index (GAZ)
iPath S&P GSCI Crude Oil Total Return Index ETN (OIL)
PowerShares DB Energy Fund ETF (DBE)
PowerShares DB Oil Fund ETF (DBO)
United States Gasoline Fund, LP ETF (UGA)
United States Oil Fund, LP ETF (USO)
United States 12 Month Oil Fund, LP ETF (USL)
United States Natural Gas Fund, LP ETF (UNG)

Commodities-Related ETFs
Van Eck Market Vectors Agribusiness ETF (MOO)
Van Eck Market Vectors Coal ETF (KOL)
Van Eck Market Vectors Gold Miners ETF (GDX)

What Are They?

Commodity ETFs (exchange traded funds) attempt to track the price of a single commodity, such as gold or oil, or a basket of commodities by holding the actual commodity in storage, or by purchasing futures contracts. Because futures provide leverage (more exposure than the actual cash invested), ETFs that use futures contracts have uninvested cash, which they usually park in interest-bearing government bonds. The interest on the bonds is used to cover the expenses of the ETF and to pay dividends to the holders.
Commodity ETNs (exchange traded notes) are non-interest paying debt instruments whose price fluctuates (by contractual commitment) with an underlying commodities index. Because they are debt obligations, ETNs are subject to the solvency of the issuer.
Commodities-related ETFs generally track the producers of commodities, such as mining companies. While the financial performance of those companies -- and thus their stocks -- may be highly leveraged to the underlying commodity, other factors can impact the profitability of production. The ETFs, therefore, may not reflect the performance of the underlying commodity. For example, gold miners are highly leveraged to the discovery of gold deposits, exchange rates and their relationships with the countries where gold deposits are found.
Why & How To Use Them

Commodities are a separate asset class from stocks and bonds, so they provide extra diversification in a portfolio.
The case for commodities: The industrialization of the China and India and the integration of Russia and Eastern Europe into the global economy are boosting demand for commodities, driving up prices. Many people believe that this will result in a long term uptrend ("super cycle") in commodity prices.
The case against commodities: In contrast to stocks and bonds, commodities are not income generating. So ownership of commodities, including via ETFs or ETNs, is a pure bet on prices. And the expenses charged by the ETF and ETN providers and in the cost of storing hard assets or trading futures eat away at the underlying value of the fund.
Commodity ETFs and ETNs can also be used as a hedge. For example, if you consume a large amount of gasoline and heating fuel and are concerned about the impact on your income of a rise in oil and gas prices, buying an oil and gas ETF can help offset your exposure.
What to Look Out For

Commodities ETFs that use futures have diverged significantly from the price of the hard commodities themselves. ETNs, in contrast, track the price of the commodity closely. See the articles in the Further Reading section below.
There are dramatic differences in structure of these ETFs and ETNs, even for the same commodities, leading to potential differences in performance and tax treatment.
ETFs and ETNs are treated differently for taxation purposes. Current opinion is that all gains on ETNs held for longer than one year are treated as long-term capital gains, whereas an investor owning a futures-based ETF is taxed on any capital gains on the underlying futures held by the fund using the taxation convention for futures, ie. at a hybrid rate of 60% long-term, 40% short-term each year on all gains, even if the investor doesn't sell the fund. (Check this carefully with your accountant.)
Further Reading

For long term investors considering including a commodity ETF in a diversified portfolio, the value of commodities as a diversifier is addressed by Nik Bienkowski in Commodities Outperform During Equity Market Downturns. Mebane Faber discusses a portfolio including commodities exposure in An Endowment Portfolio From Publicly-Traded Vehicles. For a negative view on the investment case for commodities, see Bill Miller on Oil, Silver, Other Commodities: Don't Buy!.
Roger Ehrenberg discusses using commodity ETFs to hedge real exposure to oil and gas in Think Carefully Before Macro Hedging Your Life/Work/Oil Exposures.
The underperformance of futures-based commodity ETFs relative to the actual commodity they are supposed to track is discussed in Scott Rothbort's US Oil Fund ETF Fails Investors Consistently. Richard Shaw presents the case for commodity ETNs over commodity ETFs in Troubled By ETF Tracking Failures? Try ETNs. See also The ETN Market Heats Up With Goldman Launch; More On the Way (Matt Hougan).
Should you use a broad commodities ETF or a set of ETFs or ETNs that track individual commodities? See Richard Kang's Is Commodity ETF Slicing and Dicing Necessary?.
For further analysis of these ETFs, and comparisons between them, see: Commodity ETF Overview (Tim Iacono), A Look at the New GreenHaven Commodity ETF (Hard Assets Investor), Commodity Exposure Via ETFs: A Fund Manager's Process (Keith Lenger), Ameristock Funds' New Gas Futures ETF: An Attractive Instrument In So Volatile a Market (Matt Hougan), First Gasoline ETF Comes to Market (Murray Coleman), Natural Gas ETF Is No Long Term Hold (Zman), New iShares GSCI Commodity Trust - Key Points To Understand (Market Participant), Commodities ETFs Protect The Little Guy (Tim Iacono), The New Generation of Diversified Commodity Indexes (Rich White), New ETF Tracks Oil-Prices Across 12 Months (Eli Hoffmann) and Digging Deeper Into Commodity-Based Funds (Keith Lenger).
For analysis and discussion of agricultural commoditiies ETFs see Powershares' Agricultural ETF: The Soft Commodities Slam Dunk (Nicolas Vardy), PowerShares DB Agriculture ETF: 'Optimum Yield' or Undue Risk? (Don Dion) and Which of the 6 Agriculture ETFs is Best? (Matthew D. McCall).