If a futures market is in backwardation, the opposite of contango (the future price is lower than the present/earlier prices), an ETF long those futures will benefit from rolling forward the futures contracts it holds. This can happen if, say, this year's corn harvest is poor and there will be a shortage of corn to feed livestock until the next harvest. You might see backwardation in that situation. The price will be high for futures maturing in the near future, and lower further out.
Not all futures markets are in contango by exactly the amount of the cost of storage. That is a limit on how steep contango can be, as it create arbitrage opportunities (speculators did this with oil in 2008 after the crash, storing oil in tankers), but because commodities are produced continually, it is not necessary to store everything that is sold in later futures contracts. Some of it is selling future production. This happens to be the case right now, as corn is in backwardation.
If you are curious about term structures of various commodities, check out this site:
http://www.hardassetsinvestor.com/w...-backwardation.html?showall=&start=1#wticrude
Currently, natural gas and brent crude are in backwardation, meaning that ETFs holding these commodities long are benefiting from rolling futures contracts (sell high and buy lower in the future).