The cash price for corn is testing the plunge from 2 weeks ago; however, the ETF is relatively ~2.5% above that level. I know the Corn ETF uses futures, but even futures would track better than 2.5%, especially when the premium decreases as time goes by.
No the CORN ETF is performing exactly as designed. When you mention cash; you have referred to the spot market. The Tecurium AG ETFs attempt to maintaain only a 35% postion in the second to expire CBOT contract. IF it was postioned in the front month you would see tracking closer to the basis change between the spot and the front month contract.