The CRB Index addresses that a bit. It includes a group of 19 total commodities of which grains are a part of. Includes things like copper, aluminum, etc  tho. I've also found some who post a weekly net fund ownership of corn, soybeans, and wheat. I find it's movement interesting, ( I.E. Arlan Suderman on twitter ) but have yet to find noticeable or reliable correlation to market impact yet, havent been watching it for that long either.