Economist: RISING oil prices and mounting concern about global warming may have stoked demand for biofuels, but oilmen and farmers still have relatively little to do with one another. The biggest producers of ethanol are agribusinesses or farmers’ co-operatives growing maize (corn) in America and sugarcane in Brazil. Big Western oil firms such as Exxon Mobil and Royal Dutch Shell have confined themselves to trading biofuels and blending them into their wares, while dabbling in research into a “second …