Clean diesel technology has been making its way off the highways and into farms and fields and other off-road uses since the 1990s. For growers, this has translated into more expensive and sophisticated tractors and diesel engine-driven equipment, but they will become even more so when the last tier of federal emission standards for off-road diesel engines takes effect next year. The U.S. Environmental Protection Agency published the first set (Tier 1) of emission standards for off-highway uses in 1996. Since then, emission regulations have progressively tightened, lowering the allowable levels of particulate matter and nitrogen oxide with each tier. […]