In response to farmer concerns about the effect of wheel traffic on alfalfa yields, we began a study to look at these effects. Studies were established at the UW Arlington Research Station and ABI Research Station (Napier, IA) during the spring of 2000. In a second study started in 2001, plots were established in Iowa, New York, Wisconsin, Minnesota, New York, Nebraska, Oklahoma, Kentucky and South Dakota. Wheel traffic was applied five days after cutting by driving approximately a 100-Hp tractor across the plots covering the entire plots with both wheel tracts. This was an attempt to simulate driving over the field with tractor, chopper, and wagons, or tractor baler, and wagon. In the 2001 study, wheel traffic conditions were the same except that treatments were no wheel traffic, wheel traffic 2 days after cutting and wheel traffic 5 days after cutting.