WITHIN-FIELD VARIATIONS OF MIDWEST CORN YIELDS AND PROBABLE CAUSES Yields in some sections of fields are commonly less than half of those in the best sections, as illustrated in figure 1, which shows corn yields in 2010 measured along two transects across a 10 ha (25 ac) field in Winnebago County, Wisconsin. These large yield differences are surprising because farmers usually use the same seed and rates of fertilizer, manure, and herbicide and the same equipment for tillage and seeding over a whole field. The south side of this field is 1.8 to 3.6 m (6 to 12 ft) lower than the north side, so as is commonly the case, the black topsoil layer is thicker on the south (lower and flatter) side than the north. Since yields tended to be better on the north side of this field, lower soil organic matter content was apparently not a dominant factor causing the lower yields. Installation of piezometers and monitoring of water tables during the following fall, winter, and early spring months showed water tables averaging about 80 cm (31 in) below the surface but rising in April and averaging about 40 cm (16 in) below the soil…